Glossary — Part 5

NO.

NO.

AIM 2/20/25

FIG 6−2−1

Ground−Air Visual Code for Use by Survivors

NO. MESSAGEMESSAGE CODE SYMBOLCODE SYMBOL

1 Require assistanceRequire assistance V

2 Require medical assistanceRequire medical assistance X

3 No or NegativeNo or Negative N

4 Yes or AffirmativeYes or Affirmative Y

5 Proceeding in this directionProceeding in this direction

IF IN DOUBT, USE INTERNATIONAL SYMBOLIF IN DOUBT, USE INTERNATIONAL SYMBOL S O S

INSTRUCTIONSINSTRUCTIONS

1. Lay out symbols by using strips of fabric or parachutes, pieces of wood, stones, or any available material.

2. Provide as much color contrast as possible between material used for symbols and background against which symbols are exposed.

3. Symbols should be at least 10 feet high or larger. Care should be taken to lay out symbols exactly as shown.

4. In addition to using symbols, every effort is to be made to attract attention by means of radio, flares, smoke, or other available means.

5. On snow covered ground, signals can be made by dragging, shoveling or tramping. Depressed areas forming symbols will

appear black from the air.

6. Pilot should acknowledge message by rocking wings from side to side.

FIG 6−2−2

Ground−Air Visual Code for use by Ground Search Parties

CODE SYMBOLCODE SYMBOLNO. MESSAGEMESSAGE

1 Operation completed.Operation completed. L L L

2 L LWe have found all personnel.We have found all personnel.

3 We have found only some personnel.We have found only some personnel.

We are not able to continue.We are not able to continue.4 X XReturning to base.Returning to base.

Have divided into two groups.Have divided into two groups.5 Each proceeding in direction indicated.Each proceeding in direction indicated.

6 Information received that aircraft is in this direction.Information received that aircraft is in this direction.

7 Nothing found. Will continue search.Nothing found. Will continue search. N N

Note: These visual signals have been accepted for international use and appear in Annex 12 to the Convention on International

Civil Aviation.

Emergency Services Available to Pilots6−2−8

2/20/25 AIM

FIG 6−2−3

Urgent Medical Assistance

NEED MEDICAL

ASSISTANCE-URGENT

Used only when life is at stake

NEED MEDICAL

ASSISTANCE-URGENT

Used only when life is at stake

FIG 6−2−4

All OK

ALL OK-DO NOT WAIT

Wave one arm overhead

ALL OK-DO NOT WAIT

Wave one arm overhead

Emergency Services Available to Pilots 6−2−9

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FIG 6−2−5

Short Delay

CAN PROCEED SHORTLY

WAIT IF PRACTICABLE

One arm horizontal

CAN PROCEED SHORTLY

WAIT IF PRACTICABLE

One arm horizontal

FIG 6−2−6

Long Delay

NEED MECHANICAL HELP

OR PARTS - LONG DELAY

Both arms horizontal

NEED MECHANICAL HELP

OR PARTS - LONG DELAY

Both arms horizontal

6−2−10 Emergency Services Available to Pilots

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FIG 6−2−7

Drop Message

Make throwing motion

FIG 6−2−8

Receiver Operates

OUR RECEIVER IS

OPERATING

Cup hands over ears

OUR RECEIVER IS

OPERATING

Cup hands over ears

Emergency Services Available to Pilots 6−2−11

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FIG 6−2−9

Do Not Land Here

DO NOT ATTEMPT

TO LAND HERE

Both arms waved across face

DO NOT ATTEMPT

TO LAND HERE

Both arms waved across face

FIG 6−2−10

Land Here

LAND HERE

Both arms forward horizontally,

squatting and point in direction

of landing - Repeat

LAND HERE

Both arms forward horizontally,

squatting and point in direction

of landing - Repeat

6−2−12 Emergency Services Available to Pilots

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FIG 6−2−11

Negative (Ground)

NEGATIVE (NO)

White cloth waved horizontally

NEGATIVE (NO)

White cloth waved horizontally

FIG 6−2−12

Affirmative (Ground)

AFFIRMATIVE (YES)

White cloth waved vertically

AFFIRMATIVE (YES)

White cloth waved vertically

Emergency Services Available to Pilots 6−2−13

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FIG 6−2−13

Pick Us Up

PICK US UP-

PLANE ABANDONED

Both arms vertical

PICK US UP-

PLANE ABANDONED

Both arms vertical

FIG 6−2−14

Affirmative (Aircraft)

Affirmative reply from aircraft:

AFFIRMATIVE (YES)

Dip nose of plane several times

6−2−14 Emergency Services Available to Pilots

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FIG 6−2−15

Negative (Aircraft)

Negative reply from aircraft:Negative reply from aircraft:

NEGATIVE (NO)NEGATIVE (NO)

Fishtail planeFishtail plane

FIG 6−2−16

Message received and understood (Aircraft)

Message received and understood by aircraft:Message received and understood by aircraft:

Day or moonlight - Rocking wingsDay or moonlight - Rocking wings

Night - Green flashed from signal lampNight - Green flashed from signal lamp

Emergency Services Available to Pilots 6−2−15

AIM 2/20/25

6−2−16 Emergency Services Available to Pilots

FIG 6−2−17

Message received and NOT understood (Aircraft)

Message received and NOT understood by aircraft:

Day or moonlight - Making a complete right-hand circle

Night-Red flashes from signal lamp.

Message received and NOT understood by aircraft:

Day or moonlight - Making a complete right-hand circle

Night-Red flashes from signal lamp.

2/20/25 AIM

Section 3. Distress and Urgency Procedures

6−3−1. Distress and Urgency Communications

a. A pilot who encounters a distress or urgency condition can obtain assistance simply by contacting the air

traffic facility or other agency in whose area of responsibility the aircraft is operating, stating the nature of the

difficulty, pilot’s intentions and assistance desired. Distress and urgency communications procedures are

prescribed by the International Civil Aviation Organization (ICAO), however, and have decided advantages over

the informal procedure described above.

b. Distress and urgency communications procedures discussed in the following paragraphs relate to the use

of air ground voice communications.

c. The initial communication, and if considered necessary, any subsequent transmissions by an aircraft in

distress should begin with the signal MAYDAY , preferably repeated three times. The signal PAN−PAN should

be used in the same manner for an urgency condition.

d. Distress communications have absolute priority over all other communications, and the word MAYDAY

commands radio silence on the frequency in use. Urgency communications have priority over all other

communications except distress, and the word PAN−PAN warns other stations not to interfere with urgency

transmissions.

e. Normally, the station addressed will be the air traffic facility or other agency providing air traffic services,

on the frequency in use at the time. If the pilot is not communicating and receiving services, the station to be called

will normally be the air traffic facility or other agency in whose area of responsibility the aircraft is operating,

on the appropriate assigned frequency. If the station addressed does not respond, or if time or the situation

dictates, the distress or urgency message may be broadcast, or a collect call may be used, addressing “Any Station

(Tower)(Radio)(Radar).”

f. The station addressed should immediately acknowledge a distress or urgency message, provide assistance,

coordinate and direct the activities of assisting facilities, and alert the appropriate search and rescue coordinator

if warranted. Responsibility will be transferred to another station only if better handling will result.

g. All other stations, aircraft and ground, will continue to listen until it is evident that assistance is being

provided. If any station becomes aware that the station being called either has not received a distress or urgency

message, or cannot communicate with the aircraft in difficulty, it will attempt to contact the aircraft and provide

assistance.

h. Although the frequency in use or other frequenc ies assigned by ATC are preferable, the following

emergency frequencies can be used for distress or urgency communications, if necessary or desirable:

121.5 MHz and 243.0 MHz. Both have a range generally limited to line of sight. 121.5 MHz is guarded by

direction finding stations and some military and civil aircraft. 243.0 MHz is guarded by military aircraft. Both

121.5 MHz and 243.0 MHz are guarded by military towers, most civil towers, and radar facilities. Normally

ARTCC emergency frequency capability does not extend to radar coverage limits. If an ARTCC does not respond

when called on 121.5 MHz or 243.0 MHz, call the nearest tower.

6−3−2. Obtaining Emergency Assistance

a. A pilot in any distress or urgency condition should immediately take the following action, not necessarily

in the order listed, to obtain assistance:

1. Climb, if possible, for improved communications, and better radar and direction finding detection.

However, it must be understood that unauthorized climb or descent under IFR conditions within controlled

airspace is prohibited, except as permitted by 14 CFR section 91.3(b).

Distress and Urgency Procedures 6−3−1

AIM 2/20/25

2. If equipped with a radar beacon transponder (civil) or IFF/SIF (military):

(a) Continue squawking assigned Mode A/3 discrete code/VFR code and Mode C altitude encoding when

in radio contact with an air traffic facility or other agency providing air traffic services, unless instructed to do

otherwise.

(b) If unable to immediately establish communications with an air traffic facility/agency, squawk Mode

A/3, Code 7700/Emergency and Mode C.

3. Transmit a distress or urgency message consisting of as many as necessary of the following elements,

preferably in the order listed:

(a) If distress, MAYDAY , MAYDAY , MAY−DAY; if urgency, PAN−PAN, PAN−PAN, PAN−PAN.

(b) Name of station addressed.

(c) Aircraft identification and type.

(d) Nature of distress or urgency.

(e) Weather.

(f) Pilots intentions and request.

(g) Present position, and heading; or if lost, last known position, time, and heading since that position.

(h) Altitude or flight level.

(i) Fuel remaining in minutes.

(j) Number of people on board.

(k) Any other useful information.

REFERENCE−

Pilot/Controller Glossary Term− Fuel Remaining.

b. After establishing radio contact, comply with advice and instructions received. Cooperate. Do not hesitate

to ask questions or clarify instructions when you do not understand or if you cannot comply with clearance. Assist

the ground station to control communications on the frequency in use. Silence interfering radio stations. Do not

change frequency or change to another ground station unless absolutely necessary. If you do, advise the ground

station of the new frequency and station name prior to the change, transmitting in the blind if necessary. If

two−way communications cannot be established on the new frequency, return immediately to the frequency or

station where two−way communications last existed.

c. When in a distress condition with bailout, crash landing or ditching imminent, take the following additional

actions to assist search and rescue units:

1. Time and circumstances permitting, transmit as many as necessary of the message elements in

subparagraph a3 above, and any of the following that you think might be helpful:

(a) ELT status.

(b) Visible landmarks.

(c) Aircraft color.

(d) Number of persons on board.

(e) Emergency equipment on board.

2. Actuate your ELT if the installation permits.

3. For bailout, and for crash landing or ditching if risk of fire is not a consideration, set your radio for

continuous transmission.

4. If it becomes necessary to ditch, make every effort to ditch near a surface vessel. If time permits, an FAA

facility should be able to get the position of the nearest commercial or Coast Guard vessel from a Coast Guard

Rescue Coordination Center.

6−3−2 Distress and Urgency Procedures

2/20/25 AIM

5. After a crash landing, unless you have good reason to believe that you will not be located by search

aircraft or ground teams, it is best to remain with your aircraft and prepare means for signaling search aircraft.

6−3−3. Ditching Procedures

FIG 6−3−1

Single Swell (15 knot wind)

SWELLSWELL

DITCHING

HEADING

WIND

FIG 6−3−2

Double Swell (15 knot wind)

PRIMARY

SWELL

PRIMARY

SWELL

SECONDARY

SWELL

SECONDARY

SWELL

HEADING

WIND

DITCHING

Distress and Urgency Procedures 6−3−3

AIM 2/20/25

FIG 6−3−3

Double Swell (30 knot wind)

PRIMARY

SWELL

PRIMARY

SWELL

SECONDARY

SWELL

SECONDARY

SWELL

WIND

DITCHING

HEADING

FIG 6−3−4

(50 knot wind)

SWELLSWELL

WINDWIND

Aircraft with low landing speeds - land into the wind.

Aircraft with high landing speeds - choose compromise

heading between wind and swell.

Both - land on back side of swell.

6−3−4 Distress and Urgency Procedures

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FIG 6−3−5

Wind−Swell−Ditch Heading

GOOD FAIR

BESTBEST

DIRECTION OF

SWELL MOVEMENT

GOOD

Landing parallel to the major swell

GOOD !!

BACK SIDE

POOR!!!

FACE

Landing on the face and back of swell

a. A successful aircraft ditching is dependent on three primary factors. In order of importance they are:

1. Sea conditions and wind.

2. Type of aircraft.

3. Skill and technique of pilot.

b. Common oceanographic terminology.

1. Sea. The condition of the surface that is the result of both waves and swells.

2. Wave (or Chop). The condition of the surface caused by the local winds.

3. Swell. The condition of the surface which has been caused by a distance disturbance.

4. Swell Face. The side of the swell toward the observer. The backside is the side away from the observer.

These definitions apply regardless of the direction of swell movement.

5. Primary Swell. The swell system having the greatest height from trough to crest.

6. Secondary Swells. Those swell systems of less height than the primary swell.

7. Fetch. The distance the waves have been driven by a wind blowing in a constant direction, without

obstruction.

8. Swell Period. The time interval between the passage of two successive crests at the same spot in the

water, measured in seconds.

9. Swell Velocity. The speed and direction of the swell with relation to a fixed reference point, measured

in knots. There is little movement of water in the horizontal direction. Swells move primarily in a vertical motion,

similar to the motion observed when shaking out a carpet.

10. Swell Direction. The direction from which a swell is moving. This direction is not necessarily the result

of the wind present at the scene. The swell may be moving into or across the local wind. Swells, once set in

Distress and Urgency Procedures 6−3−5

AIM 2/20/25

motion, tend to maintain their original direction for as long as they continue in deep water, regardless of changes

in wind direction.

11. Swell Height. The height between crest and trough, measured in feet. The vast majority of ocean swells

are lower than 12 to 15 feet, and swells over 25 feet are not common at any spot on the oceans. Successive swells

may differ considerably in height.

c. In order to select a good heading when ditching an aircraft, a basic evaluation of the sea is required.

Selection of a good ditching heading may well minimize damage and could save your life. It can be extremely

dangerous to land into the wind without regard to sea conditions; the swell system, or systems, must be taken

into consideration. Remember one axiom− AVOID THE FACE OF A SWELL.

1. In ditching parallel to the swell, it makes little difference whether touchdown is on the top of the crest

or in the trough. It is preferable, however, to land on the top or back side of the swell, if possible. After

determining which heading (and its reciprocal) will parallel the swell, select the heading with the most into the

wind component.

2. If only one swell system exists, the problem is relatively simple −even with a high, fast system.

Unfortunately, most cases involve two or more swell systems running in different directions. With more than one

system present, the sea presents a confused appearance. One of the most difficult situations occurs when two

swell systems are at right angles. For example, if one system is eight feet high, and the other three feet, plan to

land parallel to the primary system, and on the down swell of the secondary system. If both systems are of equal

height, a compromise may be advisable−select an intermediate heading at 45 degrees down swell to both systems.

When landing down a secondary swell, attempt to touch down on the back side, not on the face of the swell.

3. If the swell system is formidable, it is considered advisable, in landplanes, to accept more crosswind in

order to avoid landing directly into the swell.

4. The secondary swell system is often from the same direction as the wind. Here, the landing may be made

parallel to the primary system, with the wind and secondary system at an angle. There is a choice to two directions

paralleling the primary system. One direction is downwind and down the secondary swell, and the other is into

the wind and into the secondary swell, the choice will depend on the velocity of the wind versus the velocity and

height of the secondary swell.

d. The simplest method of estimating the wind direction and velocity is to examine the windstreaks on the

water. These appear as long streaks up and down wind. Some persons may have difficulty determining wind

direction after seeing the streaks on the water. Whitecaps fall forward with the wind but are overrun by the waves

thus producing the illusion that the foam is sliding backward. Knowing this, and by observing the direction of

the streaks, the wind direction is easily determined. Wind velocity can be estimated by noting the appearance

of the whitecaps, foam and wind streaks.

1. The behavior of the aircraft on making contact with the water will vary within wide limits according to

the state of the sea. If landed parallel to a single swell system, the behavior of the aircraft may approximate that

to be expected on a smooth sea. If landed into a heavy swell or into a confused sea, the deceleration forces may

be extremely great−resulting in breaking up of the aircraft. Within certain limits, the pilot is able to minimize

these forces by proper sea evaluation and selection of ditching heading.

2. When on final approach the pilot should look ahead and observe the surface of the sea. There may be

shadows and whitecaps−signs of large seas. Shadows and whitecaps close together indicate short and rough seas.

Touchdown in these areas is to be avoided. Select and touchdown in any area (only about 500 feet is needed)

where the shadows and whitecaps are not so numerous.

3. Touchdown should be at the lowest speed and rate of descent which permit safe handling and optimum

nose up attitude on impact. Once first impact has been made, there is often little the pilot can do to control a

landplane.

e. Once preditching preparations are completed, the pilot should turn to the ditching heading and commence

let−down. The aircraft should be flown low over the water, and slowed down until ten knots or so above stall.

6−3−6 Distress and Urgency Procedures

2/20/25 AIM

At this point, additional power should be used to overcome the increased drag caused by the nose up attitude.

When a smooth stretch of water appears ahead, cut power, and touchdown at the best recommended speed as fully

stalled as possible. By cutting power when approaching a relatively smooth area, the pilot will prevent

overshooting and will touchdown with less chance of planing off into a second uncontrolled landing. Most

experienced seaplane pilots prefer to make contact with the water in a semi−stalled attitude, cutting power as the

tail makes contact. This technique eliminates the chance of misjudging altitude with a resultant heavy drop in

a fully stalled condition. Care must be taken not to drop the aircraft from too high altitude or to balloon due to

excessive speed. The altitude above water depends on the aircraft. Over glassy smooth water, or at night without

sufficient light, it is very easy, for even the most experienced pilots to misjudge altitude by 50 feet or more. Under

such conditions, carry enough power to maintain nine to twelve degrees nose up attitude, and 10 to 20 percent

over stalling speed until contact is made with the water. The proper use of power on the approach is of great

importance. If power is available on one side only, a little power should be used to flatten the approach; however,

the engine should not be used to such an extent that the aircraft cannot be turned against the good engines right

down to the stall with a margin of rudder movement available. When near the stall, sudden application of

excessive unbalanced power may result in loss of directional control. If power is available on one side only, a

slightly higher than normal glide approach speed should be used. This will ensure good control and some margin

of speed after leveling off without excessive use of power. The use of power in ditching is so important that when

it is certain that the coast cannot be reached, the pilot should, if possible, ditch before fuel is exhausted. The use

of power in a night or instrument ditching is far more essential than under daylight contact conditions.

1. If no power is available, a greater than normal approach speed should be used down to the flare−out. This

speed margin will allow the glide to be broken early and more gradually, thereby giving the pilot time and distance

to feel for the surface − decreasing the possibility of stalling high or flying into the water. When landing parallel

to a swell system, little difference is noted between landing on top of a crest or in the trough. If the wings of aircraft

are trimmed to the surface of the sea rather than the horizon, there is little need to worry about a wing hitting a

swell crest. The actual slope of a swell is very gradual. If forced to land into a swell, touchdown should be made

just after passage of the crest. If contact is made on the face of the swell, the aircraft may be swamped or thrown

violently into the air, dropping heavily into the next swell. If control surfaces remain intact, the pilot should

attempt to maintain the proper nose above the horizon attitude by rapid and positive use of the controls.

f. After T ouchdown. In most cases drift, caused by crosswind can be ignored; the forces acting on the aircraft

after touchdown are of such magnitude that drift will be only a secondary consideration. If the aircraft is under

good control, the “crab” may be kicked out with rudder just prior to touchdown. This is more important with high

wing aircraft, for they are laterally unstable on the water in a crosswind and may roll to the side in ditching.

REFERENCE−

This information has been extracted from Appendix H of the “National Search and Rescue Manual.”

6−3−4. Special Emergency (Air Piracy)

a. A special emergency is a condition of air piracy, or other hostile act by a person(s) aboard an aircraft, which

threatens the safety of the aircraft or its passengers.

b. The pilot of an aircraft reporting a special emergency condition should:

1. If circumstances permit, apply distress or urgency radio−telephony procedures. Include the details of the

special emergency.

REFERENCE−

AIM, Para 6−3−1, Distress and Urgency Communications.

2. If circumstances do not permit the use of prescribed distress or urgency procedures, transmit:

(a) On the air/ground frequency in use at the time.

(b) As many as possible of the following elements spoken distinctly and in the following order:

(1) Name of the station addressed (time and circumstances permitting).

Distress and Urgency Procedures 6−3−7

AIM 2/20/25

(2) The identification of the aircraft and present position.

(3) The nature of the special emergency condition and pilot intentions (circumstances permitting).

(4) If unable to provide this information, use code words and/or transponder as follows:

Spoken Words

TRANSPONDER SEVEN FIVE ZERO ZERO

Meaning

I am being hijacked/forced to a new destination

Transponder Setting

Mode 3/A, Code 7500

NOTE−

Code 7500 will never be assigned by ATC without prior notification from the pilot that the aircraft is being subjected to

unlawful interference. The pilot should refuse the assignment of Code 7500 in any other situation and inform the controller

accordingly. Code 7500 will trigger the special emergency indicator in all radar ATC facilities.

c. Air traffic controllers will acknowledge and confirm receipt of transponder Code 7500 by asking the pilot

to verify it. If the aircraft is not being subjected to unlawful interference, the pilot should respond to the query

by broadcasting in the clear that the aircraft is not being subjected to unlawful interference. Upon receipt of this

information, the controller will request the pilot to verify the code selection depicted in the code selector windows

in the transponder control panel and change the code to the appropriate setting. If the pilot replies in the

affirmative or does not reply, the controller will not ask further questions but will flight follow, respond to pilot

requests and notify appropriate authorities.

d. If it is possible to do so without jeopardizing the safety of the flight, the pilot of a hijacked passenger aircraft,

after departing from the cleared routing over which the aircraft was operating, will attempt to do one or more

of the following things, insofar as circumstances may permit:

1. Maintain a true airspeed of no more than 400 knots, and preferably an altitude of between 10,000 and

25,000 feet.

2. Fly a course toward the destination which the hijacker has announced.

e. If these procedures result in either radio contact or air intercept, the pilot will attempt to comply with any

instructions received which may direct the aircraft to an appropriate landing field or alter the aircraft’s flight path

off its current course, away from protected airspace.

6−3−5. Fuel Dumping

a. Should it become necessary to dump fuel, the pilot should immediately advise ATC. Upon receipt of

information that an aircraft will dump fuel, ATC will broadcast or cause to be broadcast immediately and every

3 minutes thereafter the following on appropriate ATC and FSS radio frequencies:

EXAMPLE−

Attention all aircraft − fuel dumping in progress over − (location) at (altitude) by (type aircraft) (flight direction).

b. Upon receipt of such a broadcast, pilots of aircraft affected, which are not on IFR flight plans or special

VFR clearances, should clear the area specified in the advisory. Aircraft on IFR flight plans or special VFR

clearances will be provided specific separation by ATC. At the termination of the fuel dumping operation, pilots

should advise ATC. Upon receipt of such information, ATC will issue, on the appropriate frequencies, the

following:

EXAMPLE−

ATTENTION ALL AIRCRAFT − FUEL DUMPING BY − (type aircraft) − TERMINATED.

6−3−8 Distress and Urgency Procedures

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Section 4. Two-way Radio Communications Failure

6−4−1. Two-way Radio Communications Failure

a. It is virtually impossible to provide regulations an d procedures applicable to all possible situations

associated with two-way radio communications failure. During two-way radio communications failure, when

confronted by a situation not covered in the regulation, pilots are expected to exercise good judgment in whatever

action they elect to take. Should the situation so dictate they should not be reluctant to use the emergency action

contained in 14 CFR section 91.3(b).

b. Whether two-way communications failure constitutes an emergency depends on the circumstances, and in

any event, it is a determination made by the pilot. 14 CFR section 91.3(b) authorizes a pilot to deviate from any

rule in Subparts A and B to the extent required to meet an emergency.

c. In the event of two-way radio communications failure, ATC service will be provided on the basis that the

pilot is operating in accordance with 14 CFR section 91.185. A pilot experiencing two-way communications

failure should (unless emergency authority is exercised) comply with 14 CFR section 91.185 quoted below:

1. General. Unless otherwise authorized by ATC, each pilot who has two-way radio communications

failure when operating under IFR must comply with the rules of this section.

2. VFR conditions. If the failure occurs in VFR conditions, or if VFR conditions are encountered after the

failure, each pilot must continue the flight under VFR and land as soon as practicable.

NOTE−

This procedure also applies when two-way radio failure occurs while operating in Class A airspace. The primary objective

of this provision in 14 CFR section 91.185 is to preclude extended IFR operation by these aircraft within the ATC system.

Pilots should recognize that operation under these conditions may unnecessarily as well as adversely affect other users of

the airspace, since ATC may be required to reroute or delay other users in order to protect the failure aircraft. However, it

is not intended that the requirement to “land as soon as practicable” be construed to mean “as soon as possible.” Pilots

retain the prerogative of exercising their best judgment and are not required to land at an unauthorized airport, at an airport

unsuitable for the type of aircraft flown, or to land only minutes short of their intended destination.

3. IFR conditions. If the failure occurs in IFR conditions, or if subparagraph 2 above cannot be complied

with, each pilot must continue the flight according to the following:

(a) Route.

(1) By the route assigned in the last ATC clearance received;

(2) If being radar vectored, by the direct route from the point of radio failure to the fix, route, or airway

specified in the vector clearance;

(3) In the absence of an assigned route, by the route that ATC has advised may be expected in a further

clearance; or

(4) In the absence of an assigned route or a route that ATC has advised may be expected in a further

clearance by the route filed in the flight plan.

(b) Altitude. At the HIGHEST of the following altitudes or flight levels FOR THE ROUTE SEGMENT

BEING FLOWN:

(1) The altitude or flight level assigned in the last ATC clearance received;

(2) The minimum altitude (converted, if appropriate, to minimum flight level as prescribed in 14 CFR

section 91.121(c)) for IFR operations; or

(3) The altitude or flight level ATC has advised may be expected in a further clearance.

Two-way Radio Communications Failure 6−4−1

AIM 2/20/25

NOTE−

The intent of the rule is that a pilot who has experienced two-way radio failure should select the appropriate altitude for

the particular route segment being flown and make the necessary altitude adjustments for subsequent route segments. If the

pilot received an “expect further clearance” containing a higher altitude to expect at a specified time or fix, maintain the

highest of the following altitudes until that time/fix:

(1) the last assigned altitude; or

(2) the minimum altitude/flight level for IFR operations.

Upon reaching the time/fix specified, the pilot should commence climbing to the altitude advised to expect. If the radio failure

occurs after the time/fix specified, the altitude to be expected is not applicable and the pilot should maintain an altitude

consistent with 1 or 2 above. If the pilot receives an “expect further clearance” containing a lower altitude, the pilot should

maintain the highest of 1 or 2 above until that time/fix specified in subparagraph (c) Leave clearance limit, below.

EXAMPLE−

1. A pilot experiencing two-way radio failure at an assigned altitude of 7,000 feet is cleared along a direct route which will

require a climb to a minimum IFR altitude of 9,000 feet, should climb to reach 9,000 feet at the time or place where it becomes

necessary (see 14 CFR section 91.177(b)). Later while proceeding along an airway with an MEA of 5,000 feet, the pilot

would descend to 7,000 feet (the last assigned altitude), because that altitude is higher than the MEA.

2. A pilot experiencing two-way radio failure while being progressively descended to lower altitudes to begin an approach

is assigned 2,700 feet until crossing the VOR and then cleared for the approach. The MOCA along the airway is 2,700 feet

and MEA is 4,000 feet. The aircraft is within 22 NM of the VOR. The pilot should remain at 2,700 feet until crossing the VOR

because that altitude is the minimum IFR altitude for the route segment being flown.

3. The MEA between a and b: 5,000 feet. The MEA between b and c: 5,000 feet. The MEA between c and d: 11,000 feet.

The MEA between d and e: 7,000 feet. A pilot had been cleared via a, b, c, d, to e. While flying between a and b the assigned

altitude was 6,000 feet and the pilot was told to expect a clearance to 8,000 feet at b. Prior to receiving the higher altitude

assignment, the pilot experienced two-way failure. The pilot would maintain 6,000 to b, then climb to 8,000 feet (the altitude

advised to expect). The pilot would maintain 8,000 feet, then climb to 11,000 at c, or prior to c if necessary to comply with

an MCA at c. (14 CFR section 91.177(b).) Upon reaching d, the pilot would descend to 8,000 feet (even though the MEA

was 7,000 feet), as 8,000 was the highest of the altitude situations stated in the rule (14 CFR section 91.185).

(c) Leave clearance limit.

(1) When the clearance limit is a fix from which an approach begins, commence descent or descent and

approach as close as possible to the expect further clearance time if one has been received, or if one has not been

received, as close as possible to the Estimated Time of Arrival (ETA) as calculated from the filed or amended

(with ATC) Estimated Time En Route (ETE).

(2) If the clearance limit is not a fix from which an approach begins, leave the clearance limit at the

expect further clearance time if one has been received, or if none has been received, upon arrival over the

clearance limit, and proceed to a fix from which an approach begins and commence descent or descent and

approach as close as possible to the estimated time of arrival as calculated from the filed or amended (with ATC)

estimated time en route.

6−4−2. Transponder Operation During Two-way Communications Failure

a. If an aircraft with a coded radar beacon transponder experiences a loss of two-way radio capability, the pilot

should adjust the transponder to reply on Mode A/3, Code 7600.

b. The pilot should understand that the aircraft may not be in an area of radar coverage.

6−4−3. Reestablishing Radio Contact

a. In addition to monitoring the NA V AID voice feature, the pilot should attempt to reestablish

communications by attempting contact:

1. On the previously assigned frequency; or

6−4−2 Two-way Radio Communications Failure

2/20/25 AIM

2. With an FSS or with New York Radio or San Francisco Radio.

b. If communications are established with an FSS or New York Radio or San Francisco Radio, the pilot should

advise that radio communications on the previously assigned frequency have been lost giving the aircraft’s

position, altitude, last assigned frequency and then request further clearance from the controlling facility. The

preceding does not preclude the use of 121.5 MHz. There is no priority on which action should be attempted first.

If the capability exists, do all at the same time.

NOTE−

New York Radio and San Francisco Radio are operated by Collins Aerospace, Incorporated (formerly ARINC) under

contract with the F AA for communications services. These Radio facilities have the capability of relaying information

to/from ATC facilities throughout the country.

Two-way Radio Communications Failure 6−4−3

2/20/25 AIM

Section 5. Aircraft Rescue and

Fire Fighting Communications

6−5−1. Discrete Emergency Frequency

a. Direct contact between an emergency aircraft flight crew, Aircraft Rescue and Fire Fighting Incident

Commander (ARFF IC), and the Airport Traffic Control Tower (ATCT), is possible on an aeronautical radio

frequency (Discrete Emergency Frequency [DEF]), designated by Air Traffic Control (ATC) from the

operational frequencies assigned to that facility.

b. Emergency aircraft at airports without an ATCT, (or when the ATCT is closed), may contact the ARFF IC

(if ARFF service is provided), on the Common Traffic Advisory Frequency (CTAF) published for the airport

or the civil emergency frequency 121.5 MHz.

6−5−2. Radio Call Signs

Preferred radio call sign for the ARFF IC is “(location/facility) Command” when communicating with the flight

crew and the FAA ATCT.

EXAMPLE−

LAX Command.

Washington Command.

6−5−3. ARFF Emergency Hand Signals

In the event that electronic communications cannot be maintained between the ARFF IC and the flight crew,

standard emergency hand signals as depicted in FIG 6−5−1 through FIG 6−5−3 should be used. These hand

signals should be known and understood by all cockpit and cabin aircrew, and all ARFF firefighters.

FIG 6−5−1

Recommend Evacuation

Aircraft Rescue and Fire Fighting Communications 6−5−1

AIM 2/20/25

FIG 6−5−2

Recommend Stop

FIG 6−5−3

Emergency Contained

6−5−2 Aircraft Rescue and Fire Fighting Communications

2/20/25 AIM

Chapter 7. Safety of Flight

Section 1. Meteorology

7−1−1. National Weather Service Aviation Weather Service Program

a. Weather service to aviation is a joint effort of the National Oceanic and Atmospheric Administration

(NOAA), the National Weather Service (NWS), the Federal Aviation Administration (FAA), Department of

Defense, and various private sector aviation weather service providers. Requirements for all aviation weather

products originate from the FAA, which is the Meteorological Authority for the U.S.

b. NWS meteorologists are assigned to all air route traffic control centers (ARTCC) as part of the Center

Weather Service Units (CWSU) as well as the Air Traffic Control System Command Center (ATCSCC). These

meteorologists provide specialized briefings as well as tailored forecasts to support the needs of the FAA and

other users of the NAS.

c. Aviation Products

1. The NWS maintains an extensive surface, upper air, and radar weather observing program; and a

nationwide aviation weather forecasting service.

2. Airport observations (METAR and SPECI) supported by the NWS are provided by automated observing

systems.

3. Terminal Aerodrome Forecasts (TAF) are prepared by 123 NWS Weather Forecast Offices (WFOs) for

over 700 airports. These forecasts are valid for 24 or 30 hours and amended as required.

4. Inflight aviation advisories (for example, Significant Meteorological Information (SIGMETs) and

Airmen’s Meteorological Information (AIRMETs)) are issued by three NWS Meteorological Watch Offices

(MWOs); the Aviation Weather Center (AWC) in Kansas City, MO, the Alaska Aviation Weather Unit (AAWU)

in Anchorage, AK, and the Weather Service Forecast Office (WFO) in Honolulu, HI. The AWC, the AAWU,

and WSFO Honolulu issue area forecasts for selected areas. In addition, NWS meteorologists assigned to most

ARTCCs as part of the Center Weather Service Unit (CWSU) provide Center Weather Advisories (CWAs) and

gather weather information to support the needs of the FAA and other users of the system.

5. Several NWS National Centers for Environmental Production (NCEP) provide aviation specific weather

forecasts, or select public forecasts which are of interest to pilots and operators.

(a) The Aviation Weather Center (AWC) displays a variety of domestic and international aviation

forecast products over the Internet at aviationweather.gov.

(b) The NCEP Central Operations (NCO) is responsible for the operation of many numerical weather

prediction models, including those which produce the many wind and temperature aloft forecasts.

(c) The Storm Prediction Center (SPC) issues tornado and severe weather watches along with other

guidance forecasts.

(d) The National Hurricane Center (NHC) issues forecasts on tropical weather systems (for example,

hurricanes).

(e) The Space Weather Prediction Center (SWPC) provides alerts, watches, warnings and forecasts for

space weather events (for example, solar storms) affecting or expected to affect Earth’s environment.

(f) The Weather Prediction Center (WPC) provides analysis and forecast products on a national scale

including surface pressure and frontal analyses.

6. NOAA operates two V olcanic Ash Advisory Centers (V AAC) which issue forecasts of ash clouds

following a volcanic eruption in their area of responsibility.

Meteorology 7−1−1

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7. Details on the products provided by the above listed offices and centers is available in FAA−H−8083−28,

Aviation Weather Handbook.

d. Weather element values may be expressed by using different measurement systems depending on several

factors, such as whether the weather products will be used by the general public, aviation interests, international

services, or a combination of these users. FIG 7−1−1 provides conversion tables for the most used weather

elements that will be encountered by pilots.

7−1−2. FAA Weather Services

a. The FAA provides the Flight Service program, which serves the weather needs of pilots through its flight

service stations (FSS). Pilots may access weather information through www.1800wxbrief.com. To contact Flight

Service in the CONUS, Hawaii, and U.S. territories; call 1−800−WX−BRIEF (1−800−992−7433); in Alaska call

1−833−AK−BRIEF (1−833−252−7433).

b. The FAA maintains an extensive surface weather observing program. Airport observations (METAR and

SPECI) in the U.S. are provided by automated observing systems. Various levels of human oversight of the

METAR and SPECI reports and augmentation may be provided at select larger airports by either government

or contract personnel qualified to report specified weather elements that cannot be detected by the automated

observing system. The requirements to issue SPECI reports are detailed in TBL 7−1−1.

TBL 7−1−1

SPECI Issuance Table

1 Wind Shift Wind direction changes by 45° or more, in less than 15 minutes, and the wind speed is

10 kt or more throughout the wind shift.

2 Visibility

The surface visibility (as reported in the body of the report):

 Decreases to less than 3 sm, 2 sm, 1 sm, ½ sm, ¼ sm or the lowest standard instrument

approach procedure (IAP) minimum.1

 Increases to equal to or exceed 3 sm, 2 sm, 1 sm, ½ sm, ¼ sm or the lowest standard

IAP minimum.1

1 As published in the U.S. Terminal Procedures. If none published, use ½ sm.

3 RVR

The highest value from the designated RVR runway decreases to less than 2,400 ft during

the preceding 10 minutes; or, if the RVR is below 2,400 ft, increases to equal to or exceed

2,400 ft during the preceding 10 minutes. U.S. military stations may not report a SPECI

based on RVR.

Tornado, Funnel

Cloud, or

Waterspout

 Is observed.

 Disappears from sight or ends.

5 Thunderstorm

 Begins (a SPECI is not required to report the beginning of a new thunderstorm if one

is currently reported).

 Ends.

6 Precipitation

 Hail begins or ends.

 Freezing precipitation begins, ends, or changes intensity.

 Ice pellets begin, end, or change intensity.

 Snow begins, ends, or changes intensity.

7−1−2 Meteorology

AIM2/20/251/22/26 AIM

7 Squalls When a squall occurs. (Wind speed suddenly increases by at least 16 knots and is

sustained at 22 knots or more for at least one minute.)

8 Ceiling

The ceiling changes1 through:

 3,000 ft.

 1,500 ft.

 1,000 ft.

 500 ft.

 The lowest standard IAP minimum.2

1 “Ceiling change” means that it forms, dissipates below, decreases to less than, or, if

below, increases to equal or exceed the values listed.

2 As published in the U.S. Terminal Procedures. If none published, use 200 ft.

9 Sky Condition A layer of clouds or obscurations aloft is present below 1,000 ft and no layer aloft was

reported below 1,000 ft in the preceding METAR or SPECI.

10 Volcanic

Eruption When an eruption is first noted.

11 Aircraft Mishap

Upon notification of an aircraft mishap, 1 unless there has been an intervening

observation.

1 “Aircraft mishap” is an inclusive term to denote the occurrence of an aircraft accident

or incident.

12 Miscellaneous Any other meteorological situation designated by the responsible agency of which, in

the opinion of the observer, is critical.

c. Other Sources of Weather Information

1. Weather and aeronautical information are available from numerous private industry sources on an

individual or contract pay basis. Prior to every flight, pilots should gather all information vital to the nature of

the flight. Pilots can receive a regulatory compliant briefing without contacting Flight Service. Pilots are

encouraged to use automated resources and review AC 91−92, Pilot’s Guide to a Preflight Briefing, for more

information.

2. Pilots can access Leidos Flight Services via the Internet at http://www.1800wxbrief.com. Pilots can

receive preflight weather data and file VFR and IFR flight plans.

7−1−3. Use of Aviation Weather Products

a. Air carriers and operators certificated under the provisions of 14 CFR part 119 are required to use the

aeronautical weather information systems defined in the Operations Specifications issued to that certificate

holder by the FAA. These systems may utilize basic FAA/National Weather Service (NWS) weather services,

contractor− or operator−proprietary weather services and/or Enhanced Weather Information System (EWINS)

when approved in the Operations Specifications. As an integral part of this system approval, the procedures for

collecting, producing and disseminating aeronautical weather information, as well as the crew member and

dispatcher training to support the use of system weather products, must be accepted or approved.

b. Operators not certificated under the provisions of 14 CFR part 119 are encouraged to use FAA/NWS

products through Flight Service Stations, Leidos Flight Service, and/or Flight Information Services−Broadcast

(FIS−B).

c. The suite of available aviation weather product types is expanding, with the development of new sensor

systems, algorithms and forecast models. The FAA and NWS, supported by various weather research

Meteorology 7−1−3

AIM 2/20/253/15/077110.65R CHG 2AIM 1/22/26

laboratories and corporations under contract to the Government, develop and implement new aviation weather

product types. The FAA’s NextGen Aviation Weather Research Program (AWRP) facilitates collaboration

between the NWS, the FAA, and various industry and research representatives. This collaboration ensures that

user needs and technical readiness requirements are met before experimental products mature to operational

application.

d. The AWRP manages the transfer of aviation weather R&D to operational use through technical review

panels and conducting safety assessments to ensure that newly developed aviation weather products meet

regulatory requirements and enhance safety.

FIG 7−1−1

Weather Elements Conversion Tables

e. The AWRP review and decision−making process applies criteria to weather products at various stages . The

stages are composed of the following:

7−1−4 Meteorology

AIM2/20/251/22/26 AIM

1. Sponsorship of user needs.

2. R & D and controlled testing.

3. Experimental application.

4. Operational application.

f. Pilots and operators should be aware that weather services provided by entities other than FAA, NWS, or

their contractors may not meet FAA/NWS quality control standards. Hence, operators and pilots contemplating

using such services should request and/or review an appropriate description of services and provider disclosure.

This should include, but is not limited to, the type of weather product (for example, current weather or forecast

weather), the currency of the product (that is, product issue and valid times), and the relevance of the product.

Pilots and operators should be cautious when using unfamiliar products, or products not supported by FAA/NWS

technical specifications.

NOTE−

When in doubt, consult with a F AA Flight Service Station Specialist.

g. In addition, pilots and operators should be aware there are weather services and products available from

government organizations beyond the scope of the AWRP process mentioned earlier in this section. For example,

governmental agencies such as the NWS and the Aviation Weather Center (AWC), or research organizations such

as the National Center for Atmospheric Research (NCAR) display weather “model data” and “experimental”

products which require training and/or expertise to properly interpret and use. These products are developmental

prototypes that are subject to ongoing research and can change without notice. Therefore, some data on display

by government organizations, or government data on display by independent organizations may be unsuitable

for flight planning purposes. Operators and pilots contemplating using such services should request and/or

review an appropriate description of services and provider disclosure. This should include, but is not limited to,

the type of weather product (for example, current weather or forecast weather), the currency of the product (i.e.,

product issue and valid times), and the relevance of the product. Pilots and operators should be cautious when

using unfamiliar weather products.

NOTE−

When in doubt, consult with a F AA Flight Service Station Specialist.

h. With increased access to weather products via the public Internet, the aviation community has access to

an overwhelming amount of weather information and data that support self −briefing. the Aviation Weather

Handbook, FAA−H−8083−28 (current edition), describes the weather products distributed by the NWS. Pilots

and operators using the public Internet to access weather from a third party vendor should request and/or review

an appropriate description of services and provider disclosure. This should include, but is not limited to, the type

of weather product (for example, current weather or forecast weather), the currency of the product (i.e., product

issue and valid times), and the relevance of the product. Pilots and operators should be cautious when using

unfamiliar weather products and when in doubt, consult with a Flight Service Specialist.

i. The development of new weather products, coupled with the termination of some legacy textual and

graphical products may create confusion between regulatory requirements and the new products. All

flight−related, aviation weather decisions must be based on all available pertinent weather products. As every

flight is unique and the weather conditions for that flight vary hour by hour, day to day, multiple weather products

may be necessary to meet aviation weather regulatory requirements. Many new weather products now have a

Precautionary Use Statement that details the proper use or application of the specific product.

j. The FAA has identified three distinct types of weather information available to pilots and operators.

1. Observations. Raw weather data collected by some type of sensor suite including surface and airborne

observations, radar, lightning, satellite imagery, and profilers.

2. Analysis. Enhanced depiction and/or interpretation of observed weather data.

3. Forecasts . Predictions of the development and/or movement of weather phenomena based on

meteorological observations and various mathematical models.

Meteorology 7−1−5

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k. Not all sources of aviation weather information are able to provide all three types of weather information.

The FAA has determined that operators and pilots may utilize the following approved sources of aviation weather

information:

1. Federal Government. The FAA and NWS collect raw weather data, analyze the observations, and

produce forecasts. The FAA and NWS disseminate meteorological observations, analyses, and forecasts through

a variety of systems. In addition, the Federal Government is the only approval authority for sources of weather

observations; for example, contract towers and airport operators may be approved by the Federal Government

to provide weather observations.

2. Enhanced Weather Information System (EWINS). An EWINS is an FAA authorized, proprietary

system for tracking, evaluating, reporting, and forecasting the presence or lack of adverse weather phenomena.

The FAA authorizes a certificate holder to use an EWINS to produce flight movement forecasts, adverse weather

phenomena forecasts, and other meteorological advisories. For more detailed information regarding EWINS, see

FAA−H−8083−28, Aviation Weather Handbook, and the Flight Standards Information Management System

8900.1.

3. Commercial Weather Information Providers. In general, commercial providers produce proprietary

weather products based on NWS/FAA products with formatting and layout modifications but no material

changes to the weather information itself. This is also referred to as “repackaging.” In addition, commercial

providers may produce analyses, forecasts, and other proprietary weather products that substantially alter the

information contained in government−produced products. However, those proprietary weather products that

substantially alter government−produced weather products or information, may only be approved for use by 14

CFR part 121 and part 135 certificate holders if the commercial provider is EWINS qualified.

NOTE−

Commercial weather information providers contracted by F AA to provide weather observations, analyses, and forecasts

(e.g., contract towers) are included in the Federal Government category of approved sources by virtue of maintaining

required technical and quality assurance standards under Federal Government oversight.

7−1−4. Graphical Forecasts for Aviation (GFA)

a. The GFA website is intended to provide the necessary aviation weather information to give users a complete

picture of the weather that may affect flight in the continental United States (CONUS). The website includes

observational data, forecasts, and warnings that can be viewed from 14 hours in the past to 15 hours in the future,

including thunderstorms, clouds, flight category, precipitation, icing, turbulence, and wind. Hourly model data

and forecasts, including information on clouds, flight category, precipitation, icing, turbulence, wind, and

graphical output from the National Weather Service’s (NWS) National Digital Forecast Data (NDFD) are

available. Wind, icing, and turbulence forecasts are available in 3,000 ft increments from the surface up to 30,000

ft MSL, and in 6,000 ft increments from 30,000 ft MSL to 48,000 ft MSL. Turbulence forecasts are also broken

into low (below 18,000 ft MSL) and high (at or above 18,000 ft MSL) graphics. A maximum icing graphic and

maximum wind velocity graphic (regardless of altitude) are also available. Built with modern geospatial

information tools, users can pan and zoom to focus on areas of greatest interest. Target users are commercial and

general aviation pilots, operators, briefers, and dispatchers.

b. Weather Products.

1. The Aviation Forecasts include gridded displays of various weather parameters as well as NWS textual

weather observations, forecasts, and warnings. Ici ng, turbulence, and wind gridded products are

three−dimensional. Other gridded products are two −dimensional and may represent a “composite” of a

three−dimensional weather phenomenon or a surface weather variable, such as horizontal visibility. The

following are examples of aviation forecasts depicted on the GFA:

(a) Terminal Aerodrome Forecast (TAF)

(b) Ceiling & Visibility (CIG/VIS)

7−1−6 Meteorology

2/20/25 AIM

(c) Clouds

(d) Precipitation / Weather (PCPN/WX)

(e) Thunderstorm (TS)

(f) Winds

(g) Turbulence

(h) Ice

2. Observations & Warnings (Obs/Warn). The Obs/Warn option provides an option to display weather

data for the current time and the previous 14 hours (rounded to the nearest hour). Users may advance through

time using the arrow buttons or by clicking on the desired hour. Provided below are the Obs/Warn product tabs

available on the GFA website:

(a) METAR

(b) Precipitation/Weather (PCPN/WX)

(c) Ceiling & Visibility (CIG/VIS)

(d) Pilot Weather Report (PIREP)

(e) Radar & Satellite (RAD/SAT)

3. The GFA will be continuously updated and available online at http://aviationweather.gov/gfa. Upon

clicking the link above, select INFO on the top right corner of the map display. The next screen presents the option

of selecting Overview, Products, and Tutorial. Simply select the tab of interest to explore the enhanced digital

and graphical weather products designed to replace the legacy FA. Users should also refer to the Aviation Weather

Handbook, FAA−H−8083−28, Graphical Forecasts for Aviation (GFA) Tool, for more detailed information on

the GFA.

4. GFA Static Images. Some users with limited internet connectivity may access static images via the

Aviation Weather Center (AWC) Decision Support Imagery at: https://aviationweather.gov/graphics/. There are

two static graphical images available, titled Aviation Cloud Forecast and Aviation Surface Forecast. The

Aviation Cloud Forecast provides cloud coverage, bases, layers, and tops with AIRMETs for mountain

obscuration and AIRMETs for icing overlaid. The Aviation Surface Fo recast provides visibility, weather

phenomena, and winds (including wind gusts) with AIRMETs for instrument flight rules conditions and

AIRMETs for sustained surface winds of 30 knots or more overlaid. These images are presented on ten separate

maps providing forecast views for the entire contiguous United States (U.S.) on one and nine regional views

which provide more detail for the user. They are updated every 3 hours and provide forecast snapshots for 3, 6,

9, 12, 15, and 18 hours into the future. (See FIG 7−1−2 and FIG 7−1−3.)

NOTE−

The contiguous United States (U.S.) refers to the 48 adjoining U.S. states on the continent of North America that are south

of Canada and north of Mexico, plus the District of Columbia. The term excludes the states of Alaska and Hawaii, and all

off−shore U.S. territories and possessions, such as Puerto Rico.

Meteorology 7−1−7

AIM 2/20/25

FIG 7−1−2

Aviation Surface Forecast

FIG 7−1−3

Aviation Cloud Forecast

7−1−5. Preflight Briefing

a. Flight Service is one of the primary sources for obtaining preflight briefings and to file flight plans by phone

or the Internet. Flight Service Specialists are qualified and certificated as Pilot Weather Briefers by the FAA.

7−1−8 Meteorology

2/20/25 AIM

They are not authorized to make original forecasts, but are authorized to translate and interpret available forecasts

and reports directly into terms describing the weather conditions which you can expect along your flight route

and at your destination. Prior to every flight, pilots should gather all information vital to the nature of the flight.

Pilots can receive a regulatory compliant briefing without contacting Flight Service. Pilots are encouraged to use

automated resources and review AC 91−92, Pilot’s Guide to a Preflight Briefing, for more information. Pilots

who prefer to contact Flight Service are encouraged to conduct a self −brief prior to calling. Conducting a

self−brief before contacting Flight Service provides familiarity of meteorological and aeronautical conditions

applicable to the route of flight and promotes a better understanding of weather information. Three basic types

of preflight briefings (Standard, Abbreviated, and Outlook) are available to serve the pilot’s specific needs. Pilots

should specify to the briefer the type of briefing they want, along with their appropriate background information.

This will enable the briefer to tailor the information to the pilot’s intended flight. The following paragraphs

describe the types of briefings available and the information provided in each briefing.

REFERENCE−

AIM, Para 5−1−1, Preflight Preparation, for items that are required.

b. Standard Briefing. You should request a Standard Briefing any time you are planning a flight and you

have not received a previous briefing or have not received preliminary information through online resources.

International data may be inaccurate or incomplete. If you are planning a flight outside of U.S. controlled

airspace, the briefer will advise you to check data as soon as practical after entering foreign airspace, unless you

advise that you have the international cautionary advisory. The briefer will automatically provide the following

information in the sequence listed, except as noted, when it is applicable to your proposed flight.

1. Adverse Conditions. Significant meteorological and/or aeronautical information that might influence

the pilot to alter or cancel the proposed flight; for example, hazardous weather conditions, airport closures, air

traffic delays, etc. Pilots should be especially alert for current or forecast weather that could reduce flight

minimums below VFR or IFR conditions. Pilots should also be alert for any reported or forecast icing if the

aircraft is not certified for operating in icing conditions. Flying into areas of icing or weather below minimums

could have disastrous results.

2. VFR Flight Not Recommended. When VFR flight is proposed and sky conditions or visibilities are

present or forecast, surface or aloft, that, in the briefer’s judgment, would make flight under VFR doubtful, the

briefer will describe the conditions, describe the affected locations, and use the phrase “VFR flight not

recommended.” This recommendation is advisory in nature. The final decision as to whether the flight can be

conducted safely rests solely with the pilot. Upon receiving a “ VFR flight not recommended ” statement, the

non−IFR rated pilot will need to make a “go or no go” decision. This decision should be based on weighing the

current and forecast weather conditions against the pilot’s experience and ratings. The aircraft’s equipment,

capabilities and limitations should also be considered.

NOTE−

Pilots flying into areas of minimal VFR weather could encounter unforecasted lowering conditions that place the aircraft

outside the pilot’ s ratings and experience level. This could result in spatial disorientation and/or loss of control of the

aircraft.

3. Synopsis. A brief statement describing the type, location and movement of weather systems and/or air

masses which might affect the proposed flight.

NOTE−

These first 3 elements of a briefing may be combined in any order when the briefer believes it will help to more clearly

describe conditions.

4. Current Conditions. Reported weather conditions applicable to the flight will be summarized from all

available sources; e.g., METARs/ SPECIs, PIREPs, RAREPs. This element will be omitted if the proposed time

of departure is beyond 2 hours, unless the information is specifically requested by the pilot.

5. En Route Forecast. Forecast en route conditions for the proposed route are summarized in logical

order; i.e., departure/climbout, en route, and descent. (Heights are MSL, unless the contractions “AGL” or “CIG”

are denoted indicating that heights are above ground.)

Meteorology 7−1−9

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6. Destination Forecast. The destination forecast for the planned ETA. Any significant changes within

1 hour before and after the planned arrival are included.

7. Winds Aloft. Forecast winds aloft will be provided in knots and degrees, referenced to true north. The

briefer will interpolate wind directions and speeds between levels and stations as necessary to provide expected

conditions at planned altitudes. (Heights are MSL.) Temperature information will be provided on request.

8. Notices to Airmen (NOTAMs).

(a) Available NOTAM (D) information pertinent to the proposed flight, including special use airspace

(SUA) NOTAMs for restricted areas, aerial refueling, and night vision goggles (NVG).

NOTE−

Other SUA NOTAMs (D), such as military operations area (MOA), military training route (MTR), and warning area

NOTAMs, are considered “upon request” briefing items as indicated in paragraph 7−1−4b10(a).

(b) Prohibited Areas P−40, P−49, P−56, and the special flight rules area (SFRA) for Washington, DC.

(c) FSS briefers do not provide FDC NOTAM information for special instrument approach procedures

unless specifically asked. Pilots authorized by the FAA to use special instrument approach procedures must

specifically request FDC NOTAM information for these procedures.

NOTE−

1. NOTAM information may be combined with current conditions when the briefer believes it is logical to do so.

2. Airway NOTAMs, procedural NOTAMs, and NOTAMs that are general in nature and not tied to a specific airport/facility

(for example, flight advisories and restrictions, open duration special security instructions, and special flight rules areas)

are briefed solely by pilot request. For complete flight information, pilots are urged to review the Domestic Notices and

International Notices found in the External Links section of the Federal NOTAM System (FNS) NOTAM Search System and

the Chart Supplement in addition to obtaining a briefing.

9. ATC Delays. Any known ATC delays and flow control advisories which might affect the proposed

flight.

10. Pilots may obtain the following from flight service station briefers upon request:

(a) Information on SUA and SUA−related airspace, except those listed in paragraph 7−1−4b8.

NOTE−

1. For the purpose of this paragraph, SUA and related airspace includes the following types of airspace: alert area, military

operations area (MOA), warning area, and air traffic control assigned airspace (ATCAA). MTR data includes the following

types of airspace: IFR training routes (IR), VFR training routes (VR), and slow training routes (SR).

2. Pilots are encouraged to request updated information from ATC facilities while in flight.

(b) A review of airway NOTAMs, procedural NOTAMs, and NOTAMs that are general in nature and not

tied to a specific airport/facility (for example, flight advisories and restrictions, open duration special security

instructions, and special flight rules areas), Domestic Notices and International Notices. Domestic Notices and

International Notices are found in the External Links section of the Federal NOTAM System (FNS) NOTAM

Search System.

(c) Approximate density altitude data.

(d) Information regarding such items as air traffic services and rules, customs/immigration procedures,

ADIZ rules, search and rescue, etc.

(e) GPS RAIM availability for 1 hour before to 1 hour after ETA or a time specified by the pilot.

(f) Other assistance as required.

c. Abbreviated Briefing. Request an Abbreviated Briefing when you need information to supplement mass

disseminated data, update a previous briefing, or when you need only one or two specific items. Provide the

briefer with appropriate background information, the time you received the previous information, and/or the

specific items needed. You should indicate the source of the information already received so that the briefer can

7−1−10 Meteorology

AIM2/20/258/7/25 AIM

limit the briefing to the information that you have not received, and/or appreciable changes in

meteorological/aeronautical conditions since your previous briefing. To the extent possible, the briefer will

provide the information in the sequence shown for a Standard Briefing. If you request only one or two specific

items, the briefer will advise you if adverse conditions are present or forecast. (Adverse conditions contain both

meteorological and/or aeronautical information.) Details on these conditions will be provided at your request.

International data may be inaccurate or incomplete. If you are planning a flight outside of U.S. controlled

airspace, the briefer will advise you to check data as soon as practical after entering foreign airspace, unless you

advise that you have the international cautionary advisory.

d. Outlook Briefing. You should request an Outlook Briefing whenever your proposed time of departure is

six or more hours from the time of the briefing. The briefer will provide available forecast data applicable to the

proposed flight. This type of briefing is provided for planning purposes only. You should obtain a Standard or

Abbreviated Briefing prior to departure in order to obtain such items as adverse conditions, current conditions,

updated forecasts, winds aloft and NOTAMs, etc.

e. When filing a flight plan only, you will be asked if you require the latest information on adverse conditions

pertinent to the route of flight.

f. Inflight Briefing. You are encouraged to conduct a self −briefing using online resources or obtain your

preflight briefing by telephone or in person (Alaska only) before departure. In those cases where you need to

obtain a preflight briefing or an update to a previous briefing by radio, you should contact the nearest FSS to

obtain this information. After communications have been established, advise the specialist of the type briefing

you require and provide appropriate background information. You will be provided information as specified in

the above paragraphs, depending on the type of briefing requested. En Route advisories tailored to the phase of

flight that begins after climb−out and ends with descent to land are provided upon pilot request. Besides Flight

Service, there are other resources available to the pilot in flight, including:

Automatic Dependent Surveillance −Broadcast (ADS −B). Free traffic, weather, and flight information are

available on ADS−B In receivers that can receive data over 978 MHz (UAT) broadcasts. These services are

available across the nation to aircraft owners who equip with ADS−B In, with further advances coming from

airborne and runway traffic awareness. Even search −and−rescue operations benefit from accurate ADS −B

tracking.

Flight Information Services−Broadcast (FIS−B). FIS−B is a free service; but is only available to aircraft that can

receive data over 978 MHz (UA T). FIS−B automatically transmits a wide range of weather products with national

and regional focus to all equipped aircraft. Having current weather and aeronautical information in the cockpit

helps pilots plan more safe and efficient flight paths, as well as make strategic decisions during flight to avoid

potentially hazardous weather.

Pilots are encouraged to provide a continuous exchange of information on weather, winds, turbulence, flight

visibility, icing, etc., between pilots and inflight specialists. Pilots should report good weather as well as bad, and

confirm expected conditions as well as unexpected. Remember that weather conditions can change rapidly and

that a “go or no go” decision, as mentioned in paragraph 7−1−4b2, should be assessed at all phases of flight.

g. Following any briefing, feel free to ask for any information that you or the briefer may have missed or are

not understood. This way, the briefer is able to present the information in a logical sequence, and lessens the

chance of important items being overlooked.

7−1−6. Inflight Aviation Weather Advisories

a. Inflight Aviation Weather Advisories are forecasts to advise en route aircraft of development of potentially

hazardous weather. Inflight aviation weather advisories in the conterminous U.S. are issued by the Aviation

Weather Center (AWC) in Kansas City, MO, as well as 20 Center Weather Service Units (CWSU) associated with

ARTCCs. AWC also issues advisories for portions of the Gulf of America, Atlantic and Pacific Oceans, which

Meteorology 7−1−11

AIM 2/20/25

are under the control of ARTCCs with Oceanic flight information regions (FIRs). The Weather Forecast Office

(WFO) in Honolulu issues advisories for the Hawaiian Islands and a large portion of the Pacific Ocean. In Alaska,

the Alaska Aviation Weather Unit (AAWU) issues inflight aviation weather advisories along with the Anchorage

CWSU. All heights are referenced MSL, except in the case of ceilings (CIG) which indicate AGL.

b. There are four types of inflight aviation weather advisories: the SIGMET, the Convective SIGMET, the

AIRMET, and the Center Weather Advisory (CWA). All of these advisories use VORs, airports, or well−known

geographic areas to describe the hazardous weather areas.

c. The Severe Weather Watch Bulletins (WWs), (with associated Alert Messages) (AWW) supplements these

Inflight Aviation Weather Advisories.

d. SIGMET. A SIGMET is a concise description of the occurrence or expected occurrence of specified en

route weather phenomena which is expected to affect the safety of aircraft operations.

1. SIGMETs:

(a) Are intended for dissemination to all pilots in flight to enhance safety.

(b) Are issued by the responsible MWO as soon as it is practical to alert operators and aircrews of

hazardous en route conditions.

(c) Are unscheduled products that are valid for 4 hours; except SIGMETs associated with tropical

cyclones and volcanic ash clouds are valid for 6 hours. Unscheduled updates and corrections are issued as

necessary.

(d) Use geographical points to describe the hazardous weather areas. These points can reference either

VORs, airports, or latitude−longitude, depending on SIGMET location. If the total area to be affected during the

forecast period is very large, it could be that, in actuality, only a small portion of this total area would be affected

at any one time.

EXAMPLE−

Example of a SIGMET:

BOSR WS 050600

SIGMET ROMEO 2 VALID UNTIL 051000

ME NH VT

FROM CAR TO YSJ TO CON TO MPV TO CAR

OCNL SEV TURB BLW 080 EXP DUE TO STG NWLY FLOW. CONDS CONTG BYD 1000Z.

2. SIGMETs over the contiguous U.S.:

(a) Are issued corresponding to the areas described in FIG 7−1−5. and are only for non−convective

weather. The U.S. issues a special category of SIGMETs for convective weather called Convective SIGMETs.

(b) Are identified by an alphabetic designator from November through Yankee, excluding Sierra and

Tango. Issuance for the same phenomenon will be sequentially numbered, using the original designator until the

phenomenon ends. For example, the first issuance in the Chicago (CHI) area (reference FIG 7−1−5) for

phenomenon moving from the Salt Lake City (SLC) area will be SIGMET Papa 3, if the previous two issuances,

Papa 1 and Papa 2, had been in the SLC area. Note that no two different phenomena across the country can have

the same alphabetic designator at the same time.

(c) Use location identifiers (either VORs or airports) to describe the hazardous weather areas.

(d) Are issued when the following phenomena occur or are expected to occur:

(1) Severe icing not associated with thunderstorms.

(2) Severe or extreme turbulence or clear air turbulence (CAT) not associated with thunderstorms.

(3) Widespread dust storms or sandstorms lowering surface visibilities to below 3 miles.

(4) V olcanic ash.

7−1−12 Meteorology

2/20/25 AIM

3. SIGMETs over Alaska:

(a) Are issued for the Anchorage FIR including Alaska and nearby coastal waters corresponding to the

areas described in FIG 7−1−4. and are only for non−convective weather. The U.S. issues a special category of

SIGMETs for convective weather called Convective SIGMETs.

(b) Use location identifiers (either VORs or airports) to describe the hazardous weather areas.

(c) Use points of latitude and longitude over the ocean areas of the Alaska FIR.

(d) Are identified by an alphabetic designator from India through Mike.

(e) In addition to the phenomenon applicable to SIGMETs over the contiguous U.S., SIGMETs over

Alaska are also issued for:

(1) Tornadoes.

(2) Lines of thunderstorms.

(3) Embedded thunderstorms.

(4) Hail greater than or equal to ¾ inch in diameter.

FIG 7−1−4

Alaska SIGMET and Area Forecast Zones

4. SIGMETs over oceanic regions (New York Oceanic FIR, Oakland Oceanic FIR including Hawaii,

Houston Oceanic FIR, Miami Oceanic FIR, San Juan FIR), points of latitude and longitude are used to describe

the hazard area.

(a) SIGMETs over the Oakland Oceanic FIR west of 140 west and south of 30 north (including the

Hawaiian Islands) are identified by an alphabetic designator from November through Zulu.

Meteorology 7−1−13

AIM 2/20/253/15/077110.65R CHG 2AIM 7/9/26

(b) SIGMETs over the Oakland Oceanic FIR east of 140 west and north of 30 north are identified by an

alphabetic designator from Alpha through Mike.

(c) SIGMETs over the New York Oceanic FIR, Houston Oceanic FIR, Miami Oceanic FIR, and San Juan

FIR are identified by an alphabetic designator from Alpha through Mike.

(d) In addition to SIGMETs issued for the phenomenon for the contiguous U.S., SIGMETs in the oceanic

regions are also issued for:

(1) Tornadoes.

(2) Lines of thunderstorms.

(3) Embedded thunderstorms.

(4) Hail greater than or equal to ¾ inch in diameter.

e. Convective SIGMET

1. Convective SIGMETs are issued in the conterminous U.S. for any of the following:

(a) Severe thunderstorm due to:

(1) Surface winds greater than or equal to 50 knots.

(2) Hail at the surface greater than or equal to 3/4 inches in diameter.

(3) Tornadoes.

(b) Embedded thunderstorms.

(c) A line of thunderstorms.

(d) Thunderstorms producing precipitation greater than or equal to heavy precipitation affecting 40

percent or more of an area at least 3,000 square miles.

2. Any convective SIGMET implies severe or greater turbulence, severe icing, and low−level wind shear.

A convective SIGMET may be issued for any convective situation that the forecaster feels is hazardous to all

categories of aircraft.

3. Convective SIGMET bulletins are issued for the western (W), central (C), and eastern (E) United States.

(Convective SIGMETs are not issued for Alaska or Hawaii.) The areas are separated at 87 and 107 degrees west

longitude with sufficient overlap to cover most cases when the phenomenon crosses the boundaries. Bulletins

are issued hourly at H+55. Special bulletins are issued at any time as required and updated at H+55. If no criteria

meeting convective SIGMET requirements are obse rved or forecasted, the message “CONVECTIVE

SIGMET... NONE” will be issued for each area at H+55. Individual convective SIGMETs for each area (W, C,

E) are numbered sequentially from number one each day, beginning at 00Z. A convective SIGMET for a

continuing phenomenon will be reissued every hour at H+55 with a new number. The text of the bulletin consists

of either an observation and a forecast or just a forecast. The forecast is valid for up to 1 hour.

EXAMPLE−

CONVECTIVE SIGMET 44C

VALID UNTIL 1455Z

AR TX OK

FROM 40NE ADM-40ESE MLC-10W TXK-50WNW LFK-40ENE SJT-40NE ADM

AREA TS MOV FROM 26025KT. TOPS ABV FL450.

7−1−14 Meteorology

2/20/25 AIM

FIG 7−1−5

SIGMET Locations – Contiguous U.S.

FIG 7−1−6

Hawaii Area Forecast Locations

f. AIRMET. AIRMETs are a concise description of the occurrence or expected occurrence of specified en

route weather phenomena that may affect the safety of aircraft operations, but at intensities lower than those

which require the issuance of a SIGMET.

Meteorology 7−1−15

AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25

1. AIRMETs contain details about IFR conditions, extensive mountain obscuration, turbulence, strong

surface winds, icing, and freezing levels. Unscheduled updates and corrections are issued as necessary.

2. AIRMETs:

(a) Are intended to inform all pilots, but especially Visual Flight Rules pilots and operators of sensitive

aircraft, of potentially hazardous weather phenomena.

(b) Are issued on a scheduled basis every 6 hours, except every 8 hours in Alaska. Unscheduled updates

and corrections are issued as necessary.

(c) Are intended for dissemination to all pilots in the preflight and en route phase of flight to enhance

safety. En route AIRMETs are available over flight service frequencies. Over the contiguous U.S., AIRMETs

are also available on equipment intended to display weather and other non −air traffic control−related flight

information to pilots using the Flight Information Service–Broadcast (FIS−B). In Alaska and Hawaii, AIRMETs

are broadcast on air traffic frequencies.

(d) Are issued for the contiguous U.S., Alaska, and Hawaii. No AIRMETs are issued for U.S. Oceanic

FIRs in the Gulf of America, Caribbean, Western Atlantic, and Pacific Oceans.

TBL 7−1−2

U. S. AIRMET Issuance Time and Frequency

Product Type Issuance Time Issuance Frequency

AIRMETs over the Contiguous U.S. 0245, 0845, 1445, 2045 UTC Every 6 hours

AIRMETs over Alaska

0515, 1315, 2115 UTC

(standard time)

0415, 1215, 2015 UTC

(Daylight savings time)

Every 8 hours

AIRMETs over Hawaii 0400, 1000, 1600, 2200 UTC Every 6 hours

3. AIRMETs over the Contiguous U.S.:

(a) Are displayed graphically on websites, such as, aviationweather.gov and 1800wxbrief.com, and

equipment receiving FIS−B information.

(b) Provide a higher forecast resolution than AIRMETs issued in text format.

(c) Are valid at discrete times no more than 3 hours apart for a period of up to 12 hours into the future

(for example, 00, 03, 06, 09, and 12 hours). Additional forecasts may be inserted during the first 6 hours (for

example; 01, 02, 04, and 05). 00−hour represents the initial conditions, and the subsequent graphics depict the

area affected by the particular hazard at that valid time. Forecasts valid at 00 through 06 hours correspond to the

text AIRMET bulletin.

(d) Depict the following en route aviation weather hazards:

(1) Instrument flight rule conditions (ceiling < 1000’ and/or surface visibility < 3 miles).

(2) Widespread mountain obscuration.

(3) Moderate icing.

(4) Freezing levels.

(5) Moderate turbulence.

(6) Non−convective low−level wind shear potential below 2,000 feet AGL.

(7) Sustained surface winds greater than 30 knots.

7−1−16 Meteorology

2/20/25 AIM

4. Interpolation of time periods between AIRMETs over the contiguous U.S. valid times: Users must keep

in mind when using the AIRMET over the contiguous U.S. that if a 00 −hour forecast shows no significant

weather and a 03−hour forecast shows hazardous weather, they must assume a change is occurring during the

period between the two forecasts. It should be taken into consideration that the hazardous weather starts

immediately after the 00−hour forecast unless there is a defined initiation or ending time for the hazardous

weather. The same would apply after the 03 −hour forecast. The user should assume the hazardous weather

condition is occurring between the snap shots unless informed otherwise. For example, if a 00 −hour forecast

shows no hazard, a 03−hour forecast shows the presence of hazardous weather, and a 06−hour forecast shows

no hazard, the user should assume the hazard exists from the 0001 hour to the 0559 hour time period.

Meteorology 7−1−17

AIM 2/20/25

FIG 7−1−7

AIRMET over the Contiguous U.S.

7−1−18 Meteorology

2/20/25 AIM

5. AIRMETs over Alaska and Hawaii:

(a) AIRMETs over Alaska and Hawaii are in text format. The hazard areas are described using

well−known geographical areas. AIRMETs over Alaska are issued for three Alaskan regions corresponding to

Alaska area forecasts (See FIG 7−1−4).

(b) AIRMETs over Alaska are valid up to eight hours. AIRMETs over Hawaii are valid up to six hours.

Unscheduled issuances contain an update number for easier identification.

(c) AIRMET Zulu describes moderate icing and provides freezing level heights.

EXAMPLE−

Example of AIRMET Sierra issued for the Southeast Alaska area:

WAAK47 P AWU 241324

WA7O

JNUS WA 241315

AIRMET SIERRA FOR IFR AND MT OBSC VALID UNTIL 242115

LYNN CANAL AND GLACIER BAY JB

MTS OBSC BY CLDS/ISOL PCPN. NC.

CNTRL SE AK JC

MTS OCNL OBSC IN CLDS. NC.

SRN SE AK JD

PAWG−P AKT LN W OCNL CIGS BLW 010/VIS BLW 3SM BR. IMPR.

ERN GLF CST JE

OCNL CIGS BLW 010/VIS BLW 3SM BR/−RA BR. DTRT.

=JNUT WA 241315

AIRMET TANGO FOR TURB/STG SFC WINDS VALID UNTIL 242115

ERN GLF CST JE

OFSHR ICY BAY W SUSTAINED SFC WND 30 KTS

OR GTR. SPRDG E. INTSF .

=JNUZ WA 241315

AIRMET ZULU FOR ICING VALID UNTIL 242115

ERN GLF CST JE

16Z TO 19Z ALG CST W ICY BAY OCNL MOD ICEIC 080−160.

FZLVL 045 EXC 015 INLAND. WKN.

EXAMPLE−

Example of AIRMET Tango issued for Hawaii F A area:

WAHW31 PHFO 241529

WA0HI

HNLS WA 241600

AIRMET SIERRA UPDATE 2 FOR IFR VALID UNTIL 242200

NO SIGNIFICANT IFR EXP .

=HNLT WA 241600

AIRMET TANGO UPDATE 3 FOR TURB VALID UNTIL 242200

Meteorology 7−1−19

AIM 2/20/25

AIRMET TURB...HI

OVER AMD IMT S THRU W OF MTN.

TEMPO MOD TURB BLW 070.

COND CONT BEYOND 2200Z.

=HNLZ WA 241600

AIRMET ZULU UPDATE 2 FOR ICE AND FZLVL VALID UNTIL 242200

NO SIGNIFICANT ICE EXP

g. Watch Notification Messages

The Storm Prediction Center (SPC) in Norman, OK, issues Watch Notification Messages to provide an area threat

alert for forecast organized severe thunderstorms that may produce tornadoes, large hail, and/or convective

damaging winds within the CONUS. SPC issues three types of watch notification messages: Aviation Watch

Notification Messages, Public Severe Thunderstorm Watch Notification Messages, and Public Tornado Watch

Notification Messages.

It is important to note the difference between a Severe T hunderstorm (or Tornado) Watch and a Severe

Thunderstorm (or Tornado) Warning. A watch means severe weather is possible during the next few hours, while

a warning means that severe weather has been observed, or is expected within the hour. Only the SPC issues

Severe Thunderstorm and Tornado Watches, while only NWS Weather Forecasts Offices issue Severe

Thunderstorm and Tornado Warnings.

1. The Aviation Watch Notification Message. The Aviation Watch Notification Message product is an

approximation of the area of the Public Severe Thunderstorm Watch or Public Tornado Watch. The area may be

defined as a rectangle or parallelogram using VOR navigational aides as coordinates.

The Aviation Watch Notification Message was formerly known as the Alert Severe Weather Watch Bulletin

(AWW). The NWS no longer uses that title or acronym for this product. The NWS uses the acronym SAW for

the Aviation Watch Notification Message, but retains AWW in the product header for processing by weather data

systems.

EXAMPLE−

Example of an Aviation Watch Notification Message:

WWUS30 KWNS 271559

SAW2

SPC AWW 271559

WW 568 TORNADO AR LA MS 271605Z - 280000Z

AXIS..65 STATUTE MILES EAST AND WEST OF LINE..

45ESE HEZ/NATCHEZ MS/ - 50N TUP/TUPELO MS/

..AVIATION COORDS.. 55NM E/W /18WNW MCB - 60E MEM/

HAIL SURF ACE AND ALOFT..3 INCHES. WIND GUSTS..70 KNOTS. MAX TOPS TO 550. MEAN STORM MOTION

VECTOR 26030.

LAT...LON 31369169 34998991 34998762 31368948

THIS IS AN APPROXIMATION TO THE WATCH AREA. FOR A COMPLETE DEPICTION OF THE WATCH SEE

WOUS64 KWNS FOR WOU2.

2. Public Severe Thunderstorm Watch Notification Messages describe areas of expected severe

thunderstorms. (Severe thunderstorm criteria are 1-inch hail or larger and/or wind gusts of 50 knots [58 mph]

or greater). A Public Severe Thunderstorm Watch Notification Message contains the area description and axis,

the watch expiration time, a description of hail size and thunderstorm wind gusts expected, the definition of the

watch, a call to action statement, a list of other valid watches, a brief discussion of meteorological reasoning and

technical information for the aviation community.

3. Public Tornado Watch Notification Messages describe areas where the threat of tornadoes exists. A

Public Tornado Watch Notification Message contains the area description and axis, watch expiration time, the

term “damaging tornadoes,” a description of the largest hail size and strongest thunderstorm wind gusts expected,

the definition of the watch, a call to action statement, a list of other valid watches, a brief discussion of

7−1−20 Meteorology

2/20/25 AIM

meteorological reasoning and technical information for the aviation community. SPC may enhance a Public

Tornado Watch Notification Message by using the words “THIS IS A PARTICULARLY DANGEROUS

SITUATION” when there is a likelihood of multiple strong (damage of EF2 or EF3) or violent (damage of EF4

or EF5) tornadoes.

4. Public severe thunderstorm and tornado watch notification messages were formerly known as the Severe

Weather Watch Bulletins (WW). The NWS no longer uses that title or acronym for this product but retains WW

in the product header for processing by weather data systems.

EXAMPLE−

Example of a Public Tornado Watch Notification Message:

WWUS20 KWNS 050550

SEL2

SPC WW 051750

URGENT - IMMEDIATE BROADCAST REQUESTED

TORNADO WATCH NUMBER 243

NWS STORM PREDICTION CENTER NORMAN OK

1250 AM CDT MON MAY 5 2011

THE NWS STORM PREDICTION CENTER HAS ISSUED A

*TORNADO WATCH FOR PORTIONS OF

WESTERN AND CENTRAL ARKANSAS

SOUTHERN MISSOURI

F AR EASTERN OKLAHOMA

*EFFECTIVE THIS MONDAY MORNING FROM 1250 AM UNTIL 600 AM CDT.

...THIS IS A P ARTICULARLY DANGEROUS SITUATION...

*PRIMARY THREATS INCLUDE

NUMEROUS INTENSE TORNADOES LIKELY

NUMEROUS SIGNIFICANT DAMAGING WIND GUSTS TO 80 MPH LIKELY

NUMEROUS VERY LARGE HAIL TO 4 INCHES IN DIAMETER LIKELY

THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 100 STATUTE MILES EAST AND WEST OF A LINE

FROM 15 MILES WEST NORTHWEST OF FORT LEONARD WOOD MISSOURI TO 45 MILES SOUTHWEST OF HOT

SPRINGS ARKANSAS. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE

UPDATE (WOUS64 KWNS WOU2).

REMEMBER...A TORNADO WATCH M EANS CONDITIONS ARE F AVORABLE FOR TORNADOES AND SEVERE

THUNDERSTORMS IN AND CLOSE TO THE WATCH AREA. PERSONS IN THESE AREAS SHOULD BE ON THE

LOOKOUT FOR THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS AND POSSIBLE

WARNINGS.

OTHER WATCH INFORMATION...THIS TORNADO WATCH REPLACES TORNADO WATCH NUMBER 237. WATCH

NUMBER 237 WILL NOT BE IN EFFECT AFTER

1250 AM CDT. CONTINUE...WW 239...WW 240...WW 241...WW 242...

DISCUSSION...SRN MO SQUALL LINE EXPECTED TO CONTINUE EWD...WHERE LONG/HOOKED HODOGRAPHS

SUGGEST THREAT FOR EMBEDDED SUPERCELLS/PO SSIBLE TORNADOES. F ARTHER S...MORE WIDELY

SCATTERED

SUPERCELLS WITH A THREAT FOR TORNADOES WILL PERSIST IN VERY STRONGLY DEEP SHEARED/LCL

ENVIRONMENT IN AR.

AVIATION...TORNADOES AND A FEW SEVERE THUNDERS TORMS WITH HAIL SURF ACE AND ALOFT TO 4

INCHES. EXTREME TURBULENCE AND SURF ACE WIND GUSTS TO 70 KNOTS. A FEW CUMULONIMBI WITH

MAXIMUM TOPS TO 500. MEAN STORM MOTION VECTOR 26045.

5. Status reports are issued as needed to show progress of storms and to delineate areas no longer under the

threat of severe storm activity. Cancellation bulletins are issued when it becomes evident that no severe weather

will develop or that storms have subsided and are no longer severe.

h. Center Weather Advisories (CWAs)

1. CWAs are unscheduled inflight, flow control, air traffic, and air crew advisory. By nature of its short lead

time, the CW A is not a flight planning product. It is generally a nowcast for conditions beginning within the next

two hours. CWAs will be issued:

Meteorology 7−1−21

AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25

(a) As a supplement to an existing SIGMET, Convective SIGMET or AIRMET.

(b) When an Inflight Advisory has not been issued but observed or expected weather conditions meet

SIGMET/AIRMET criteria based on current pilot reports and reinforced by other sources of information about

existing meteorological conditions.

(c) When observed or developing weather conditions do not meet SIGMET, Convective SIGMET, or

AIRMET criteria; e.g., in terms of intensity or area coverage, but current pilot reports or other weather

information sources indicate that existing or anticipated meteorological phenomena will adversely affect the safe

flow of air traffic within the ARTCC area of responsibility.

2. The following example is a CWA issued from the Kansas City, Missouri, ARTCC. The “3” after ZKC

in the first line denotes this CWA has been issued for the third weather phenomena to occur for the day. The “301”

in the second line denotes the phenomena number again (3) and the issuance number (01) for this phenomena.

The CWA was issued at 2140Z and is valid until 2340Z.

EXAMPLE−

ZKC3 CWA 032140

ZKC CWA 301 VALID UNTIL 032340

ISOLD SVR TSTM over KCOU MOVG SWWD 10 KTS ETC.

7−1−7. Categorical Ceiling and Visibility Conditions

a. Categorical terms, describing either reported or forecast general ceiling and visibility conditions, are

defined as follows:

1. LIFR (Low IFR). Ceiling less than 500 feet and/or visibility less than 1 mile.

2. IFR. Ceiling 500 to less than 1,000 feet and/or visibility 1 to less than 3 miles.

3. MVFR (Marginal VFR). Ceiling 1,000 to 3,000 feet and/or visibility 3 to 5 miles inclusive.

4. VFR. Ceiling greater than 3,000 feet and visibility greater than 5 miles; includes sky clear.

b. The cause of LIFR, IFR, or MVFR is indicated by either ceiling or visibility restrictions or both. The

contraction “CIG” and/or weather and obstruction to vision symbols are used. If winds or gusts of 25 knots or

greater are forecast for the outlook period, the word “WIND” is also included for all categories including VFR.

EXAMPLE−

1. LIFR CIG−low IFR due to low ceiling.

2. IFR FG−IFR due to visibility restricted by fog.

3. MVFR CIG HZ FU−marginal VFR due to both ceiling and visibility restricted by haze and smoke.

4. IFR CIG RA WIND−IFR due to both low ceiling and visibility restricted by rain; wind expected to be 25 knots or greater .

7−1−8. Inflight Weather Advisory Broadcasts

a. ARTCCs broadcast a Convective SIGMET, SIGMET, AIRMET (except in the contiguous U.S.), Urgent

Pilot Report, or CWA alert once on all frequencies, except emergency frequencies, when any part of the area

described is within 150 miles of the airspace under their jurisdiction. These broadcasts advise pilots of the

availability of hazardous weather advisories and to contact the nearest flight service facility for additional details.

EXAMPLE−

1. Attention all aircraft, SIGMET Delta Three, from Myton to Tuba City to Milford, severe turbulence and severe clear icing

below one zero thousand feet. Expected to continue beyond zero three zero zero zulu.

2. Attention all aircraft, convective SIGMET Two Seven Eastern. From the vicinity of Elmira to Phillipsburg. Scattered

embedded thunderstorms moving east at one zero knots. A few intense level five cells, maximum tops four five zero.

3. Attention all aircraft, Kansas City Center weather advisory one zero three. Numerous reports of moderate to severe icing

from eight to niner thousand feet in a three zero mile radius of St. Louis. Light or negative icing reported from four thousand

to one two thousand feet remainder of Kansas City Center area.

7−1−22 Meteorology

2/20/25 AIM

NOTE−

Terminal control facilities have the option to limit hazardous weather information broadcast as follows: Tower cab and

approach control positions may opt to broadcast hazardous weather information alerts only when any part of the area

described is within 50 miles of the airspace under their jurisdiction.

REFERENCE−

F AA Order JO 7110.65, Para 2−6−6, Hazardous Inflight Weather Advisory.

7−1−9. Flight Information Services (FIS)

FIS is a method of disseminating meteorological (MET) and aeronautical information (AI) to displays in the

cockpit in order to enhance pilot situational awareness, provide decision support tools, and improve safety. FIS

augments traditional pilot voice communication with Flight Service Stations (FSSs), ATC facilities, or Airline

Operations Control Centers (AOCCs). FIS is not intended to replace traditional pilot and controller/flight service

specialist/aircraft dispatcher preflight briefings or inflight voice communications. FIS, however, can provide

textual and graphical information that can help abbreviate and improve the usefulness of such communications.

FIS enhances pilot situational awareness and improves safety.

a. Data link Service Providers (DSPs). DSPs deploy and maintain airborne, ground−based, and, in some cases,

space−based infrastructure that supports the transmission of AI/MET information over one or more physical

links. A DSP may provide a free of charge or a for−fee service that permits end users to uplink and downlink

AI/MET and other information. The following are examples of DSPs:

1. FAA FIS-B. A ground-based broadcast service provided through the ADS-B Universal Access

Transceiver (UAT) network. The service provides users with a 978 MHz data link capability when operating

within range and line-of-sight of a transmitting ground station. FIS-B enables users of properly equipped aircraft

to receive and display a suite of broadcast weather and aeronautical information products.

2. Non-FAA FIS Systems. Several commercial vendors provide customers with FIS data over both the

aeronautical spectrum and on other frequencies using a variety of data link protocols. Services available from

these providers vary greatly and may include tier based subscriptions. Advancements in bandwidth technology

permits preflight as well as inflight access to the same MET and AI information available on the ground. Pilots

and operators using non-FAA FIS for MET and AI information should be knowledgeable regarding the weather

services being provided as some commercial vendors may be repackaging NWS sourced weather, while other

commercial vendors may alter the weather information to produce vendor−tailored or vendor−specific weather

reports and forecasts.

b. Three Data Link Modes. There are three data link modes that may be used for transmitting AI and MET

information to aircraft. The intended use of the AI and/or MET information will determine the most appropriate

data link service.

1. Broadcast Mode: A one-way interaction in which AI and/or MET updates or changes applicable to a

designated geographic area are continuously transmitted (or transmitted at repeated periodic intervals) to all

aircraft capable of receiving the broadcast within the service volume defined by the system network architecture.

2. Contract/Demand Mode: A two-way interaction in which AI and/or MET information is transmitted to

an aircraft in response to a specific request.

3. Contract/Update Mode: A two-way interaction that is an extension of the Demand Mode. Initial AI

and/or MET report(s) are sent to an aircraft and subsequent updates or changes to the AI and/or MET information

that meet the contract criteria are automatically or manually sent to an aircraft.

c. To ensure airman compliance with Federal Aviation Regulations, manufacturer’s operating manuals should

remind airmen to contact ATC controllers, FSS specialists, operator dispatchers, or airline operations control

centers for general and mission critical aviation weather information and/or NAS status conditions (such as

NOTAMs, Special Use Airspace status, and other gove rnment flight information). If FIS products are

systemically modified (for example, are displayed as abbreviated plain text and/or graphical depictions), the

modification process and limitations of the resultant product should be clearly described in the vendor’s user

guidance.

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AIM 2/20/25

d. Operational Use of FIS. Regardless of the type of FIS system being used, several factors must be considered

when using FIS:

1. Before using FIS for inflight operations, pilots and other flight crewmembers should become familiar

with the operation of the FIS system to be used, the airborne equipment to be used, including its system

architecture, airborne system components, coverage service volume and other limitations of the particular

system, modes of operation and indications of various system failures. Users should also be familiar with the

specific content and format of the services available from the FIS provider(s). Sources of information that may

provide this specific guidance include manufacturer’s manuals, training programs, and reference guides.

2. FIS should not serve as the sole source of aviation weather and other operational information. ATC, FSSs,

and, if applicable, AOCC VHF/HF voice remain as a redundant method of communicating aviation weather,

NOTAMs, and other operational information to aircraft in flight. FIS augments these traditional

ATC/FSS/AOCC services and, for some products, offers the advantage of being displayed as graphical

information. By using FIS for orientation, the usefulness of information received from conventional means may

be enhanced. For example, FIS may alert the pilot to specific areas of concern that will more accurately focus

requests made to FSS or AOCC for inflight updates or similar queries made to ATC.

3. The airspace and aeronautical environment is constantly changing. These changes occur quickly and

without warning. Critical operational decisions should be based on use of the most current and appropriate data

available. When differences exist between FIS and information obtained by voice communication with ATC,

FSS, and/or AOCC (if applicable), pilots are cautioned to use the most recent data from the most authoritative

source.

4. FIS aviation weather products (for example, graphical ground−based radar precipitation depictions) are

not appropriate for tactical (typical timeframe of less than 3 minutes) avoidance of severe weather such as

negotiating a path through a weather hazard area. FIS supports strategic (typical timeframe of 20 minutes or

more) weather decision−making such as route selection to avoid a weather hazard area in its entirety. The misuse

of information beyond its applicability may place the pilot and aircraft in jeopardy. In addition, FIS should never

be used in lieu of an individual preflight weather and flight planning briefing.

5. DSPs offer numerous MET and AI products with information that can be layered on top of each other.

Pilots need to be aware that too much information can have a negative effect on their cognitive work load. Pilots

need to manage the amount of information to a level that offers the most pertinent information to that specific

flight without creating a cockpit distraction. Pilots may need to adjust the amount of information based on

numerous factors including, but not limited to, the phase of flight, single pilot operation, autopilot availability,

class of airspace, and the weather conditions encountered.

6. FIS NOTAM products, including Temporary Flight Restriction (TFR) information, are advisory −use

information and are intended for situational awareness purposes only. Cockpit displays of this information are

not appropriate for tactical navigation − pilots should stay clear of any geographic area displayed as a TFR

NOTAM. Pilots should contact FSSs and/or ATC while en route to obtain updated information and to verify the

cockpit display of NOTAM information.

7. FIS supports better pilot decision−making by increasing situational awareness. Better decision−making

is based on using information from a variety of sources. In addition to FIS, pilots should take advantage of other

weather/NAS status sources, including, briefings from Flight Service Stations, data from other air traffic control

facilities, airline operation control centers, pilot reports, as well as their own observations.

e. FAA’s Flight Information Service−Broadcast (FIS−B).

1. FIS−B is a ground− based broadcast service provided through the FAA’s Automatic Dependent

Surveillance–Broadcast (ADS−B) Services Universal Access Transceiver (UAT) network. The service provides

users with a 978 MHz data link capability when operating within range and line−of−sight of a transmitting ground

station. FIS−B enables users of properly−equipped aircraft to receive and display a suite of broadcast weather

and aeronautical information products.

7−1−24 Meteorology

AIM2/20/258/7/25 AIM

2. TBL 7−1−3 lists the text and graphical products available through FIS−B and provided free−of−charge.

Detailed information concerning FIS−B meteorological products can be found in FAA−H−8083−28, Aviation

Weather Handbook, and AC 00–63, Use of Cockpit Displays of Digital Weather and Aeronautical Information.

Information on Special Use Airspace (SUA), Temporary Flight Restriction (TFR), and Notice to Airmen

(NOTAM) products can be found in Chapters 3, 4, and 5 of this manual.

3. Users of FIS−B should familiarize themselves with the operational characteristics and limitations of the

system, including: system architecture; service envi ronment; product lifecycles; modes of operation; and

indications of system failure.

NOTE−

The NOTAM−D and NOTAM−FDC products broadcast via FIS−B are limited to those issued or effective within the past 30

days. Except for TFRs, NOTAMs older than 30 days are not provided. The pilot in command is responsible for reviewing

all necessary information prior to flight.

4. FIS−B products are updated and transmitted at specific intervals based primarily on product issuance

criteria. Update intervals are defined as the rate at which the product data is available from the source for

transmission. Transmission intervals are defined as the amount of time within which a new or updated product

transmission must be completed and/or the rate or repetition interval at which the product is rebroadcast. Update

and transmission intervals for each product are provided in TBL 7−1−3.

5. Where applicable, FIS−B products include a look−ahead range expressed in nautical miles (NM) for three

service domains: Airport Surface; Terminal Airspace; and En Route/Gulf of America. TBL 7 −1−4 provides

service domain availability and look−ahead ranging for each FIS−B product.

6. Prior to using this capability, users should familiarize themselves with the operation of FIS−B avionics

by referencing the applicable User’s Guides. Guidance concerning the interpretation of information displayed

should be obtained from the appropriate avionics manufacturer.

7. FIS−B malfunctions not attributed to aircraft system failures or covered by active NOTAM should be

reported by radio or telephone to the nearest FSS facility, or by sending an email to the ADS−B help desk at

[email protected]. Reports should include:

(a) Condition observed;

(b) Date and time of observation;

(c) Altitude and location of observation;

(d) Type and call sign of the aircraft; and

(e) Type and software version of avionics system.

f. Non−FAA FIS Systems. Several commercial vendors also provide customers with FIS data over both the

aeronautical spectrum and on other frequencies using a variety of data link protocols. In some cases, the vendors

provide only the communications system that carries customer messages, such as the Aircraft Communications

Addressing and Reporting System (ACARS) used by many air carrier and other operators.

1. Operators using non−FAA FIS data for inflight weather and other operational information should ensure

that the products used conform to FAA/NWS standa rds. Specifically, aviation weather and NAS status

information should meet the following criteria:

(a) The products should be either FAA/NWS “accepted” aviation weather reports or products, or based

on FAA/NWS accepted aviation weather reports or products. If products are used which do not meet this criteria,

they should be so identified. The operator must determine the applicability of such products to their particular

flight operations.

(b) In the case of a weather product which is the result of the application of a process which alters the

form, function or content of the base FAA/NWS accepted weather product(s), that process, and any limitations

to the application of the resultant product, should be described in the vendor’s user guidance material. An

Meteorology 7−1−25

AIM 2/20/25

example would be a NEXRAD radar composite/mosaic map, which has been modified by changing the scaling

resolution. The methodology of assigning reflectivity values to the resultant image components should be

described in the vendor’s guidance material to ensure that the user can accurately interpret the displayed data.

TBL 7−1−3

FIS−B Over UAT Product Update and Transmission Intervals

Product Update Interval1 Transmission

Interval (95%)2

Basic

Product

AIRMET As Available 5 minutes Yes

AWW/WW As Available, then at 15 minute

intervals for 1 hour 5 minutes No

Ceiling As Available 10 minutes No

Convective SIGMET As Available, then at 15 minute

intervals for 1 hour 5 minutes Yes

D−A TIS As Available 1 minute No

Echo Top 5 minutes 5 minutes No

METAR/SPECI 1 minute (where available), As

Available otherwise 5 minutes Yes

MRMS NEXRAD (CONUS) 2 minutes 15 minutes Yes

MRMS NEXRAD (Regional) 2 minutes 2.5 minutes Yes

NOTAMs−D/FDC As Available 10 minutes Yes

NOTAMs−TFR As Available 10 minutes Yes

PIREP As Available 10 minutes Yes

SIGMET As Available, then at 15 minute

intervals for 1 hour 5 minutes Yes

SUA Status As Available 10 minutes Yes

TAF/AMEND 6 Hours (±15 minutes) 10 minutes Yes

Temperature Aloft 12 Hours (±15 minutes) 10 minutes Yes

TWIP As Available 1 minute No

Winds aloft 12 Hours (±15 minutes) 10 minutes Yes

Lightning strikes 3 5 minutes 5 minutes Yes

Turbulence 3 1 minute 15 minutes Yes

Icing, Forecast Potential (FIP) 3 60 minutes 15 minutes Yes

Cloud tops 3 30 minutes 15 minutes Yes

1 Minute AWOS 3 1 minute 10 minutes No

Graphical−AIRMET 3 As Available 5 minutes Yes

Center Weather Advisory (CWA) 3 As Available 10 minutes Yes

Temporary Restricted Areas (TRA) As Available 10 minutes Yes

Temporary Military Operations Areas

(TMOA) As Available 10 minutes Yes

1 The Update Interval is the rate at which the product data is available from the source.

7−1−26 Meteorology

2/20/25 AIM

2 The Transmission Interval is the amount of time within which a new or updated product transmission must be completed

(95%) and the rate or repetition interval at which the product is rebroadcast (95%).

3 The transmission and update intervals for the expanded set of basic meteorological products may be adjusted based on FAA

and vendor agreement on the final product formats and performance requirements.

NOTE−

1. Details concerning the content, format, and symbols of the various data link products provided should be obtained from

the specific avionics manufacturer.

2. NOTAM−D and NOTAM−FDC products broadcast via FIS−B are limited to those issued or effective within the past 30

days.

TBL 7−1−4

Product Parameters for Low/Medium/High Altitude Tier Radios

Product Surface Radios Low Altitude Tier Medium Altitude

Tier

High Altitude Tier

CONUS NEXRAD N/A CONUS NEXRAD CONUS NEXRAD CONUS NEXRAD

not provided imagery imagery

Winds & Temps 500 NM look−ahead 500 NM look−ahead 750 NM look−ahead 1,000 NM look−

Aloft range range range ahead range

METAR 100 NM look−ahead 250 NM look−ahead 375 NM look−ahead CONUS: CONUS

range range range Class B & C airport

METARs and 500

NM look−ahead

range

Outside of CONUS:

500 NM look-ahead

range

TAF 100 NM look−ahead 250 NM look−ahead 375 NM look−ahead CONUS: CONUS

range range range Class B & C airport

TAFs and 500 NM

look−ahead range

Outside of CONUS:

500 NM look-ahead

range

AIRMET, SIGMET, 100 NM look−ahead 250 NM look−ahead 375 NM look−ahead 500 NM look−ahead

PIREP, and SUA/ range. PIREP/SUA/ range range range

SAA SAA is N/A.

Regional NEXRAD 150 NM look−ahead 150 NM look−ahead 200 NM look−ahead 250 NM look−ahead

range range range range

NOTAMs D, FDC, 100 NM look−ahead 100 NM look−ahead 100 NM look−ahead 100 NM look−ahead

and TFR range range range range

7−1−10. Weather Observing Programs

a. Manual Observations. With only a few exceptions, these reports are from airport locations staffed by

FAA personnel who manually observe, perform calculations, and enter these observations into the (WMSCR)

communication system. The format and coding of these observations are contained in paragraph 7−1−28 , Key

to Aviation Routine Weather Report (METAR) and Aerodrome Forecasts (TAF).

b. Automated Weather Observing System (AWOS).

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AIM 2/20/25

1. Automated weather reporting systems are increasingly being installed at airports. These systems consist

of various sensors, a processor, a computer-generated voice subsystem, and a transmitter to broadcast local,

minute-by-minute weather data directly to the pilot.

NOTE−

When the barometric pressure exceeds 31.00 inches Hg., see AIM, Para 7−2−3, Altimeter Errors.

2. The AWOS observations will include the prefix “AUTO” to indicate that the data are derived from an

automated system. Some AWOS locations will be augmented by certified observers who will provide weather

and obstruction to vision information in the remarks of the report when the reported visibility is less than 7 miles.

These sites, along with the hours of augmentation, are to be published in the Chart Supplement. Augmentation

is identified in the observation as “OBSERVER WEATHER.” The AWOS wind speed, direction and gusts,

temperature, dew point, and altimeter setting are exactly the same as for manual observations. The AWOS will

also report density altitude when it exceeds the field elevation by more than 1,000 feet. The reported visibility

is derived from a sensor near the touchdown of the primary instrument runway. The visibility sensor output is

converted to a visibility value using a 10−minute harmonic average. The reported sky condition/ceiling is derived

from the ceilometer located next to the visibility sensor. The AWOS algorithm integrates the last 30 minutes of

ceilometer data to derive cloud layers and heights. This output may also differ from the observer sky condition

in that the AWOS is totally dependent upon the cloud advection over the sensor site.

3. These real-time systems are operationally classified into nine basic levels:

(a) AWOS−A only reports altimeter setting;

NOTE−

Any other information is advisory only.

(b) AWOS−AV reports altimeter and visibility;

NOTE−

Any other information is advisory only.

(c) AWOS−l usually reports altimeter setting, wind data, temperature, dew point, and density altitude;

(d) AWOS−2 provides the information provided by AWOS−l plus visibility; and

(e) AWOS−3 provides the information provided by AWOS−2 plus cloud/ceiling data.

(f) AWOS− 3P provides reports the same as the AWOS 3 system, plus a precipitation identification

sensor.

(g) AWOS− 3PT reports the same as the AWOS 3P System, plus thunderstorm/lightning reporting

capability.

(h) AWOS− 3T reports the same as AWOS 3 system and includes a thunderstorm/lightning reporting

capability.

(i) AWOS− 4 reports the same as the AWOS 3 system, plus precipitation occurrence, type and

accumulation, freezing rain, thunderstorm, and runway surface sensors.

4. The information is transmitted over a discrete VHF radio frequency or the voice portion of a local

NA V AID. AWOS transmissions on a discrete VHF radio frequency are engineered to be receivable to a maximum

of 25 NM from the AWOS site and a maximum altitude of 10,000 feet AGL. At many locations, AWOS signals

may be received on the surface of the airport, but local conditions may limit the maximum AWOS reception

distance and/or altitude. The system transmits a 20 to 30 second weather message updated each minute. Pilots

should monitor the designated frequency for the automated weather broadcast. A description of the broadcast

is contained in subparagraph c. There is no two-way communication capability. Most AWOS sites also have a

dial-up capability so that the minute-by-minute weather messages can be accessed via telephone.

5. AWOS information (system level, frequency, phone number, etc.) concerning specific locations is

published, as the systems become operational, in the Chart Supplement, and where applicable, on published

7−1−28 Meteorology

2/20/25 AIM

Instrument Approach Procedures. Selected individual systems may be incorporated into nationwide data

collection and dissemination networks in the future.

c. AWOS Broadcasts. Computer-generated voice is used in AWOS to automate the broadcast of the

minute-by-minute weather observations. In addition, some systems are configured to permit the addition of an

operator-generated voice message; e.g., weather remarks following the automated parameters. The phraseology

used generally follows that used for other weather broadcasts. Following are explanations and examples of the

exceptions.

1. Location and Time. The location/name and the phrase “AUTOMATED WEATHER OBSERV A-

TION,” followed by the time are announced.

(a) If the airport’s specific location is included in the airport’s name, the airport’s name is announced.

EXAMPLE−

“Bremerton National Airport automated weather observation, one four five six zulu;”

“Ravenswood Jackson County Airport automated weather observation, one four five six zulu.”

(b) If the airport’s specific location is not included in the airport’s name, the location is announced

followed by the airport’s name.

EXAMPLE−

“Sault Ste. Marie, Chippewa County International Airport automated weather observation;”

“Sandusky, Cowley Field automated weather observation.”

(c) The word “TEST” is added following “OBSERV ATION” when the system is not in commissioned

status.

EXAMPLE−

“Bremerton National Airport automated weather observation test, one four five six zulu.”

(d) The phrase “TEMPORARILY INOPERATIVE” is added when the system is inoperative.

EXAMPLE−

“Bremerton National Airport automated weather observing system temporarily inoperative.”

2. Visibility.

(a) The lowest reportable visibility value in AWOS is “less than 1/4.” It is announced as “VISIBILITY

LESS THAN ONE QUARTER.”

(b) A sensor for determining visibility is not included in some AWOS. In these systems, visibility is not

announced. “VISIBILITY MISSING” is announced only if the system is configured with a visibility sensor and

visibility information is not available.

3. Weather. In the future, some AWOSs are to be configured to determine the occurrence of precipitation.

However, the type and intensity may not always be determined. In these systems, the word “PRECIPITATION”

will be announced if precipitation is occurring, but the type and intensity are not determined.

4. Ceiling and Sky Cover.

(a) Ceiling is announced as either “CEILING” or “INDEFINITE CEILING.” With the exception of

indefinite ceilings, all automated ceiling heights are measured.

EXAMPLE−

“Bremerton National Airport automated weather observation, one four five six zulu. Ceiling two thousand overcast;”

“Bremerton National Airport automated weather observation, one four five six zulu. Indefinite ceiling two hundred, sky

obscured.”

(b) The word “Clear” is not used in AWOS due to limitations in the height ranges of the sensors. No

clouds detected is announced as “NO CLOUDS BELOW XXX” or, in newer systems as “CLEAR BELOW

XXX” (where XXX is the range limit of the sensor).

Meteorology 7−1−29

AIM 2/20/25

EXAMPLE−

“No clouds below one two thousand.”

“Clear below one two thousand.”

(c) A sensor for determining ceiling and sky cover is not included in some AWOS. In these systems,

ceiling and sky cover are not announced. “SKY CONDITION MISSING” is announced only if the system is

configured with a ceilometer and the ceiling and sky cover information is not available.

5. Remarks. If remarks are included in the observation, the word “REMARKS” is announced following

the altimeter setting.

(a) Automated “Remarks.”

(1) Density Altitude.

(2) Variable Visibility.

(3) Variable Wind Direction.

(b) Manual Input Remarks. Manual input remarks are prefaced with the phrase “OBSERVER

WEATHER.” As a general rule the manual remarks are limited to:

(1) Type and intensity of precipitation.

(2) Thunderstorms and direction; and

(3) Obstructions to vision when the visibility is 3 miles or less.

EXAMPLE−

“Remarks ... density altitude, two thousand five hundred ... visibility variable between one and two ... wind direction variable

between two four zero and three one zero ...observed we ather ... thunderstorm moderate rain showers and fog ...

thunderstorm overhead.”

(c) If an automated parameter is “missing” and no manual input for that parameter is available, the

parameter is announced as “MISSING.” For example, a report with the dew point “missing” and no manual input

available, would be announced as follows:

EXAMPLE−

“Ceiling one thousand overcast ... visibility three ... precipitation ... temperature three zero, dew point missing ... wind calm

... altimeter three zero zero one.”

(d) “REMARKS” are announced in the following order of priority:

(1) Automated “REMARKS.”

[a] Density Altitude.

[b] Variable Visibility.

[c] Variable Wind Direction.

(2) Manual Input “REMARKS.”

[a] Sky Condition.

[b] Visibility.

[c] Weather and Obstructions to Vision.

[d] Temperature.

[e] Dew Point.

[f] Wind; and

[g] Altimeter Setting.

EXAMPLE−

“Remarks ... density altitude, two thousand five hundred ... visibility variable between one and two ... wind direction variable

7−1−30 Meteorology

2/20/25 AIM

between two four zero and three one zero ... observer ceiling estimated two thousand broken ... observer temperature two,

dew point minus five.”

d. Automated Surface Observing System (ASOS)/Automated Weather Observing System

(AWOS) The ASOS/AWOS is the primary surface weather observing system of the U.S. (See Key to Decode

an ASOS/AWOS (METAR) Observation, FIG 7−1−8 and FIG 7−1−9.) The program to install and operate these

systems throughout the U.S. is a joint effort of the NWS, the FAA and the Department of Defense. ASOS/AWOS

is designed to support aviation operations and weather forecast activities. The ASOS/AWOS will provide

continuous minute-by-minute observations and perform the basic observing functions necessary to generate an

aviation routine weather report (METAR) and other aviation weather information. The information may be

transmitted over a discrete VHF radio frequency or the voice portion of a local NA V AID. ASOS/AWOS

transmissions on a discrete VHF radio frequency are engineered to be receivable to a maximum of 25 NM from

the ASOS/AWOS site and a maximum altitude of 10,000 feet AGL. At many locations, ASOS/AWOS signals

may be received on the surface of the airport, but local conditions may limit the maximum reception distance

and/or altitude. While the automated system and the human may differ in their methods of data collection and

interpretation, both produce an observation quite similar in form and content. For the “objective” elements such

as pressure, ambient temperature, dew point temperature, wind, and precipitation accumulation, both the

automated system and the observer use a fixed location and time-averaging technique. The quantitative

differences between the observer and the automated observation of these elements are negligible. For the

“subjective” elements, however, observers use a fixed time, spatial averaging technique to describe the visual

elements (sky condition, visibility and present weather), while the automated systems use a fixed location, time

averaging technique. Although this is a fundamental change, the manual and automated techniques yield

remarkably similar results within the limits of their respective capabilities.

1. System Description.

(a) The ASOS/AWOS at each airport location consists of these main components:

(1) Individual weather sensors.

(2) Data collection and processing units.

(3) Peripherals and displays.

(b) The ASOS/AWOS sensors perform the basic function of data acquisition. They continuously sample

and measure the ambient environment, derive raw sensor data and make them available to the collection and

processing units.

2. Every ASOS/AWOS will contain the following basic set of sensors:

(a) Cloud height indicator (one or possibly three).

(b) Visibility sensor (one or possibly three).

(c) Precipitation identification sensor.

(d) Freezing rain sensor (at select sites).

(e) Pressure sensors (two sensors at small airports; three sensors at large airports).

(f) Ambient temperature/Dew point temperature sensor.

(g) Anemometer (wind direction and speed sensor).

(h) Rainfall accumulation sensor.

(i) Automated Lightning Detection and Reporting System (ALDARS) (excluding Alaska and Pacific

Island sites).

3. The ASOS/AWOS data outlets include:

(a) Those necessary for on-site airport users.

Meteorology 7−1−31

AIM 2/20/25

(b) National communications networks.

(c) Computer-generated voice (available through FAA radio broadcast to pilots, and dial-in telephone

line).

NOTE−

Wind direction is reported relative to magnetic north in ATIS as well as ASOS and AWOS radio (voice) broadcasts.

4. An ASOS/AWOS report without human intervention will contain only that weather data capable of being

reported automatically. The modifier for this METAR report is “AUTO.” When an observer augments or

backs−up an ASOS/AWOS site, the “AUTO” modifier disappears.

5. There are two types of automated stations, AO1 for automated weather reporting stations without a

precipitation discriminator, and AO2 for automated stations with a precipitation discriminator. As appropriate,

“AO1” and “AO2” must appear in remarks. (A precipitation discriminator can determine the difference between

liquid and frozen/freezing precipitation).

NOTE−

To decode an ASOS/AWOS report, refer to FIG 7−1−8 and FIG 7−1−9.

REFERENCE−

A complete explanation of METAR terminology is located in AIM, Para 7−1−28, Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report

(METAR).

7−1−32 Meteorology

2/20/25 AIM

FIG 7−1−8

Key to Decode an ASOS/AWOS (METAR) Observation (Front)

Meteorology 7−1−33

AIM 2/20/25

FIG 7−1−9

Key to Decode an ASOS/AWOS (METAR) Observation (Back)

7−1−34 Meteorology

2/20/25 AIM

e. TBL 7−1−5 contains a comparison of weather observing programs and the elements reported.

f. Service Standards. During 1995, a government/industry team worked to comprehensively reassess the

requirements for surface observations at the nation’s airports. That work resulted in agreement on a set of service

standards, and the FAA and NWS ASOS sites to which the standards would apply. The term “Service Standards”

refers to the level of detail in weather observation. The service standards consist of four different levels of service

(A, B, C, and D) as described below. Specific observational elements included in each service level are listed

in TBL 7−1−6.

1. Service Level D defines the minimum acceptable level of service. It is a completely automated service

in which the ASOS/AWOS observation will constitute the entire observation, i.e., no additional weather

information is added by a human observer. This service is referred to as a stand alone D site.

2. Service Level C is a service in which the human observer, usually an air traffic controller, augments or

adds information to the automated observation. Service Level C also includes backup of ASOS/AWOS elements

in the event of an ASOS/AWOS malfunction or an unrepresentative ASOS/AWOS report. In backup, the human

observer inserts the correct or missing value for the automated ASOS/AWOS elements. This service is provided

by air traffic controllers under the Limited Aviation Weather Reporting Station (LAWRS) process, FSS and NWS

observers, and, at selected sites, Non−Federal Observation Program observers.

Two categories of airports require detail beyond Service Level C in order to enhance air traffic control efficiency

and increase system capacity. Services at these airports are typically provided by contract weather observers,

NWS observers, and, at some locations, FSS observers.

3. Service Level B is a service in which weather observations consist of all elements provided under Service

Level C, plus augmentation of additional data beyond the capability of the ASOS/AWOS. This category of

airports includes smaller hubs or special airports in other ways that have worse than average bad weather

operations for thunderstorms and/or freezing/frozen precipitation, and/or that are remote airports.

4. Service Level A, the highest and most demanding category, includes all the data reported in Service

Standard B, plus additional requirements as specified. Service Level A covers major aviation hubs and/or high

volume traffic airports with average or worse weather.

TBL 7−1−5

Weather Observing Programs

Element

Reported

Type

Wind

Visibility

Temperature

Dew Point

Altimeter

Density

Altimeter

Cloud/Ceiling

Precipitation

Identification

Thunderstorm/

Lightning

Precipitation

Occurrence

Rainfall

Accumulation

Runway Surface

Condition

Freezing Rain

Occurrence

Remarks

ASOS X X X X X X X X X X

AWOS−A X

AWOS−A/V X X

AWOS−1 X X X X

AWOS−2 X X X X X

AWOS−3 X X X X X X

AWOS−3P X X X X X X X

AWOS−3T X X X X X X X

AWOS−3P/T X X X X X X X X

AWOS−4 X X X X X X X X X X X X

Manual X X X X X X X

REFERENCE− FAA Order JO 7900.5, Surface Weather Observing, for element reporting.

Meteorology 7−1−35

AIM 2/20/25

TBL 7−1−6

SERVICE LEVEL A

Service Level A consists of all the elements of

Service Levels B, C and D plus the elements

listed to the right, if observed.

10 minute longline RVR at precedented sites or

additional visibility increments of 1/8, 1/16 and 0

Sector visibility

Variable sky condition

Cloud layers above 12,000 feet and cloud types

Widespread dust, sand and other obscurations

V olcanic eruptions

SERVICE LEVEL B

Service Level B consists of all the elements of

Service Levels C and D plus the elements listed to

the right, if observed.

Longline RVR at precedented sites

(may be instantaneous readout)

Freezing drizzle versus freezing rain

Ice pellets

Snow depth & snow increasing rapidly remarks

Thunderstorm and lightning location remarks

Observed significant weather not at the station

remarks

SERVICE LEVEL C

Service Level C consists of all the elements of Service

Level D plus augmentation and backup by a human

observer or an air traffic control specialist on location

nearby. Backup consists of inserting the correct value if

the system malfunctions or is unrepresentative.

Augmentation consists of adding the elements listed to

the right, if observed. During hours that the observing

facility is closed, the site reverts to Service Level D.

Thunderstorms

Tornadoes

Hail

Virga

V olcanic ash

Tower visibility

Operationally significant remarks as deemed

appropriate by the observer

SERVICE LEVEL D

This level of service consists of an ASOS or AWOS

continually measuring the atmosphere at a point near the

runway. The ASOS or AWOS senses and measures the

weather parameters listed to the right.

Wind

Visibility

Precipitation/Obstruction to vision

Cloud height

Sky cover

Temperature

Dew point

Altimeter

7−1−11. Weather Radar Services

a. The National Weather Service operates a network of radar sites for detecting coverage, intensity, and

movement of precipitation. The network is supplemented by FAA and DoD radar sites in the western sections

of the country. Local warning radar sites augment the network by operating on an as needed basis to support

warning and forecast programs.

b. Scheduled radar observations are taken hourly and transmitted in alpha-numeric format on weather

telecommunications circuits for flight planning purposes. Under certain conditions, special radar reports are

issued in addition to the hourly transmittals. Data contained in the reports are also collected by the National

Center for Environmental Prediction and used to prepare national radar summary charts for dissemination on

facsimile circuits.

c. A clear radar display (no echoes) does not mean that there is no significant weather within the coverage of

the radar site. Clouds and fog are not detected by the radar. However, when echoes are present, turbulence can

be implied by the intensity of the precipitation, and icing is implied by the presence of the precipitation at

temperatures at or below zero degrees Celsius. Used in conjunction with other weather products, radar provides

invaluable information for weather avoidance and flight planning.

Meteorology7−1−36

2/20/25 AIM

FIG 7−1−10

NEXRAD Coverage

Meteorology 7−1−37

AIM 2/20/25

FIG 7−1−11

NEXRAD Coverage

7−1−38 Meteorology

2/20/25 AIM

FIG 7−1−12

NEXRAD Coverage

Meteorology 7−1−39

AIM 2/20/25

d. All En Route Flight Advisory Service facilities and FSSs have equipment to directly access the radar

displays from the individual weather radar sites. Specialists at these locations are trained to interpret the display

for pilot briefing and inflight advisory services. The Center Weather Service Units located in ARTCCs also have

access to weather radar displays and provide support to all air traffic facilities within their center’s area.

e. For more detailed information on PIREPS, users can refer to the current version of the Aviation Weather

Handbook, FAA−H−8083−28.

REFERENCE−

Pilot/Controller Glossary Term− Precipitation Radar Weather Descriptions.

AIM, Para 7−1−26, Thunderstorms.

Chart Supplement, Charts, NWS Upper Air Observing Stations and Weather Network for the location of specific radar sites.

7−1−12. ATC Inflight Weather Avoidance Assistance

a. ATC Radar Weather Display.

1. ATC radars are able to display areas of precipitation by sending out a beam of radio energy that is reflected

back to the radar antenna when it strikes an object or moisture which may be in the form of rain drops, hail, or

snow. The larger the object is, or the more dense its reflective surface, the stronger the return will be presented.

Radar weather processors indicate the intensity of reflective returns in terms of decibels (dBZ). ATC systems

cannot detect the presence or absence of clouds. The ATC systems can often determine the intensity of a

precipitation area, but the specific character of that area (snow, rain, hail, VIRGA, etc.) cannot be determined.

For this reason, ATC refers to all weather areas displayed on ATC radar scopes as “precipitation.”

2. All A TC facilities using radar weather processors with the ability to determine precipitation intensity, will

describe the intensity to pilots as:

(a) “LIGHT” (< 26 dBZ)

(b) “MODERATE” (26 to 40 dBZ)

(c) “HEA VY” (> 40 to 50 dBZ)

(d) “EXTREME” (> 50 dBZ)

NOTE−

En route ATC radar’ s Weather and Radar Processor (WARP) does not display light precipitation intensity.

3. ATC facilities that, due to equipment limitations, cannot display the intensity levels of precipitation, will

describe the location of the precipitation area by geographic position, or position relative to the aircraft. Since

the intensity level is not available, the controller will state “INTENSITY UNKNOWN.”

4. ARTCC facilities normally use a Weather and Radar Processor (WARP) to display a mosaic of data

obtained from multiple NEXRAD sites. There is a time delay between actual conditions and those displayed to

the controller. For example, the precipitation data on the ARTCC controller’s display could be up to 6 minutes

old. When the WARP is not available, a second system, the narrowband Air Route Surveillance Radar (ARSR)

can display two distinct levels of precipitation intensity that will be described to pilots as “MODERATE” (30

to 40 dBZ) and “HEA VY TO EXTREME” ( > 40 dBZ ). The WARP processor is only used in ARTCC facilities.

5. ATC radar is not able to detect turbulence. Generally, turbulence can be expected to occur as the rate of

rainfall or intensity of precipitation increases. Turbulence associated with greater rates of rainfall/precipitation

will normally be more severe than any associated with lesser rates of rainfall/precipitation. Turbulence should

be expected to occur near convective activity, even in clear air. Thunderstorms are a form of convective activity

that imply severe or greater turbulence. Operation within 20 miles of thunderstorms should be approached with

great caution, as the severity of turbulence can be markedly greater than the precipitation intensity might indicate.

b. Weather Avoidance Assistance.

1. To the extent possible, controllers will issue pertinent information on weather or chaff areas and assist

pilots in avoiding such areas when requested. Pilo ts should respond to a weather advisory by either

7−1−40 Meteorology

2/20/25 AIM

acknowledging the advisory or by acknowledging the advisory and requesting an alternative course of action as

follows:

(a) Request to deviate off course by stating a heading or degrees, direction of deviation, and approximate

number of miles. In this case, when the requested deviation is approved, navigation is at the pilot’s prerogative,

but must maintain the altitude assigned, and remain within the lateral restrictions issued by ATC.

(b) An approval for lateral deviation authorizes the pilot to maneuver left or right within the lateral limits

specified in the clearance.

NOTE−

1. It is often necessary for ATC to restrict the amount of lateral deviation (“twenty degrees right,” “up to fifteen degrees

left,” “up to ten degrees left or right of course”).

2. The term “when able, proceed direct,” in an ATC weather deviation clearance, refers to the pilot’ s ability to remain clear

of the weather when returning to course/route.

(c) Request a new route to avoid the affected area.

(d) Request a change of altitude.

(e) Request radar vectors around the affected areas.

2. For obvious reasons of safety, an IFR pilot must not deviate from the course or altitude or flight level

without a proper ATC clearance. When weather conditions encountered are so severe that an immediate deviation

is determined to be necessary and time will not permit approval by ATC, the pilot’s emergency authority may

be exercised.

3. When the pilot requests clearance for a route deviation or for an ATC radar vector, the controller must

evaluate the air traffic picture in the affected area, and coordinate with other controllers (if ATC jurisdictional

boundaries may be crossed) before replying to the request.

4. It should be remembered that the controller’s primary function is to provide safe separation between

aircraft. Any additional service, such as weather avoidance assistance, can only be provided to the extent that

it does not derogate the primary function. It’s also worth noting that the separation workload is generally greater

than normal when weather disrupts the usual flow of traffic. ATC radar limitations and frequency congestion may

also be a factor in limiting the controller’s capability to provide additional service.

5. It is very important, therefore, that the request for deviation or radar vector be forwarded to ATC as far

in advance as possible. Delay in submitting it may delay or even preclude ATC approval or require that additional

restrictions be placed on the clearance. Insofar as possible the following information should be furnished to A TC

when requesting clearance to detour around weather activity:

(a) Proposed point where detour will commence.

(b) Proposed route and extent of detour (direction and distance).

(c) Point where original route will be resumed.

(d) Flight conditions (IFR or VFR).

(e) Any further deviation that may become necessary as the flight progresses.

(f) Advise if the aircraft is equipped with functioning airborne radar.

6. To a large degree, the assistance that might be rendered by ATC will depend upon the weather information

available to controllers. Due to the extremely transitory nature of severe weather situations, the controller’s

weather information may be of only limited value if based on weather observed on radar only. Frequent updates

by pilots giving specific information as to the area affected, altitudes, intensity and nature of the severe weather

can be of considerable value. Such reports are relayed by radio or phone to other pilots and controllers and also

receive widespread teletypewriter dissemination.

7. Obtaining IFR clearance or an ATC radar vector to circumnavigate severe weather can often be

accommodated more readily in the en route areas away from terminals because there is usually less congestion

Meteorology 7−1−41

AIM 2/20/25

and, therefore, offer greater freedom of action. In terminal areas, the problem is more acute because of traffic

density, ATC coordination requirements, complex departure and arrival routes, adjacent airports, etc. As a

consequence, controllers are less likely to be able to accommodate all requests for weather detours in a terminal

area or be in a position to volunteer such routing to the pilot. Nevertheless, pilots should not hesitate to advise

controllers of any observed severe weather and should specifically advise controllers if they desire

circumnavigation of observed weather.

7−1−13. Runway Visual Range (RVR)

There are currently two configurations of RVR in the NAS commonly identified as Taskers and New Generation

RVR. The Taskers are the existing configuration which uses transmissometer technology. The New Generation

RVRs were deployed in November 1994 and use forward scatter technology. The New Generation RVRs are

currently being deployed in the NAS to replace the existing Taskers.

a. RVR values are measured by transmissometers mounted on 14−foot towers along the runway. A full RVR

system consists of:

1. Transmissometer projector and related items.

2. Transmissometer receiver (detector) and related items.

3. Analog recorder.

4. Signal data converter and related items.

5. Remote digital or remote display programmer.

b. The transmissometer projector and receiver are mounted on towers 250 feet apart. A known intensity of

light is emitted from the projector and is measured by the receiver. Any obscuring matter such as rain, snow, dust,

fog, haze or smoke reduces the light intensity arriving at the receiver. The resultant intensity measurement is then

converted to an RVR value by the signal data converter. These values are displayed by readout equipment in the

associated air traffic facility and updated approximately once every minute for controller issuance to pilots.

c. The signal data converter receives information on the high intensity runway edge light setting in use (step 3,

4, or 5); transmission values from the transmissometer and the sensing of day or night conditions. From the three

data sources, the system will compute appropriate RVR values.

d. An RVR transmissometer established on a 250 foot baseline provides digital readouts to a minimum of 600

feet, which are displayed in 200 foot increments to 3,000 feet and in 500 foot increments from 3,000 feet to a

maximum value of 6,000 feet.

e. RVR values for Category IIIa operations extend down to 700 feet RVR; however, only 600 and 800 feet

are reportable RVR increments. The 800 RVR reportable value covers a range of 701 feet to 900 feet and is

therefore a valid minimum indication of Category IIIa operations.

f. Approach categories with the corresponding minimum RVR values. (See TBL 7−1−7.)

TBL 7−1−7

Approach Category/Minimum RVR Table

Category Visibility (RVR)

Nonprecision 2,400 feet

Category I 1,800 feet*

Category II 1,000 feet

Category IIIa 700 feet

Category IIIb 150 feet

Category IIIc 0 feet

* 1,400 feet with special equipment and authorization

7−1−42 Meteorology

2/20/25 AIM

g. Ten minute maximum and minimum RVR values for the designated RVR runway are reported in the body

of the aviation weather report when the prevailing visibility is less than one mile and/or the RVR is 6,000 feet

or less. ATCTs report RVR when the prevailing visibility is 1 mile or less and/or the RVR is 6,000 feet or less.

h. Details on the requirements for the operational use of RVR are contained in FAA AC 97−1, Runway Visual

Range (RVR). Pilots are responsible for compliance with minimums prescribed for their class of operations in

the appropriate CFRs and/or operations specifications.

i. RVR values are also measured by forward scatter meters mounted on 14−foot frangible fiberglass poles. A

full RVR system consists of:

1. Forward scatter meter with a transmitter, receiver and associated items.

2. A runway light intensity monitor (RLIM).

3. An ambient light sensor (ALS).

4. A data processor unit (DPU).

5. Controller display (CD).

j. The forward scatter meter is mounted on a 14−foot frangible pole. Infrared light is emitted from the

transmitter and received by the receiver. Any obscuring matter such as rain, snow, dust, fog, haze or smoke

increases the amount of scattered light reaching the receiver. The resulting measurement along with inputs from

the runway light intensity monitor and the ambient light sensor are forwarded to the DPU which calculates the

proper RVR value. The RVR values are displayed locally and remotely on controller displays.

k. The runway light intensity monitors both the runway edge and centerline light step settings (steps 1 through

5). Centerline light step settings are used for CAT IIIb operations. Edge Light step settings are used for CAT I,

II, and IIIa operations.

l. New Generation RVRs can measure and display RVR values down to the lowest limits of Category IIIb

operations (150 feet RVR). RVR values are displayed in 100 feet increments and are reported as follows:

1. 100−feet increments for products below 800 feet.

2. 200−feet increments for products between 800 feet and 3,000 feet.

3. 500−feet increments for products between 3,000 feet and 6,500 feet.

4. 25−meter increments for products below 150 meters.

5. 50−meter increments for products between 150 meters and 800 meters.

6. 100−meter increments for products between 800 meters and 1,200 meters.

7. 200−meter increments for products between 1,200 meters and 2,000 meters.

7−1−14. Reporting of Cloud Heights

a. Ceiling, by definition in the CFRs and as used in aviation weather reports and forecasts, is the height above

ground (or water) level of the lowest layer of clouds or obscuring phenomenon that is reported as “broken,”

“overcast,” or “obscuration,” e.g., an aerodrome forecast (TAF) which reads “BKN030” refers to height above

ground level. An area forecast which reads “BKN030” indicates that the height is above mean sea level.

REFERENCE−

AIM, Para 7−1−28, Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR), defines “broken,” “overcast,” and “obscuration.”

b. Pilots usually report height values above MSL, since they determine heights by the altimeter. This is taken

in account when disseminating and otherwise applying information received from pilots. (“Ceiling” heights are

always above ground level.) In reports disseminated as PIREPs, height references are given the same as received

from pilots, that is, above MSL.

c. In area forecasts or inflight advisories, ceilings are denoted by the contraction “CIG” when used with sky

cover symbols as in “LWRG TO CIG OVC005,” or the contraction “AGL” after, the forecast cloud height value.

Meteorology 7−1−43

AIM 2/20/25

When the cloud base is given in height above MSL, it is so indicated by the contraction “MSL” or “ASL”

following the height value. The heights of clouds tops, freezing level, icing, and turbulence are always given in

heights above ASL or MSL.

7−1−15. Reporting Prevailing Visibility

a. Surface (horizontal) visibility is reported in METAR reports in terms of statute miles and increments

3/ 5/ 3/ 1/ 5/thereof; e.g., 1/16, 1/8, 16, 1/4, 16, 8, 2, 8, 3/4, 7/8, 1, 1 1/8, etc. (Visibility reported by an unaugmented automated

site is reported differently than in a manual report, i.e., ASOS/AWOS: 0, 1/16, 1/8, 1/4, 1/2, 3/4, 1, 1 1/4, 1 1/2, 1 3/4, 2,

2 1/2, 3, 4, 5, etc., AWOS: M1/4, 1/4, 1/2, 3/4, 1, 1 1/4, 1 1/2, 1 3/4, 2, 2 1/2, 3, 4, 5, etc.) Visibility is determined through

the ability to see and identify preselected and prominent objects at a known distance from the usual point of

observation. Visibilities which are determined to be less than 7 miles, identify the obscuring atmospheric

condition; e.g., fog, haze, smoke, etc., or combinations thereof.

b. Prevailing visibility is the greatest visibility equaled or exceeded throughout at least one half of the horizon

circle, not necessarily contiguous. Segments of the horizon circle which may have a significantly different

visibility may be reported in the remarks section of the weather report; i.e., the southeastern quadrant of the

horizon circle may be determined to be 2 miles in mist while the remaining quadrants are determined to be 3 miles

in mist.

c. When the prevailing visibility at the usual point of observation, or at the tower level, is less than 4 miles,

certificated tower personnel will take visibility observations in addition to those taken at the usual point of

observation. The lower of these two values will be used as the prevailing visibility for aircraft operations.

7−1−16. Estimating Intensity of Rain and Ice Pellets

a. Rain

1. Light. From scattered drops that, regardless of duration, do not completely wet an exposed surface up

to a condition where individual drops are easily seen.

2. Moderate. Individual drops are not clearly identifiable; spray is observable just above pavements and

other hard surfaces.

3. Heavy. Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray to height of

several inches is observed over hard surfaces.

b. Ice Pellets

1. Light. Scattered pellets that do not completely cover an exposed surface regardless of duration.

Visibility is not affected.

2. Moderate. Slow accumulation on ground. Visibility reduced by ice pellets to less than 7 statute miles.

3. Heavy. Rapid accumulation on ground. Visibility reduced by ice pellets to less than 3 statute miles.

7−1−17. Estimating Intensity of Snow or Drizzle (Based on Visibility)

a. Light. Visibility more than 1/2 statute mile.

b. Moderate. Visibility from more than 1/4 statute mile to 1/2 statute mile.

c. Heavy. Visibility 1/4 statute mile or less.

7−1−18. Pilot Weather Reports (PIREPs)

a. FAA air traffic facilities are required to solicit PIREPs when the following conditions are reported or

forecast: ceilings at or below 5,000 feet; visibility at or below 5 miles (surface or aloft); thunderstorms and related

phenomena; icing of light degree or greater; turbulence of moderate degree or greater; wind shear and reported

or forecast volcanic ash clouds.

7−1−44 Meteorology

2/20/25 AIM

b. Pilots are urged to cooperate and promptly volunteer reports of these conditions and other atmospheric data

such as: cloud bases, tops and layers; flight visibility; precipitation; visibility restrictions such as haze, smoke

and dust; wind at altitude; and temperature aloft.

c. PIREPs should be given to the ground facility with which communications are established; i.e., FSS,

ARTCC, or terminal ATC. One of the primary duties of the Inflight position is to serve as a collection point for

the exchange of PIREPs with en route aircraft.

d. If pilots are not able to make PIREPs by radio, reporting upon landing of the inflight conditions encountered

to the nearest FSS or Weather Forecast Office will be helpful. Some of the uses made of the reports are:

1. The A TCT uses the reports to expedite the flow of air traffic in the vicinity of the field and for hazardous

weather avoidance procedures.

2. The FSS uses the reports to brief other pilots, to provide inflight advisories, and weather avoidance

information to en route aircraft.

3. The ARTCC uses the reports to expedite the flow of en route traffic, to determine most favorable

altitudes, and to issue hazardous weather information within the center’s area.

4. The NWS uses the reports to verify or amend conditions contained in aviation forecast and advisories.

In some cases, pilot reports of hazardous conditions are the triggering mechanism for the issuance of advisories.

They also use the reports for pilot weather briefings.

5. The NWS, other government organizations, the military, and private industry groups use PIREPs for

research activities in the study of meteorological phenomena.

6. All air traffic facilities and the NWS forward the reports received from pilots into the weather distribution

system to assure the information is made available to all pilots and other interested parties.

e. The FAA, NWS, and other organizations that enter PIREPs into the weather reporting system use the format

listed in TBL 7−1−8. Items 1 through 6 are included in all transmitted PIREPs along with one or more of items

7 through 13. Although the PIREP should be as complete and concise as possible, pilots should not be overly

concerned with strict format or phraseology. The important thing is that the information is relayed so other pilots

may benefit from your observation. If a portion of the report needs clarification, the ground station will request

the information. Completed PIREPs will be transmitted to weather circuits as in the following examples:

EXAMPLE−

1. KCMH UA /OV APE 230010/TM 1516/FL085/TP BE20/SK BKN065/WX FV03SM HZ FU/TA 20/TB LGT

NOTE−

1. One zero miles southwest of Appleton VOR; time 1516 UTC; altitude eight thousand five hundred; aircraft type BE200;

bases of the broken cloud layer is six thousand five hundred; flight visibility 3 miles with haze and smoke; air temperature

20 degrees Celsius; light turbulence.

EXAMPLE−

2. KCRW UV /OV KBKW 360015 −KCRW/TM 1815/FL120//TP BE99/SK IMC/WX RA/TA M08 /WV 290030/TB

LGT−MDT/IC LGT RIME/RM MDT MXD ICG DURC KROA NWBND FL080−100 1750Z

NOTE−

2. From 15 miles north of Beckley VOR to Charleston VOR; time 1815 UTC; altitude 12,000 feet; type aircraft, BE−99; in

clouds; rain; temperature minus 8 Celsius; wind 290 degrees magnetic at 30 knots; light to moderate turbulence; light rime

icing during climb northwestbound from Roanoke, VA, between 8,000 and 10,000 feet at 1750 UTC.

f. For more detailed information on PIREPs, users can refer to the current version of the Aviation Weather

Handbook, FAA−H−8083−28.

Meteorology 7−1−45

AIM 2/20/25

TBL 7−1−8

PIREP Element Code Chart

PIREP ELEMENT PIREP CODE CONTENTS

1. 3−letter station identifier XXX Nearest weather reporting location to the reported phenomenon

2. Report type UA or UUA Routine or Urgent PIREP

3. Location /OV In relation to a VOR

4. Time /TM Coordinated Universal Time

5. Altitude /FL Essential for turbulence and icing reports

6. Type Aircraft /TP Essential for turbulence and icing reports

7. Sky cover /SK Cloud height and coverage (sky clear, few, scattered, broken, or

overcast)

8. Weather /WX Flight visibility, precipitation, restrictions to visibility, etc.

9. Temperature /TA Degrees Celsius

10. Wind /WV Direction in degrees magnetic north and speed in knots

11. Turbulence /TB See AIM paragraph 7−1−21

12. Icing /IC See AIM paragraph 7−1−19

13. Remarks /RM For reporting elements not included or to clarify previously

reported items

7−1−19. PIREPs Relating to Airframe Icing

a. The effects of ice on aircraft are cumulative-thrust is reduced, drag increases, lift lessens, and weight

increases. The results are an increase in stall speed and a deterioration of aircraft performance. In extreme cases,

2 to 3 inches of ice can form on the leading edge of the airfoil in less than 5 minutes. It takes but 1/2 inch of ice

to reduce the lifting power of some aircraft by 50 percent and increases the frictional drag by an equal percentage.

b. A pilot can expect icing when flying in visible precipitation, such as rain or cloud droplets, and the

temperature is between +02 and −10 degrees Celsius. When icing is detected, a pilot should do one of two things,

particularly if the aircraft is not equipped with deicing equipment; get out of the area of precipitation; or go to

an altitude where the temperature is above freezing. This “warmer” altitude may not always be a lower altitude.

Proper preflight action includes obtaining information on the freezing level and the above freezing levels in

precipitation areas. Report icing to ATC, and if operating IFR, request new routing or altitude if icing will be a

hazard. Be sure to give the type of aircraft to ATC when reporting icing. The following describes how to report

icing conditions.

1. Trace. Ice becomes noticeable. The rate of accumulation is slightly greater than the rate of sublimation.

A representative accretion rate for reference purposes is less than ¼ inch (6 mm) per hour on the outer wing. The

pilot should consider exiting the icing conditions before they become worse.

2. Light. The rate of ice accumulation requires occasional cycling of manual deicing systems to minimize

ice accretions on the airframe. A representative accretion rate for reference purposes is ¼ inch to 1 inch (0.6 to

2.5 cm) per hour on the unprotected part of the outer wing. The pilot should consider exiting the icing condition.

3. Moderate. The rate of ice accumulation requires frequent cycling of manual deicing systems to

minimize ice accretions on the airframe. A representative accretion rate for reference purposes is 1 to 3 inches

(2.5 to 7.5 cm) per hour on the unprotected part of the outer wing. The pilot should consider exiting the icing

condition as soon as possible.

4. Severe. The rate of ice accumulation is such that ice protection systems fail to remove the accumulation

of ice and ice accumulates in locations not normally prone to icing, such as areas aft of protected surfaces and

any other areas identified by the manufacturer. A representative accretion rate for reference purposes is more than

3 inches (7.5 cm) per hour on the unprotected part of the outer wing. By regulation, immediate exit is required.

7−1−46 Meteorology

2/20/25 AIM

NOTE−

Severe icing is aircraft dependent, as are the other categories of icing intensity. Severe icing may occur at any

ice accumulation rate when the icing rate or ice accumulations exceed the tolerance of the aircraft.

EXAMPLE−

Pilot report: give aircraft identification, location, time (UTC), intensity of type, altitude/FL, aircraft type, indicated air

speed (IAS), and outside air temperature (OAT).

NOTE−

1. Rime ice. Rough, milky, opaque ice formed by the instantaneous freezing of small supercooled water droplets.

2. Clear ice. A glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled water droplets.

3. The OAT should be requested by the FSS or ATC if not included in the PIREP .

7−1−20. Definitions of Inflight Icing Terms

See TBL 7−1−9, Icing Types, and TBL 7−1−10, Icing Conditions.

TBL 7−1−9

Icing Types

Clear Ice See Glaze Ice.

Glaze Ice Ice, sometimes clear and smooth, but usually containing some air pockets, which results in a

lumpy translucent appearance. Glaze ice results from supercooled drops/droplets striking a

surface but not freezing rapidly on contact. Glaze ice is denser, harder, and sometimes more

transparent than rime ice. Factors, which favor glaze formation, are those that favor slow

dissipation of the heat of fusion (i.e., slight supercooling and rapid accretion). With larger

accretions, the ice shape typically includes “horns” protruding from unprotected leading edge

surfaces. It is the ice shape, rather than the clarity or color of the ice, which is most likely to

be accurately assessed from the cockpit. The terms “clear” and “glaze” have been used for

essentially the same type of ice accretion, although some reserve “clear” for thinner accretions

which lack horns and conform to the airfoil.

Intercycle Ice Ice which accumulates on a protected surface between actuation cycles of a deicing system.

Known or Observed or

Detected Ice Accretion

Actual ice observed visually to be on the aircraft by the flight crew or identified by on−board

sensors.

Mixed Ice Simultaneous appearance or a combination of rime and glaze ice characteristics. Since the

clarity, color, and shape of the ice will be a mixture of rime and glaze characteristics, accurate

identification of mixed ice from the cockpit may be difficult.

Residual Ice Ice which remains on a protected surface immediately after the actuation of a deicing system.

Rime Ice A rough, milky, opaque ice formed by the rapid freezing of supercooled drops/droplets after

they strike the aircraft. The rapid freezing results in air being trapped, giving the ice its opaque

appearance and making it porous and brittle. Rime ice typically accretes along the stagnation

line of an airfoil and is more regular in shape and conformal to the airfoil than glaze ice. It is

the ice shape, rather than the clarity or color of the ice, which is most likely to be accurately

assessed from the cockpit.

Runback Ice Ice which forms from the freezing or refreezing of water leaving protected surfaces and

running back to unprotected surfaces.

Note−

Ice types are difficult for the pilot to discern and have uncertain effects on an airplane in flight. Ice type definitions will

be included in the AIM for use in the “Remarks” section of the PIREP and for use in forecasting.

Meteorology 7−1−47

AIM 2/20/25

TBL 7−1−10

Icing Conditions

Appendix C Icing Conditions Appendix C (14 CFR, part 25 and 29) is the certification icing condition standard

for approving ice protection provisions on aircraft. The conditions are specified in

terms of altitude, temperature, liquid water content (LWC), representative droplet

size (mean effective drop diameter [MED]), and cloud horizontal extent.

Forecast Icing Conditions Environmental conditions expected by a National Weather Service or an

FAA−approved weather provider to be conducive to the formation of inflight icing

on aircraft.

Freezing Drizzle (FZDZ) Drizzle is precipitation at ground level or aloft in the form of liquid water drops

which have diameters less than 0.5 mm and greater than 0.05 mm. Freezing drizzle

is drizzle that exists at air temperatures less than 0 C (supercooled), remains in

liquid form, and freezes upon contact with objects on the surface or airborne.

Freezing Precipitation Freezing precipitation is freezing rain or freezing drizzle falling through or outside

of visible cloud.

Freezing Rain (FZRA) Rain is precipitation at ground level or aloft in the form of liquid water drops which

have diameters greater than 0.5 mm. Freezing rain is rain that exists at air

temperatures less than 0C (supercooled), remains in liquid form, and freezes upon

contact with objects on the ground or in the air.

Icing in Cloud Icing occurring within visible cloud. Cloud droplets (diameter < 0.05 mm) will be

present; freezing drizzle and/or freezing rain may or may not be present.

Icing in Precipitation Icing occurring from an encounter with freezing precipitation, that is, supercooled

drops with diameters exceeding 0.05 mm, within or outside of visible cloud.

Known Icing Conditions Atmospheric conditions in which the formation of ice is observed or detected in

flight.

Note−

Because of the variability in space and time of atmospheric conditions, the existence

of a report of observed icing does not assure the presence or intensity of icing

conditions at a later time, nor can a report of no icing assure the absence of icing

conditions at a later time.

Potential Icing Conditions Atmospheric icing conditions that are typically defined by airframe manufacturers

relative to temperature and visible moisture that may result in aircraft ice accretion

on the ground or in flight. The potential icing conditions are typically defined in the

Airplane Flight Manual or in the Airplane Operation Manual.

Supercooled Drizzle Drops

(SCDD)

Synonymous with freezing drizzle aloft.

Supercooled Drops or /Droplets Water drops/droplets which remain unfrozen at temperatures below 0 C.

Supercooled drops are found in clouds, freezing drizzle, and freezing rain in the

atmosphere. These drops may impinge and freeze after contact on aircraft surfaces.

Supercooled Large Drops (SLD) Liquid droplets with diameters greater than 0.05 mm at temperatures less than

0C, i.e., freezing rain or freezing drizzle.

7−1−21. PIREPs Relating to Turbulence

a. When encountering turbulence, pilots are urgently requested to report such conditions to ATC as soon as

practicable. PIREPs relating to turbulence should state:

1. Aircraft location.

2. Time of occurrence in UTC.

3. Turbulence intensity.

4. Whether the turbulence occurred in or near clouds.

5. Aircraft altitude or flight level.

7−1−48 Meteorology

2/20/25 AIM

6. Type of aircraft.

7. Duration of turbulence.

EXAMPLE−

1. Over Omaha, 1232Z, moderate turbulence in clouds at Flight Level three one zero, Boeing 707.

2. From five zero miles south of Albuquerque to three zero miles north of Phoenix, 1250Z, occasional moderate chop at

Flight Level three three zero, DC8.

b. Duration and classification of intensity should be made using TBL 7−1−11.

TBL 7−1−11

Turbulence Reporting Criteria Table

Intensity Aircraft Reaction Reaction Inside Aircraft Reporting Term−Definition

Light Turbulence that momentarily causes

slight, erratic changes in altitude and/or

attitude (pitch, roll, yaw). Report as

Light Turbulence; 1

or

Turbulence that causes slight, rapid and

somewhat rhythmic bumpiness without

appreciable changes in altitude or

attitude. Report as Light Chop.

Occupants may feel a slight strain

against seat belts or shoulder straps.

Unsecured objects may be displaced

slightly. Food service may be con -

ducted and little or no difficulty is

encountered in walking.

Occasional−Less than 1/3 of the time.

Intermittent−1/3 to 2/3.

Continuous−More than 2/3.

Moderate Turbulence that is similar to Light

Turbulence but of greater intensity.

Changes in altitude and/or attitude occur

but the aircraft remains in positive

control at all times. It usually causes

variations in indicated airspeed. Report

as Moderate Turbulence; 1

or

Turbulence that is similar to Light Chop

but of greater intensity. It causes rapid

bumps or jolts without appreciable

changes in aircraft altitude or attitude.

Report as Moderate Chop.1

Occupants feel definite strains against

seat belts or shoulder straps. Unse -

cured objects are dislodged. Food

service and walking are difficult.

NOTE

1. Pilots should report location(s),

time (UTC), intensity, whether in or

near clouds, altitude, type of aircraft

and, when applicable, duration of

turbulence.

2. Duration may be based on time

between two locations or over a single

location. All locations should be

readily identifiable.

EXAMPLES:

a. Over Omaha. 1232Z, Moderate

Turbulence, in cloud, Flight

Level 310, B707.

b. From 50 miles south of Albuquer-

que to 30 miles north of Phoenix,

1210Z to 1250Z, occasional Moderate

Chop, Flight Level 330, DC8.

Severe Turbulence that causes large, abrupt

changes in altitude and/or attitude. It

usually causes large variations in

indicated airspeed. Aircraft may be

momentarily out of control. Report as

Severe Turbulence. 1

Occupants are forced violently against

seat belts or shoulder straps. Unse -

cured objects are tossed about. Food

Service and walking are impossible.

Extreme Turbulence in which the aircraft is

violently tossed about and is practically

impossible to control. It may cause

structural damage. Report as Extreme

Turbulence. 1

1 High level turbulence (normally above 15,000 feet ASL) not associated with cumuliform cloudiness, including thunderstorms,

should be reported as CAT (clear air turbulence) preceded by the appropriate intensity, or light or moderate chop.

7−1−22. Wind Shear PIREPs

a. Because unexpected changes in wind speed and direction can be hazardous to aircraft operations at low

altitudes on approach to and departing from airports, pilots are urged to promptly volunteer reports to controllers

of wind shear conditions they encounter. An advance warning of this information will assist other pilots in

avoiding or coping with a wind shear on approach or departure.

b. When describing conditions, use of the terms “negative” or “positive” wind shear should be avoided.

PIREPs of “negative wind shear on final,” intended to describe loss of airspeed and lift, have been interpreted

to mean that no wind shear was encountered. The recommended method for wind shear reporting is to state the

loss or gain of airspeed and the altitudes at which it was encountered.

Meteorology 7−1−49

AIM 2/20/25

EXAMPLE−

1. Denver Tower , Cessna 1234 encountered wind shear, loss of 20 knots at 400.

2. Tulsa Tower, American 721 encountered wind shear on final, gained 25 knots between 600 and 400 feet followed by loss

of 40 knots between 400 feet and surface.

1. Pilots who are not able to report wind shear in these specific terms are encouraged to make reports in

terms of the effect upon their aircraft.

EXAMPLE−

Miami Tower , Gulfstream 403 Charlie encountered an abrupt wind shear at 800 feet on final, max thrust required.

2. Pilots using Inertial Navigation Systems (INSs) should report the wind and altitude both above and below

the shear level.

c. Wind Shear Escape

1. Pilots should report to ATC when they are performing a wind shear escape maneuver. This report should

be made as soon as practicable, but not until aircraft safety and control is assured, which may not be satisfied

until the aircraft is clear of the wind shear or microburst. ATC should provide safety alerts and traffic advisories,

as appropriate.

EXAMPLE−

“Denver Tower , United 1154, wind shear escape.”

2. Once the pilot initiates a wind shear escape maneuver, ATC is not responsible for providing approved

separation between the aircraft and any other aircraft, airspace, terrain, or obstacle until the pilot reports that the

escape procedure is complete and approved separation has been re−established. Pilots should advise ATC that

they are resuming the previously assigned clearance or should request an alternate clearance.

EXAMPLE−

“Denver Tower , United ll54, wind shear escape complete, resuming last assigned heading/(name) DP/clearance.”

or

“Denver Tower , United ll54, wind shear escape complete, request further instructions.”

7−1−23. Clear Air Turbulence (CAT) PIREPs

CAT has become a very serious operational factor to flight operations at all levels and especially to jet traffic

flying in excess of 15,000 feet. The best available information on this phenomenon must come from pilots via

the PIREP reporting procedures. All pilots encountering CAT conditions are urgently requested to report time,

location, and intensity (light, moderate, severe, or extreme) of the element to the FAA facility with which they

are maintaining radio contact. If time and conditions permit, elements should be reported according to the

standards for other PIREPs and position reports.

REFERENCE−

AIM, Para 7−1−21, PIREPs Relating to Turbulence.

7−1−24. Microbursts

a. Relatively recent meteorological studies have c onfirmed the existence of microburst phenomenon.

Microbursts are small scale intense downdrafts which, on reaching the surface, spread outward in all directions

from the downdraft center. This causes the presence of both vertical and horizontal wind shears that can be

extremely hazardous to all types and categories of aircraft, especially at low altitudes. Due to their small size,

short life span, and the fact that they can occur over areas without surface precipitation, microbursts are not easily

detectable using conventional weather radar or wind shear alert systems.

b. Parent clouds producing microburst activity can be any of the low or middle layer convective cloud types.

Note, however, that microbursts commonly occur within the heavy rain portion of thunderstorms, and in much

weaker, benign appearing convective cells that have little or no precipitation reaching the ground.

7−1−50 Meteorology

e T

2/20/25 AIM

FIG 7−1−13

Evolution of a Microburst

Meteorology

HEIGHT (feet)

10,00010,000

5,0005,000

HE

IG

HT

(f

et)

WIND SPEEDWIND SPEED

10-20 knots10-20 knots

> 20 knots> 20 knots

T-5 Min T-2 Min T T + 5 MinT + 5 MinT-5 Min T-2 Min T + 10 MinT + 10 Min

0 1 2

SCALE (miles)SCALE (miles)

Vertical cross section of the evolution of a microburst wind field. T is the time of initial divergence at

the surface. The shading refers to the vector wind speeds. Figure adapted from Wilson et al., 1984,

Microburst Wind Structure and Evaluation of Doppler Radar for Wind Shear Detection, DOT/FAA

Report No. DOT/FAA/PM-84/29, National Technical Information Service, Springfield, VA 37 pp.

c. The life cycle of a microburst as it descends in a convective rain shaft is seen in FIG 7−1−13. An important

consideration for pilots is the fact that the microburst intensifies for about 5 minutes after it strikes the ground.

d. Characteristics of microbursts include:

1. Size. The microburst downdraft is typically less than 1 mile in diameter as it descends from the cloud

base to about 1,000−3,000 feet above the ground. In the transition zone near the ground, the downdraft changes

to a horizontal outflow that can extend to approximately 2 1/2 miles in diameter.

2. Intensity. The downdrafts can be as strong as 6,000 feet per minute. Horizontal winds near the surface

can be as strong as 45 knots resulting in a 90 knot shear (headwind to tailwind change for a traversing aircraft)

across the microburst. These strong horizontal winds occur within a few hundred feet of the ground.

3. Visual Signs. Microbursts can be found almost anywhere that there is convective activity. They may be

embedded in heavy rain associated with a thunderstorm or in light rain in benign appearing virga. When there

is little or no precipitation at the surface accompanying the microburst, a ring of blowing dust may be the only

visual clue of its existence.

4. Duration. An individual microburst will seldom last longer than 15 minutes from the time it strikes the

ground until dissipation. The horizontal winds continue to increase during the first 5 minutes with the maximum

intensity winds lasting approximately 2−4 minutes. Sometimes microbursts are concentrated into a line structure,

and under these conditions, activity may continue for as long as an hour. Once microburst activity starts, multiple

microbursts in the same general area are not uncommon and should be expected.

7−1−51

AIM 2/20/25

FIG 7−1−14

Microburst Encounter During Takeoff

A microburst encounter during takeoff. The airplane first encounters a headwind and experiences increasing

performance (1), this is followed in short succession by a decreasing headwind component (2), a downdraft

(3), and finally a strong tailwind (4), where 2 through 5 all result in decreasing performance of the airplane.

Position (5) represents an extreme situation just prior to impact. Figure courtesy of Walter Frost, FWG

Associates, Inc., Tullahoma, Tennessee.

e. Microburst wind shear may create a severe hazard for aircraft within 1,000 feet of the ground, particularly

during the approach to landing and landing and take-off phases. The impact of a microburst on aircraft which

have the unfortunate experience of penetrating one is characterized in FIG 7−1−14. The aircraft may encounter

a headwind (performance increasing) followed by a downdraft and tailwind (both performance decreasing),

possibly resulting in terrain impact.

Meteorology7−1−52

2/20/25 AIM

FIG 7−1−15

NAS Wind Shear Product Systems

f. Detection of Microbursts, Wind Shear and Gust Fronts.

1. FAA’s Integrated Wind Shear Detection Plan.

(a) The FAA currently employs an integrated plan for wind shear detection that will significantly improve

both the safety and capacity of the majority of the airports currently served by the air carriers. This plan integrates

several programs, such as the Integrated Terminal Weather System (ITWS), Terminal Doppler Weather Radar

(TDWR), Weather Systems Processor (WSP), and Low Level Wind Shear Alert Systems (LLWAS) into a single

strategic concept that significantly improves the aviation weather information in the terminal area. (See

FIG 7−1−15.)

(b) The wind shear/microburst information and warnings are displayed on the ribbon display terminals

(RBDT) located in the tower cabs. They are identical (and standardized) in the LLWAS, TDWR and WSP

systems, and so designed that the controller does not need to interpret the data, but simply read the displayed

information to the pilot. The RBDTs are constantly monitored by the controller to ensure the rapid and timely

dissemination of any hazardous event(s) to the pilot.

Meteorology 7−1−53

AIM 2/20/25

FIG 7−1−16

LLWAS Siting Criteria

(c) The early detection of a wind shear/micro−burst event, and the subsequent warning(s) issued to an

aircraft on approach or departure, will alert the pilot/crew to the potential of, and to be prepared for, a situation

that could become very dangerous! Without these warnings, the aircraft may NOT be able to climb out of, or

safely transition, the event, resulting in a catastrophe. The air carriers, working with the FAA, have developed

specialized training programs using their simulators to train and prepare their pilots on the demanding aircraft

procedures required to escape these very dangerous wind shear and/or microburst encounters.

2. Low Level Wind Shear Alert System (LLWAS).

(a) The LLWAS provides wind data and software processes to detect the presence of hazardous wind

shear and microbursts in the vicinity of an airport. Wind sensors, mounted on poles sometimes as high as 150

feet, are (ideally) located 2,000 − 3,500 feet, but not more than 5,000 feet, from the centerline of the runway. (See

FIG 7−1−16.)

7−1−54 Meteorology

2/20/25 AIM

FIG 7−1−17

Warning Boxes

(b) LLWAS was fielded in 1988 at 110 airports across the nation. Many of these systems have been

replaced by new TDWR and WSP technology. While all legacy LLWAS systems will eventually be phased out,

39 airports will be upgraded to LLWAS−NE (Network Expansion) system. The new LLWAS−NE systems not

only provide the controller with wind shear warnings and alerts, including wind shear/microburst detection at

the airport wind sensor location, but also provide the location of the hazards relative to the airport runway(s).

It also has the flexibility and capability to grow with the airport as new runways are built. As many as 32 sensors,

strategically located around the airport and in relationship to its runway configuration, can be accommodated

by the LLWAS−NE network.

3. Terminal Doppler Weather Radar (TDWR).

(a) TDWRs have been deployed at 45 locations across the U.S. Optimum locations for TDWRs are 8 to

12 miles off of the airport proper, and designed to look at the airspace around and over the airport to detect

microbursts, gust fronts, wind shifts, and precipitation intensities. TDWR products advise the controller of wind

shear and microburst events impacting all runways and the areas 1/2 mile on either side of the extended centerline

of the runways out to 3 miles on final approach and 2 miles out on departure. (FIG 7−1−17 is a theoretical view

of the warning boxes, including the runway, that the software uses in determining the location(s) of wind shear

or microbursts). These warnings are displayed (as depicted in the examples in subparagraph 5) on the RBDT.

(b) It is very important to understand what TDWR does NOT DO:

(1) It DOES NOT warn of wind shear outside of the alert boxes (on the arrival and departure ends of

the runways);

(2) It DOES NOT detect wind shear that is NOT a microburst or a gust front;

(3) It DOES NOT detect gusty or cross wind conditions; and

(4) It DOES NOT detect turbulence.

However, research and development is continuing on these systems. Future improvements may include such

areas as storm motion (movement), improved gust front detection, storm growth and decay, microburst

prediction, and turbulence detection.

Meteorology 7−1−55

AIM 2/20/25

(c) TDWR also provides a geographical situation display (GSD) for supervisors and traffic management

specialists for planning purposes. The GSD displays (in color) 6 levels of weather (precipitation), gust fronts and

predicted storm movement(s). This data is used by the tower supervisor(s), traffic management specialists and

controllers to plan for runway changes and arrival/departure route changes in order to both reduce aircraft delays

and increase airport capacity.

4. Weather Systems Processor (WSP).

(a) The WSP provides the controller, supervisor, traffic management specialist, and ultimately the pilot,

with the same products as the terminal doppler weather radar (TDWR) at a fraction of the cost of a TDWR. This

is accomplished by utilizing new technologies to access the weather channel capabilities of the existing ASR−9

radar located on or near the airport, thus eliminating the requirements for a separate radar location, land

acquisition, support facilities and the associated communication landlines and expenses.

(b) The WSP utilizes the same RBDT display as the TDWR and LLWAS, and, just like TDWR, also has

a GSD for planning purposes by supervisors, traffic management specialists and controllers. The WSP GSD

emulates the TDWR display, i.e., it also depicts 6 levels of precipitation, gust fronts and predicted storm

movement, and like the TDWR GSD, is used to plan for runway changes and arrival/departure route changes

in order to reduce aircraft delays and to increase airport capacity.

(c) This system is installed at 34 airports across the nation, substantially increasing the safety of flying.

5. Operational aspects of LLWAS, TDWR and WSP.

To demonstrate how this data is used by both the controller and the pilot, 3 ribbon display examples and their

explanations are presented:

(a) MICROBURST ALERTS

EXAMPLE−

This is what the controller sees on his/her ribbon display in the tower cab.

27A MBA 35K− 2MF 250 20

NOTE−

(See FIG 7−1−18 to see how the TDWR/WSP determines the microburst location).

This is what the controller will say when issuing the alert.

PHRASEOLOGY−

RUNWAY 27 ARRIVAL, MICROBURST ALERT, 35 KT LOSS 2 MILE FINAL, THRESHOLD WIND 250 AT 20.

In plain language, the controller is telling the pilot that on approach to runway 27, there is a microburst alert on

the approach lane to the runway, and to anticipate or expect a 35 knot loss of airspeed at approximately 2 miles

out on final approach (where it will first encounter the phenomena). With that information, the aircrew is

forewarned, and should be prepared to apply wind shear/microburst escape procedures should they decide to

continue the approach. Additionally, the surface winds at the airport for landing runway 27 are reported as

250 degrees at 20 knots.

NOTE−

Threshold wind is at pilot’ s request or as deemed appropriate by the controller .

REFERENCE−

F AA Order JO 7110.65, Para 3−1−8b2(a), Air Traffic Control, Low Level Wind Shear/Microburst Advisories.

Meteorology7−1−56

2/20/25 AIM

FIG 7−1−18

Microburst Alert

(b) WIND SHEAR ALERTS

EXAMPLE−

This is what the controller sees on his/her ribbon display in the tower cab.

27A WSA 20K− 3MF 200 15

NOTE−

(See FIG 7−1−19 to see how the TDWR/WSP determines the wind shear location).

This is what the controller will say when issuing the alert.

PHRASEOLOGY−

RUNWAY 27 ARRIVAL, WIND SHEAR ALERT, 20 KT LOSS 3 MILE FINAL, THRESHOLD WIND 200 AT 15.

In plain language, the controller is advising the aircraft arriving on runway 27 that at about 3 miles out they can

expect to encounter a wind shear condition that will decrease their airspeed by 20 knots and possibly encounter

turbulence. Additionally, the airport surface winds for landing runway 27 are reported as 200 degrees at 15 knots.

NOTE−

Threshold wind is at pilot’ s request or as deemed appropriate by the controller .

REFERENCE−

F AA Order JO 7110.65, Para 3−1−8, Low Level Wind Shear/Microburst Advisories, Subpara b2(a).

Meteorology 7−1−57

AIM 2/20/25

FIG 7−1−19

Weak Microburst Alert

7−1−58 Meteorology

2/20/25 AIM

FIG 7−1−20

Gust Front Alert

(c) MULTIPLE WIND SHEAR ALERTS

EXAMPLE−

This is what the controller sees on his/her ribbon display in the tower cab.

27A WSA 20K+ RWY 250 20

27D WSA 20K+ RWY 250 20

NOTE−

(See FIG 7−1−20 to see how the TDWR/WSP determines the gust front/wind shear location.)

This is what the controller will say when issuing the alert.

PHRASEOLOGY−

MULTIPLE WIND SHEAR ALERTS. RUNWAY 27 ARRIVAL, WIND SHEAR ALERT, 20 KT GAIN ON RUNWAY; RUNWAY

27 DEP ARTURE, WIND SHEAR ALERT, 20 KT GAIN ON RUNWAY, WIND 250 AT 20.

EXAMPLE−

In this example, the controller is advising arriving and departing aircraft that they could encounter a wind shear condition

right on the runway due to a gust front (significant change of wind direction) with the possibility of a 20 knot gain in airspeed

associated with the gust front. Additionally, the airport surface winds (for the runway in use) are reported as 250 degrees

at 20 knots.

REFERENCE−

F AA Order 7110.65, Para 3−1−8, Low Level Wind Shear/Microburst Advisories, Subpara b2(d).

6. The Terminal Weather Information for Pilots System (TWIP).

(a) With the increase in the quantity and quality of terminal weather information available through

TDWR, the next step is to provide this information directly to pilots rather than relying on voice communications

Meteorology 7−1−59

AIM 2/20/25

from A TC. The NAS has long been in need of a means of delivering terminal weather information to the cockpit

more efficiently in terms of both speed and accuracy to enhance pilot awareness of weather hazards and reduce

air traffic controller workload. With the TWIP capability, terminal weather information, both alphanumerically

and graphically, is now available directly to the cockpit for 46 airports in the U.S. NAS. (See FIG 7−1−21.)

FIG 7−1−21

TWIP Image of Convective Weather at MCO International

(b) TWIP products are generated using weather data from the TDWR or the Integrated Terminal W eather

System (ITWS). These products can then be accessed by pilots using the Aircraft Communications Addressing

and Reporting System (ACARS) data link services. Airline dispatchers can also access this database and send

messages to specific aircraft whenever wind shear activity begins or ends at an airport.

(c) TWIP products include descriptions and character graphics of microburst alerts, wind shear alerts,

significant precipitation, convective activity within 30 NM surrounding the terminal area, and expected weather

that will impact airport operations. During inclement weather, i.e., whenever a predetermined level of

precipitation or wind shear is detected within 15 miles of the terminal area, TWIP products are updated once each

minute for text messages and once every five minutes for character graphic messages. During good weather

(below the predetermined precipitation or wind shear parameters) each message is updated every 10 minutes.

These products are intended to improve the situational awareness of the pilot/flight crew, and to aid in flight

planning prior to arriving or departing the terminal area. It is important to understand that, in the context of TWIP,

the predetermined levels for inclement versus good weather has nothing to do with the criteria for

VFR/MVFR/IFR/LIFR; it only deals with precipitation, wind shears and microbursts.

7−1−60 Meteorology

AIM2/20/257/9/26 AIM

TBL 7−1−12

TWIP−Equipped Airports

Airport Identifier

Andrews AFB, MD KADW

Hartsfield−Jackson Atlanta Intl Airport KATL

Nashville Intl Airport KBNA

Logan Intl Airport KBOS

Baltimore/Washington Intl Airport KBWI

Hopkins Intl Airport KCLE

Charlotte/Douglas Intl Airport KCLT

Port Columbus Intl Airport KCMH

Cincinnati/Northern Kentucky Intl Airport KCVG

Dallas Love Field Airport KDAL

James M. Cox Intl Airport KDAY

Ronald Reagan Washington National Air-

port

KDCA

Denver Intl Airport KDEN

Dallas−Fort Worth Intl Airport KDFW

Detroit Metro Wayne County Airport KDTW

Newark Liberty Intl Airport KEWR

Fort Lauderdale−Hollywood Intl Airport KFLL

William P. Hobby Airport KHOU

Washington Dulles Intl Airport KIAD

George Bush Intercontinental Airport KIAH

Wichita Mid−Continent Airport KICT

Indianapolis Intl Airport KIND

John F. Kennedy Intl Airport KJFK

Airport Identifier

Harry Reid Intl Airport KLAS

LaGuardia Airport KLGA

Kansas City Intl Airport KMCI

Orlando Intl Airport KMCO

Midway Intl Airport KMDW

Memphis Intl Airport KMEM

Miami Intl Airport KMIA

General Mitchell Intl Airport KMKE

Minneapolis St. Paul Intl Airport KMSP

Louis Armstrong New Orleans Intl Air-

port

KMSY

Will Rogers World Airport KOKC

O’Hare Intl Airport KORD

President Donald J. Trump Intl KDJT

Philadelphia Intl Airport KPHL

Phoenix Sky Harbor Intl Airport KPHX

Pittsburgh Intl Airport KPIT

Raleigh−Durham Intl Airport KRDU

Louisville Intl Airport KSDF

Salt Lake City Intl Airport KSLC

Lambert−St. Louis Intl Airport KSTL

Tampa Intl Airport KTPA

Tulsa Intl Airport KTUL

Luis Munoz Marin Intl Airport TJSJ

7−1−25. PIREPs Relating to Volcanic Ash Activity

a. V olcanic eruptions which send ash into the upper atmosphere occur somewhere around the world several

times each year. Flying into a volcanic ash cloud can be extremely dangerous. At least two B747s have lost all

power in all four engines after such an encounter. Regardless of the type aircraft, some damage is almost certain

to ensue after an encounter with a volcanic ash cloud. Additionally, studies have shown that volcanic eruptions

are the only significant source of large quantities of sulphur dioxide (SO2) gas at jet-cruising altitudes. Therefore,

the detection and subsequent reporting of SO 2 is of significant importance. Although SO 2 is colorless, its

presence in the atmosphere should be suspected when a sulphur-like or rotten egg odor is present throughout the

cabin.

b. While some volcanoes in the U.S. are monitored, many in remote areas are not. These unmonitored

volcanoes may erupt without prior warning to the aviation community. A pilot observing a volcanic eruption who

has not had previous notification of it may be the only witness to the eruption. Pilots are strongly encouraged

to transmit a PIREP regarding volcanic eruptions and any observed volcanic ash clouds or detection of sulphur

dioxide (SO2) gas associated with volcanic activity.

Meteorology 7−1−61

AIM 2/20/25

c. Pilots should submit PIREPs regarding volcanic activity using the V olcanic Activity Reporting (V AR) form

as illustrated in Appendix 2. If a V AR form is not immediately available, relay enough information to identify

the position and type of volcanic activity.

d. Pilots should verbally transmit the data required in items 1 through 8 of the V AR as soon as possible. The

data required in items 9 through 16 of the V AR should be relayed after landing if possible.

7−1−26. Thunderstorms

a. Turbulence, hail, rain, snow, lightning, sustained updrafts and downdrafts, icing conditions−all are present

in thunderstorms. While there is some evidence that maximum turbulence exists at the middle level of a

thunderstorm, recent studies show little variation of turbulence intensity with altitude.

b. There is no useful correlation between the external visual appearance of thunderstorms and the severity or

amount of turbulence or hail within them. The visible thunderstorm cloud is only a portion of a turbulent system

whose updrafts and downdrafts often extend far beyond the visible storm cloud. Severe turbulence can be

expected up to 20 miles from severe thunderstorms. This distance decreases to about 10 miles in less severe

storms.

c. Weather radar, airborne or ground based, will normally reflect the areas of moderate to heavy precipitation

(radar does not detect turbulence). The frequency and severity of turbulence generally increases with the radar

reflectivity which is closely associated with the areas of highest liquid water content of the storm. NO FLIGHT

PATH THROUGH AN AREA OF STRONG OR VERY STRONG RADAR ECHOES SEPARATED BY 20−30

MILES OR LESS MAY BE CONSIDERED FREE OF SEVERE TURBULENCE.

d. Turbulence beneath a thunderstorm should not be minimized. This is especially true when the relative

humidity is low in any layer between the surface and 15,000 feet. Then the lower altitudes may be characterized

by strong out flowing winds and severe turbulence.

e. The probability of lightning strikes occurring to aircraft is greatest when operating at altitudes where

temperatures are between minus 5 degrees Celsius and plus 5 degrees Celsius. Lightning can strike aircraft flying

in the clear in the vicinity of a thunderstorm.

f. METAR reports do not include a descriptor for severe thunderstorms. However, by understanding severe

thunderstorm criteria, i.e., 50 knot winds or 3/4 inch hail, the information is available in the report to know that

one is occurring.

g. Current weather radar systems are able to objectively determine precipitation intensity. These precipitation

intensity areas are described as “light,” “moderate,” “heavy,” and “extreme.”

REFERENCE−

Pilot/Controller Glossary− Precipitation Radar Weather Descriptions

EXAMPLE−

1. Alert provided by an ATC facility to an aircraft:

(aircraft identification) EXTREME precipitation between ten o’clock and two o’clock, one five miles. Precipitation area is

two five miles in diameter.

2. Alert provided by an FSS:

(aircraft identification) EXTREME precipitation two zero miles west of Atlanta V−O−R, two five miles wide, moving east

at two zero knots, tops flight level three niner zero.

7−1−27. Thunderstorm Flying

a. Thunderstorm Avoidance. Never regard any thunderstorm lightly, even when radar echoes are of light

intensity. Avoiding thunderstorms is the best policy. Following are some Do’s and Don’ts of thunderstorm

avoidance:

1. Don’t land or takeoff in the face of an approaching thunderstorm. A sudden gust front of low level

turbulence could cause loss of control.

7−1−62 Meteorology

2/20/25 AIM

2. Don’t attempt to fly under a thunderstorm even if you can see through to the other side. Turbulence and

wind shear under the storm could be hazardous.

3. Don’t attempt to fly under the anvil of a thunderstorm. There is a potential for severe and extreme clear

air turbulence.

4. Don’t fly without airborne radar into a cloud mass containing scattered embedded thunderstorms.

Scattered thunderstorms not embedded usually can be visually circumnavigated.

5. Don’t trust the visual appearance to be a reliable indicator of the turbulence inside a thunderstorm.

6. Don’t assume that ATC will offer radar navigation guidance or deviations around thunderstorms.

7. Don’t use data-linked weather next generation weather radar (NEXRAD) mosaic imagery as the sole

means for negotiating a path through a thunderstorm area (tactical maneuvering).

8. Do remember that the data−linked NEXRAD mosaic imagery shows where the weather was, not where

the weather is. The weather conditions depicted may be 15 to 20 minutes older than indicated on the display.

9. Do listen to chatter on the A TC frequency for Pilot Weather Reports (PIREP) and other aircraft requesting

to deviate or divert.

10. Do ask ATC for radar navigation guidance or to approve deviations around thunderstorms, if needed.

11. Do use data-linked weather NEXRAD mosaic imagery (for example, Flight Information

Service-Broadcast (FIS-B)) for route selection to avoid thunderstorms entirely (strategic maneuvering).

12. Do advise ATC, when switched to another controller, that you are deviating for thunderstorms before

accepting to rejoin the original route.

13. Do ensure that after an authorized weather deviation, before accepting to rejoin the original route, that

the route of flight is clear of thunderstorms.

14. Do avoid by at least 20 miles any thunderstorm identified as severe or giving an intense radar echo. This

is especially true under the anvil of a large cumulonimbus.

15. Do circumnavigate the entire area if the area has 6/10 thunderstorm coverage.

16. Do remember that vivid and frequent lightning indicates the probability of a severe thunderstorm.

17. Do regard as extremely hazardous any thunderstorm with tops 35,000 feet or higher whether the top is

visually sighted or determined by radar.

18. Do give a PIREP for the flight conditions.

19. Do divert and wait out the thunderstorms on the ground if unable to navigate around an area of

thunderstorms.

20. Do contact Flight Service for assistance in avoiding thunderstorms. Flight Service specialists have

NEXRAD mosaic radar imagery and NEXRAD single site radar with unique features such as base and composite

reflectivity, echo tops, and V AD wind profiles.

b. If you cannot avoid penetrating a thunderstorm, following are some Do’s before entering the storm:

1. Tighten your safety belt, put on your shoulder harness (if installed), if and secure all loose objects.

2. Plan and hold the course to take the aircraft through the storm in a minimum time.

3. To avoid the most critical icing, establish a penetration altitude below the freezing level or above the level

of -15ºC.

4. Verify that pitot heat is on and turn on carburetor heat or jet engine anti-ice. Icing can be rapid at any

altitude and cause almost instantaneous power failure and/or loss of airspeed indication.

5. Establish power settings for turbulence penetration airspeed recommended in the aircraft manual.

Meteorology 7−1−63

AIM 2/20/25

6. Turn up cockpit lights to highest intensity to lessen temporary blindness from lightning.

7. If using automatic pilot, disengage Altitude Hold Mode and Speed Hold Mode. The automatic altitude

and speed controls will increase maneuvers of the aircraft thus increasing structural stress.

8. If using airborne radar, tilt the antenna up and down occasionally. This will permit the detection of other

thunderstorm activity at altitudes other than the one being flown.

c. Following are some Do’s and Don’ts during the thunderstorm penetration:

1. Do keep your eyes on your instruments. Looking outside the cockpit can increase danger of temporary

blindness from lightning.

2. Don’t change power settings; maintain settings for the recommended turbulence penetration airspeed.

3. Do maintain constant attitude. Allow the altitude and airspeed to fluctuate.

4. Don’t turn back once you are in the thunderstorm. A straight course through the storm most likely will

get the aircraft out of the hazards most quickly. In addition, turning maneuvers increase stress on the aircraft.

7−1−64 Meteorology

2/20/25 AIM

7−1−28. Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR)

FIG 7−1−22

Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR) (Front)

Key to Aerodrome Forecast (TAF) and Aviation

Routine Weather Report (METAR) (Front)

TAF KPIT 091730Z 0918/1024 15005KT 5SM HZ FEW020 WS010/31022KT

FM091930 30015G25KT 3SM SHRA OVC015

TEMPO 0920/0922 1/2SM +TSRA OVC008CB

FM100100 27008KT 5SM SHRA BKN020 OVC040

PROB30 1004/1007 1SM -RA BR

FM101015 18005KT 6SM -SHRA OVC020

BECMG 1013/1015 P6SM NSW SKC

NOTE: Users are cautioned to confirm DATE and TIME of the TAF. For example FM100000 is

0000Z on the 10th. Do not confuse with 1000Z!

METAR KPIT 091955Z COR 22015G25KT 3/4SM R28L/2600FT TSRA OVC010CB 18/16 A2992 RMK

SLP045 T01820159

Forecast Explanation Report

TAF Message type: TAF-routine or TAF AMD-amended forecast, METAR-

hourly, SPECI-special or TESTM-non-commissioned ASOS report

METAR

KPIT ICAO location indicator KPIT

091730Z Issuance time: ALL times in UTC “Z ”, 2-digit date, 4-digit time 091955Z

0918/1024 Valid period, either 24 hours or 30 hours. The first two digits of EACH

four digit number indicate the date of the valid period, the final two di­

gits indicate the time (valid from 18Z on the 9th to 24Z on the 10th).

In U.S. METAR: CORrected ob; or AUTOmated ob for automated re­

port with no human intervention; omitted when observer logs on.

COR

15005KT Wind: 3 digit true-north direction, nearest 10 degrees (or VaRiaBle);

next 2-3 digits for speed and unit, KT (KMH or MPS); as needed, Gust

and maximum speed; 00000KT for calm; for METAR, if direction varies

60 degrees or more, Variability appended, e.g., 180V260

22015G25KT

5SM Prevailing visibility; in U.S., Statute Miles & fractions; above 6 miles in

TAF Plus6SM. (Or, 4-digit minimum visibility in meters and as re­

quired, lowest value with direction)

¾SM

Runway Visual Range: R; 2-digit runway designator Left, Center, or

Right as needed; “/”, Minus or Plus in U.S., 4-digit value, FeeT in U.S.,

(usually meters elsewhere); 4-digit value Variability 4-digit value (and

tendency Down, Up or No change)

R28L/2600FT

HZ Significant present, forecast and recent weather: see table (on back) TSRA

FEW020 Cloud amount, height and type: Sky Clear 0/8, FEW >0/8-2/8, ScaTtered

3/8-4/8, BroKeN 5/8-7/8, OverCast 8/8; 3-digit height in hundreds of ft;

Towering Cumulus or CumulonimBus in METAR; in TAF, only CB.

Vertical Visibility for obscured sky and height “VV004”. More than 1

layer may be reported or forecast. In automated METAR reports only,

CleaR for “clear below 12,000 feet”

OVC 010CB

Temperature: degrees Celsius; first 2 digits, temperature “/” last 2 digits,

dew-point temperature; Minus for below zero, e.g., M06

18/16

Altimeter setting: indicator and 4 digits; in U.S., A-inches and hun­

dredths; (Q-hectoPascals, e.g., Q1013)

WS010/31022KT In U.S. TAF, non-convective low-level (≤2,000 ft) Wind Shear; 3-digit

height (hundreds of ft); “/”; 3-digit wind direction and 2-3 digit wind

speed above the indicated height, and unit, KT

Meteorology 7−1−65

AIM 2/20/25

FIG 7−1−23

Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR) (Back)

Key to Aerodrome Forecast (TAF) and Aviation

Routine Weather Report (METAR) (Back)

In METAR, ReMarK indicator & remarks. For example: Sea- Level

Pressure in hectoPascals & tenths, as shown: 1004.5 hPa; Temp/dew-

point in tenths °C, as shown: temp. 18.2°C, dew-point 15.9°C

RMK SLP045

FM091930 FroM: changes are expected at: 2-digit date, 2-digit hour, and 2-digit

minute beginning time: indicates significant change. Each FM starts on a

new line, indented 5 spaces

TEMPO

0920/0922

TEMPOrary: changes expected for <1 hour and in total, < half of the

period between the 2-digit date and 2-digit hour beginning, and 2-digit

date and 2-digit hour ending time

PROB30

1004/1007

PROBability and 2-digit percent (30 or 40): probable condition in the

period between the 2-digit date & 2-digit hour beginning time, and the

2-digit date and 2-digit hour ending time

BECMG

1013/1015

BECoMinG: change expected in the period between the 2-digit date and

2-digit hour beginning time, and the 2-digit date and 2-digit hour ending

time

Table of Significant Present, Forecast and Recent Weather - Grouped in categories and

used in the order listed below; or as needed in TAF, No Significant Weather.

Qualifiers

Intensity or Proximity

“-” = Light No sign = Moderate “+” = Heavy

“VC” = Vicinity, but not at aerodrome. In the US METAR, 5 to 10 SM from the point of observation. In the US

TAF, 5 to 10 SM from the center of the runway complex. Elsewhere, within 8000m.

Descriptor

BC – Patches BL – Blowing DR – Drifting FZ – Freezing

MI – Shallow PR – Partial SH – Showers TS – Thunderstorm

Weather Phenomena

Precipitation

DZ – Drizzle GR – Hail GS – Small Hail/Snow Pellets

IC – Ice Crystals PL – Ice Pellets RA – Rain SG – Snow Grains

SN – Snow UP – Unknown Precipitation in automated observations

Obscuration

BR – Mist (≥5/8SM) DU – Widespread Dust FG – Fog (<5/8SM) FU – Smoke

HZ – Haze PY – Spray SA – Sand VA – Volcanic Ash

Other

DS – Dust Storm FC – Funnel Cloud +FC – Tornado or Waterspout

PO – Well developed dust or sand whirls SQ – Squall SS – Sandstorm

- Explanations in parentheses “()” indicate different worldwide practices.

- Ceiling is not specified; defined as the lowest broken or overcast layer, or the vertical visibility.

- NWS TAFs exclude BECMG groups and temperature forecasts, NWS TAFS do not use PROB in the first 9

hours of a TAF; NWS METARs exclude trend forecasts. US Military TAFs include Turbulence and Icing groups.

7−1−66 Meteorology

2/20/25 AIM

7−1−29. International Civil Aviation Organization (ICAO) Weather Formats

The U.S. uses the ICAO world standard for aviation weather reporting and forecasting. The World

Meteorological Organization’s (WMO) publication No. 782 “Aerodrome Reports and Forecasts” contains the

base METAR and TAF code as adopted by the WMO member countries.

a. Although the METAR code is adopted worldwide, each country is allowed to make modifications or

exceptions to the code for use in their particular country , e.g., the U.S. will continue to use statute miles for

visibility, feet for RVR values, knots for wind speed, and inches of mercury for altimetry. However, temperature

and dew point will be reported in degrees Celsius. The U.S reports prevailing visibility rather than lowest sector

visibility. The elements in the body of a METAR report are separated with a space. The only exceptions are RVR,

temperature, and dew point which are separated with a solidus (/). When an element does not occur, or cannot

be observed, the preceding space and that element are omitted from that particular report. A METAR report

contains the following sequence of elements in the following order:

1. Type of report.

2. ICAO Station Identifier.

3. Date and time of report.

4. Modifier (as required).

5. Wind.

6. Visibility.

7. Runway Visual Range (RVR).

8. Weather phenomena.

9. Sky conditions.

10. Temperature/dew point group.

11. Altimeter.

12. Remarks (RMK).

b. The following paragraphs describe the elements in a METAR report.

1. Type of report. There are two types of report:

(a) Aviation Routine Weather Report (METAR); and

(b) Nonroutine (Special) Aviation Weather Report (SPECI).

The type of report (METAR or SPECI) will always appear as the lead element of the report.

2. ICAO Station Identifier. The METAR code uses ICAO 4−letter station identifiers. In the contiguous

48 States, the 3−letter domestic station identifier is prefixed with a “K;” i.e., the domestic identifier for Seattle

is SEA while the ICAO identifier is KSEA. Elsewhere, the first two letters of the ICAO identifier indicate what

region of the world and country (or state) the station is in. For Alaska, all station identifiers start with “PA;” for

Hawaii, all station identifiers start with “PH.” Canadian station identifiers start with “CU,” “CW,” “CY ,” and

“CZ.” Mexican station identifiers start with “MM.” The identifier for the western Caribbean is “M” followed

by the individual country’s letter; i.e., Cuba is “MU;” Dominican Republic “MD;” the Bahamas “MY .” The

identifier for the eastern Caribbean is “T” followed by the individual country’s letter; i.e., Puerto Rico is “TJ.”

For a complete worldwide listing see ICAO Document 7910, Location Indicators.

3. Date and Time of Report. The date and time the observation is taken are transmitted as a six −digit

date/time group appended with Z to denote Coordinated Universal Time (UTC). The first two digits are the date

followed with two digits for hour and two digits for minutes.

EXAMPLE−

172345Z (the 17th day of the month at 2345Z)

Meteorology 7−1−67

AIM 2/20/25

4. Modifier (As Required). “AUTO” identifies a METAR/SPECI report as an automated weather report

with no human intervention. If “AUTO” is shown in the body of the report, the type of sensor equipment used

at the station will be encoded in the remarks section of the report. The absence of “AUTO” indicates that a report

was made manually by an observer or that an automated report had human augmentation/backup. The modifier

“COR” indicates a corrected report that is sent out to replace an earlier report with an error.

NOTE−

There are two types of automated stations, AO1 for au tomated weather reporting stations without a precipitation

discriminator , and AO2 for automated stations with a precipitation discriminator. (A precipitation discriminator can

determine the difference between liquid and frozen/freezing precipitation). This information appears in the remarks section

of an automated report.

5. Wind. The wind is reported as a five digit group (six digits if speed is over 99 knots). The first three digits

are the direction the wind is blowing from, in tens of degrees referenced to true north, or “VRB” if the direction

is variable. The next two digits is the wind speed in knots, or if over 99 knots, the next three digits. If the wind

is gusty, it is reported as a “G” after the speed followed by the highest gust reported. The abbreviation “KT” is

appended to denote the use of knots for wind speed.

EXAMPLE−

13008KT − wind from 130 degrees at 8 knots

08032G45KT − wind from 080 degrees at 32 knots with gusts to 45 knots

VRB04KT − wind variable in direction at 4 knots

00000KT − wind calm

210103G130KT − wind from 210 degrees at 103 knots with gusts to 130 knots

If the wind direction is variable by 60 degrees or more and the speed is greater than 6 knots, a variable group consisting

of the extremes of the wind direction separated by a “v” will follow the prevailing wind group.

32012G22KT 280V350

(a) Peak Wind. Whenever the peak wind exceeds 25 knots “PK WND” will be included in Remarks,

e.g., PK WND 28045/1955 “Peak wind two eight zero at four five occurred at one niner five five.” If the hour

can be inferred from the report time, only the minutes will be appended, e.g., PK WND 34050/38 “Peak wind

three four zero at five zero occurred at three eight past the hour.”

(b) Wind shift. Whenever a wind shift occurs, “WSHFT” will be included in remarks followed by the

time the wind shift began, e.g., WSHFT 30 FROPA “Wind shift at three zero due to frontal passage.”

6. Visibility. Prevailing visibility is reported in statute miles with “SM” appended to it.

EXAMPLE−

7SM − seven statute miles

15SM − fifteen statute miles

1/2SM − one−half statute mile

(a) Tower/surface visibility. If either visibility (tower or surface) is below four statute miles, the lesser

of the two will be reported in the body of the report; the greater will be reported in remarks.

(b) Automated visibility. ASOS/AWOS visibility stations will show visibility 10 or greater than 10

miles as “10SM.” AWOS visibility stations will show visibility less than 1/4 statute mile as “M 1/4SM” and

visibility 10 or greater than 10 miles as “10SM.”

NOTE−

Automated sites that are augmented by human observer to meet service level requirements can report 0, 1/16 SM, and 1/8

SM visibility increments.

(c) Variable visibility. Variable visibility is shown in remarks (when rapid increase or decrease by 1/2

statute mile or more and the average prevailing visibility is less than three miles) e.g., VIS 1V2 “visibility variable

between one and two.”

(d) Sector visibility. Sector visibility is shown in remarks when it differs from the prevailing visibility,

and either the prevailing or sector visibility is less than three miles.

7−1−68 Meteorology

2/20/25 AIM

EXAMPLE−

VIS N2 − visibility north two

7. Runway Visual Range (When Reported). “R” identifies the group followed by the runway heading

(and parallel runway designator, if needed) “/” and the visual range in feet (meters in other countries) followed

with “FT” (feet is not spoken).

(a) Variability Values. When RVR varies (by more than on reportable value), the lowest and highest

values are shown with “V” between them.

(b) Maximum/Minimum Range. “P” indicates an observed RVR is above the maximum value for this

system (spoken as “more than”). “M” indicates an observed RVR is below the minimum value which can be

determined by the system (spoken as “less than”).

EXAMPLE−

R32L/1200FT − runway three two left R−V−R one thousand two hundred.

R27R/M1000V4000FT − runway two seven right R−V−R variable from less than one thousand to four thousand.

8. Weather Phenomena. The weather as reported in the METAR code represents a significant change in

the way weather is currently reported. In METAR, weather is reported in the format:

Intensity/Proximity/Descriptor/Precipitation/Obstruction to visibility/Other

NOTE−

The “/” above and in the following descriptions (except as the separator between the temperature and dew point) are for

separation purposes in this publication and do not appear in the actual METARs.

(a) Intensity applies only to the first type of precipitation reported. A “ −” denotes light, no symbol

denotes moderate, and a “+” denotes heavy.

(b) Proximity applies to and reported only for weather occurring in the vicinity of the airport (between

5 and 10 miles of the point(s) of observation). It is denoted by the letters “VC.” (Intensity and “VC” will not

appear together in the weather group).

(c) Descriptor. These eight descriptors apply to the precipitation or obstructions to visibility:

TS ........... thunderstorm

DR ........... low drifting

SH ........... showers

MI ........... shallow

FZ ........... freezing

BC ........... patches

BL ........... blowing

PR ........... partial

NOTE−

Although “TS” and “SH” are used with precipitation and may be preceded with an intensity symbol, the intensity still applies

to the precipitation, not the descriptor.

(d) Precipitation. There are nine types of precipitation in the METAR code:

RA .......... rain

DZ .......... drizzle

SN .......... snow

GR .......... hail (1/4” or greater)

GS .......... small hail/snow pellets

PL .......... ice pellets

SG .......... snow grains

IC ........... ice crystals (diamond dust)

UP .......... unknown precipitation (automated stations only)

Meteorology 7−1−69

AIM 2/20/25

(e) Obstructions to visibility. There are eight types of obscuration phenomena in the METAR code

(obscurations are any phenomena in the atmosphere, other than precipitation, that reduce horizontal visibility):

FG .......... fog (vsby less than 5/8 mile)

HZ .......... haze

FU .......... smoke

PY .......... spray

BR .......... mist (vsby 5/8 − 6 miles)

SA .......... sand

DU .......... dust

VA .......... volcanic ash

NOTE−

Fog (FG) is observed or forecast only when the visibility is less than five−eighths of mile, otherwise mist (BR) is observed

or forecast.

(f) Other. There are five categories of other weather phenomena which are reported when they occur:

SQ ........... squall

SS ........... sandstorm

DS ........... duststorm

PO .......... dust/sand whirls

FC ........... funnel cloud

+FC ......... tornado/waterspout

Examples:

TSRA ......... thunderstorm with moderate rain

+SN .......... heavy snow

−RA FG ....... light rain and fog

BRHZ ........ mist and haze (visibility 5/8 mile or greater)

FZDZ ......... freezing drizzle

VCSH ........ rain shower in the vicinity

+SHRASNPL .. heavy rain showers, snow, ice pellets (intensity indicator refers to the predominant rain)

9. Sky Condition. The sky condition as reported in METAR represents a significant change from the way

sky condition is currently reported. In METAR, sky condition is reported in the format:

Amount/Height/(Type) or Indefinite Ceiling/Height

(a) Amount. The amount of sky cover is reported in eighths of sky cover, using the contractions:

SKC ......... clear (no clouds)

FEW ........ >0 to 2/8

SCT ......... scattered (3/8s to 4/8s of clouds)

BKN ......... broken (5/8s to 7/8s of clouds)

OVC ......... overcast (8/8s clouds)

CB .......... Cumulonimbus when present

TCU ......... Towering cumulus when present

NOTE−

1. “SKC” will be reported at manual stations. “CLR” will be used at automated stations when no clouds below 12,000 feet

are reported.

2. A ceiling layer is not designated in the METAR code. For aviation purposes, the ceiling is the lowest broken or overcast

layer , or vertical visibility into an obscuration. Also there is no provision for reporting thin layers in the METAR code. When

clouds are thin, that layer must be reported as if it were opaque.

(b) Height. Cloud bases are reported with three digits in hundreds of feet above ground level (AGL).

(Clouds above 12,000 feet cannot be reported by an automated station).

7−1−70 Meteorology

2/20/25 AIM

(c) (Type). If Towering Cumulus Clouds (TCU) or Cumulonimbus Clouds (CB) are present, they are

reported after the height which represents their base.

EXAMPLE−

(Reported as) SCT025TCU BKN080 BKN250 (spoken as) “TWO THOUSAND FIVE HUNDRED SCATTERED

TOWERING CUMULUS, CEILING EIGHT THOUSAND BROKEN, TWO FIVE THOUSAND BROKEN.”

(Reported as) SCT008 OVC012CB (spoken as) “EIGHT HUNDRED SCATTERED CEILING ONE THOUSAND TWO

HUNDRED OVERCAST CUMULONIMBUS CLOUDS.”

(d) Vertical Visibility (indefinite ceiling height). The height into an indefinite ceiling is preceded by

“VV” and followed by three digits indicating the vertical visibility in hundreds of feet. This layer indicates total

obscuration.

EXAMPLE−

1/8 SM FG VV006 − visibility one eighth, fog, indefinite ceiling six hundred.

(e) Obscurations are reported when the sky is partially obscured by a ground−based phenomena by

indicating the amount of obscuration as FEW, SCT, BKN followed by three zeros (000). In remarks, the

obscuring phenomenon precedes the amount of obscuration and three zeros.

EXAMPLE−

BKN000 (in body) ........ “sky partially obscured”

FU BKN000 (in remarks) ... “smoke obscuring five− to seven−eighths of the sky”

(f) When sky conditions include a layer aloft, other than clouds, such as smoke or haze the type of

phenomena, sky cover and height are shown in remarks.

EXAMPLE−

BKN020 (in body) ........ “ceiling two thousand broken”

RMK FU BKN020 ........ “broken layer of smoke aloft, based at two thousand”

(g) Variable ceiling. When a ceiling is below three thousand and is variable, the remark “CIG” will be

shown followed with the lowest and highest ceiling heights separated by a “V .”

EXAMPLE−

CIG 005V010 ............ “ceiling variable between five hundred and one thousand”

(h) Second site sensor. When an automated station uses meteorological discontinuity sensors, remarks

will be shown to identify site specific sky conditions which differ and are lower than conditions reported in the

body.

EXAMPLE−

CIG 020 RY11 ........... “ceiling two thousand at runway one one”

(i) Variable cloud layer. When a layer is varying in sky cover, remarks will show the variability range.

If there is more than one cloud layer, the variable layer will be identified by including the layer height.

EXAMPLE−

SCT V BKN ............. “scattered layer variable to broken”

BKN025 V OVC ......... “broken layer at two thousand five hundred variable to overcast”

(j) Significant clouds. When significant clouds are observed, they are shown in remarks, along with the

specified information as shown below:

(1) Cumulonimbus (CB), or Cumulonimbus Mammatus (CBMAM), distance (if known), direction

from the station, and direction of movement, if known. If the clouds are beyond 10 miles from the airport, DSNT

will indicate distance.

EXAMPLE−

CB W MOV E ....... “cumulonimbus west moving east”

CBMAM DSNT S .... “cumulonimbus mammatus distant south”

(2) Towering Cumulus (TCU), location, (if known), or direction from the station.

EXAMPLE−

TCU OHD ......... “towering cumulus overhead”

TCU W ............ “towering cumulus west”

Meteorology 7−1−71

AIM 2/20/25

(3) Altocumulus Castellanus (ACC), Stratocumulus Standing Lenticular (SCSL), Altocumulus

Standing Lenticular (ACSL), Cirrocumulus Standing Lenticular (CCSL) or rotor clouds, describing the clouds

(if needed) and the direction from the station.

EXAMPLE−

ACC W ............. “altocumulus castellanus west”

ACSL SW−S ......... “standing lenticular altocumulus southwest through south”

APRNT ROTOR CLD S “apparent rotor cloud south”

CCSL OVR MT E ..... “standing lenticular cirrocumulus over the mountains east”

10. Temperature/Dew Point. Temperature and dew point are reported in two, two-digit groups in degrees

Celsius, separated by a solidus (“/”). Temperatures below zero are prefixed with an “M.” If the temperature is

available but the dew point is missing, the temperature is shown followed by a solidus. If the temperature is

missing, the group is omitted from the report.

EXAMPLE−

15/08 .............. “temperature one five, dew point 8”

00/M02 ............ “temperature zero, dew point minus 2”

M05/ ............... “temperature minus five, dew point missing”

11. Altimeter. Altimeter settings are reported in a four-digit format in inches of mercury prefixed with an

“A” to denote the units of pressure.

EXAMPLE−

A2995 − “Altimeter two niner niner five”

12. Remarks. Remarks will be included in all observations, when appropriate. The contraction “RMK”

denotes the start of the remarks section of a METAR report.

Except for precipitation, phenomena located within 5 statute miles of the point of observation will be reported

as at the station. Phenomena between 5 and 10 statute miles will be reported in the vicinity, “VC.” Precipitation

not occurring at the point of observation but within 10 statute miles is also reported as in the vicinity, “VC.”

Phenomena beyond 10 statute miles will be shown as distant, “DSNT.” Distances are in statute miles except for

automated lightning remarks which are in nautical miles. Movement of clouds or weather will be indicated by

the direction toward which the phenomena is moving.

(a) There are two categories of remarks:

(1) Automated, manual, and plain language.

(2) Additive and automated maintenance data.

(b) Automated, Manual, and Plain Language. This group of remarks may be generated from either

manual or automated weather reporting stations and generally elaborate on parameters reported in the body of

the report. (Plain language remarks are only provided by manual stations).

(1) V olcanic eruptions.

(2) Tornado, Funnel Cloud, Waterspout.

(3) Station Type (AO1 or AO2).

(4) PK WND.

(5) WSHFT (FROPA).

(6) TWR VIS or SFC VIS.

(7) VRB VIS.

(8) Sector VIS.

(9) VIS @ 2nd Site.

(10) Lightning. When lightning is observed at a manual location, the frequency and location is

reported.

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When cloud−to−ground lightning is detected by an automated lightning detection system, such as ALDARS:

[a] Within 5 nautical miles (NM) of the Airport Reference Point (ARP), it will be reported as “TS”

in the body of the report with no remark;

[b] Between 5 and 10 NM of the ARP, it will be reported as “VCTS” in the body of the report with

no remark;

[c] Beyond 10 but less than 30 NM of the ARP, it will be reported in remarks as “DSNT” followed

by the direction from the ARP.

EXAMPLE−

LTG DSNT W or LTG DSNT ALQDS

(11) Beginning/Ending of Precipitation/TSTMS.

(12) TSTM Location MVMT.

(13) Hailstone Size (GR).

(14) Virga.

(15) VRB CIG (height).

(16) Obscuration.

(17) VRB Sky Condition.

(18) Significant Cloud Types.

(19) Ceiling Height 2nd Location.

(20) PRESFR PRESRR.

(21) Sea−Level Pressure.

(22) ACFT Mishap (not transmitted).

(23) NOSPECI.

(24) SNINCR.

(25) Other SIG Info.

(c) Additive and Automated Maintenance Data.

(1) Hourly Precipitation.

(2) 3− and 6−Hour Precipitation Amount.

(3) 24−Hour Precipitation.

(4) Snow Depth on Ground.

(5) Water Equivalent of Snow.

(6) Cloud Type.

(7) Duration of Sunshine.

(8) Hourly Temperature/Dew Point (Tenths).

(9) 6−Hour Maximum Temperature.

(10) 6−Hour Minimum Temperature.

(11) 24−Hour Maximum/Minimum Temperature.

(12) Pressure Tendency.

Meteorology 7−1−73

AIM 2/20/25

(13) Sensor Status.

PWINO

FZRANO

TSNO

RVRNO

PNO

VISNO

Examples of METAR reports and explanation:

METAR KBNA 281250Z 33018KT 290V360 1/2SM R31/2700FT SN BLSN FG VV008 00/M03 A2991 RMK

RAE42SNB42

METAR ...... aviation routine weather report

KBNA ........ Nashville, TN

281250Z ...... date 28th, time 1250 UTC

(no modifier) .. This is a manually generated report, due to the absence of “AUTO” and “AO1 or AO2”

in remarks

33018KT ...... wind three three zero at one eight

290V360 ...... wind variable between two nine zero and three six zero

1/2SM ........ visibility one half

R31/2700FT ... Runway three one RVR two thousand seven hundred

SN ........... moderate snow

BLSN FG ..... visibility obscured by blowing snow and fog

VV008 ........ indefinite ceiling eight hundred

00/M03 ....... temperature zero, dew point minus three

A2991 ........ altimeter two niner niner one

RMK ......... remarks

RAE42 ....... rain ended at four two

SNB42 ........ snow began at four two

METAR KSFO 041453Z AUTO VRB02KT 3SM BR CLR 15/12 A3012 RMK AO2

METAR ...... aviation routine weather report

KSFO ........ San Francisco, CA

041453Z ...... date 4th, time 1453 UTC

AUTO ....... fully automated; no human intervention

VRB02KT .... wind variable at two

3SM ......... visibility three

BR .......... visibility obscured by mist

CLR ......... no clouds below one two thousand

15/12 ......... temperature one five, dew point one two

A3012 ........ altimeter three zero one two

RMK ........ remarks

AO2 ......... this automated station has a weather discriminator (for precipitation)

SPECI KCVG 152224Z 28024G36KT 3/4SM +TSRA BKN008 OVC020CB 28/23 A3000 RMK TSRAB24 TS

W MOV E

SPECI ....... (nonroutine) aviation special weather report

KCVG ....... Cincinnati, OH

152228Z ...... date 15th, time 2228 UTC

(no modifier) .. This is a manually generated report due to the absence of “AUTO” and “AO1 or AO2”

in remarks

7−1−74 Meteorology

2/20/25 AIM

28024G36KT .. wind two eight zero at two four gusts three six

3/4SM ........ visibility three fourths

+TSRA ....... thunderstorms, heavy rain

BKN008 ceiling eight hundred broken

OVC020CB ... two thousand overcast cumulonimbus clouds

28/23 ......... temperature two eight, dew point two three

A3000 ........ altimeter three zero zero zero

RMK ........ remarks

TSRAB24 ..... thunderstorm and rain began at two four

TS W MOV E thunderstorm west moving east

c. Aerodrome Forecast (TAF). A concise statement of the expected meteorological conditions at an airport

during a specified period. At most locations, TAFs have a 24 hour forecast period. However, TAFs for some

locations have a 30 hour forecast period. These forecast periods may be shorter in the case of an amended TAF.

TAFs use the same codes as METAR weather reports. They are scheduled four times daily for 24−hour periods

beginning at 0000Z, 0600Z, 1200Z, and 1800Z.

Forecast times in the TAF are depicted in two ways. The first is a 6−digit number to indicate a specific point in

time, consisting of a two−digit date, two−digit hour, and two−digit minute (such as issuance time or FM). The

second is a pair of four−digit numbers separated by a “/” to indicate a beginning and end for a period of time.

In this case, each four−digit pair consists of a two−digit date and a two−digit hour.

TAFs are issued in the following format:

TYPE OF REPORT/ICAO STATION IDENTIFIER/DATE AND TIME OF ORIGIN/V ALID PERIOD DATE

AND TIME/FORECAST METEOROLOGICAL CONDITIONS

NOTE−

The “/” above and in the following descriptions are for separation purposes in this publication and do not appear in the

actual TAFs.

TAF KORD 051130Z 0512/0618 14008KT 5SM BR BKN030

TEMPO 0513/0516 1 1/2SM BR

FM051600 16010KT P6SM SKC

FM052300 20013G20KT 4SM SHRA OVC020

PROB40 0600/0606 2SM TSRA OVC008CB

BECMG 0606/0608 21015KT P6SM NSW SCT040

TAF format observed in the above example:

TAF = type of report

KORD = ICAO station identifier

051130Z = date and time of origin (issuance time)

0512/0618 = valid period date and times

14008KT 5SM BR BKN030 = forecast meteorological conditions

Explanation of TAF elements:

1. Type of Report. There are two types of TAF issuances, a routine forecast issuance (TAF) and an

amended forecast (TAF AMD). An amended TAF is issued when the current TAF no longer adequately describes

the on-going weather or the forecaster feels the TAF is not representative of the current or expected weather.

Corrected (COR) or delayed (RTD) TAFs are identified only in the communications header which precedes the

actual forecasts.

2. ICAO Station Identifier. The TAF code uses ICAO 4−letter location identifiers as described in the

METAR section.

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AIM 2/20/25

3. Date and Time of Origin. This element is the date and time the forecast is actually prepared. The format

is a two−digit date and four−digit time followed, without a space, by the letter “Z.”

4. Valid Period Date and Time. The UTC valid period of the forecast consists of two four −digit sets,

separated by a “/”. The first four−digit set is a two−digit date followed by the two−digit beginning hour, and the

second four−digit set is a two−digit date followed by the two−digit ending hour. Although most airports have a

24−hour TAF, a select number of airports have a 30−hour TAF. In the case of an amended forecast, or a forecast

which is corrected or delayed, the valid period may be for less than 24 hours. Where an airport or terminal

operates on a part −time basis (less than 24 hours/day), the TAFs issued for those locations will have the

abbreviated statement “AMD NOT SKED” added to the end of the forecasts. The time observations are

scheduled to end and/or resume will be indicated by expanding the AMD NOT SKED statement. Expanded

statements will include:

(a) Observation ending time (AFT DDHHmm; for example, AFT 120200)

(b) Scheduled observations resumption time (TIL DDHHmm; for example, TIL 171200Z) or

(c) Period of observation unavailability (DDHH/DDHH); for example, 2502/2512).

5. Forecast Meteorological Conditions. This is the body of the TAF. The basic format is:

WIND/VISIBILITY/WEATHER/SKY CONDITION/OPTIONAL DATA (WIND SHEAR)

The wind, visibility, and sky condition elements are always included in the initial time group of the forecast.

Weather is included only if significant to aviation. If a significant, lasting change in any of the elements is

expected during the valid period, a new time period with the changes is included. It should be noted that with

the exception of a “FM” group the new time period will include only those elements which are expected to

change, i.e., if a lowering of the visibility is expected but the wind is expected to remain the same, the new time

period reflecting the lower visibility would not include a forecast wind. The forecast wind would remain the same

as in the previous time period. Any temporary conditions expected during a specific time period are included

with that time period. The following describes the elements in the above format.

(a) Wind. This five (or six) digit group includes the expected wind direction (first 3 digits) and speed

(last 2 digits or 3 digits if 100 knots or greater). The contraction “KT” follows to denote the units of wind speed.

Wind gusts are noted by the letter “G” appended to the wind speed followed by the highest expected gust. A

variable wind direction is noted by “VRB” where the three digit direction usually appears. A calm wind (3 knots

or less) is forecast as “00000KT.”

EXAMPLE−

18010KT ..... wind one eight zero at one zero (wind is blowing from 180).

35012G20KT .. wind three five zero at one two gust two zero.

(b) Visibility. The expected prevailing visibility up to and including 6 miles is forecast in statute miles,

including fractions of miles, followed by “SM” to note the units of measure. Expected visibilities greater than

6 miles are forecast as P6SM (plus six statute miles).

EXAMPLE−

1/2SM − visibility one−half

4SM − visibility four

P6SM − visibility more than six

(c) Weather Phenomena. The expected weather phenomena is coded in TAF reports using the same

format, qualifiers, and phenomena contractions as METAR reports (except UP). Obscurations to vision will be

forecast whenever the prevailing visibility is forecast to be 6 statute miles or less. If no significant weather is

expected to occur during a specific time period in the forecast, the weather phenomena group is omitted for that

time period. If, after a time period in which significant weather phenomena has been forecast, a change to a

forecast of no significant weather phenomena occurs, the contraction NSW (No Significant Weather) will appear

as the weather group in the new time period. (NSW is included only in TEMPO groups).

7−1−76 Meteorology

2/20/25 AIM

NOTE−

It is very important that pilots understand that NSW only refers to weather phenomena, i.e., rain, snow, drizzle, etc. Omitted

conditions, such as sky conditions, visibility, winds, etc., are carried over from the previous time group.

(d) Sky Condition. TAF sky condition forecasts use the METAR format described in the METAR

section. Cumulonimbus clouds (CB) are the only cloud type forecast in TAFs. When clear skies are forecast, the

contraction “SKC” will always be used. The contraction “CLR” is never used in the TAF. When the sky is

obscured due to a surface −based phenomenon, vertical visibility (VV) into the obscuration is forecast. The

format for vertical visibility is “VV” followed by a three−digit height in hundreds of feet.

NOTE−

As in METAR, ceiling layers are not designated in the TAF code. For aviation purposes, the ceiling is the lowest broken or

overcast layer or vertical visibility into a complete obscuration.

SKC .............. “sky clear”

SCT005 BKN025CB . “five hundred scattered, ceiling two thousand five hundred broken cumulonimbus

clouds”

VV008 ............ “indefinite ceiling eight hundred”

(e) Optional Data (Wind Shear). Wind shear is the forecast of nonconvective low level winds (up to

2,000 feet). The forecast includes the letters “WS” followed by the height of the wind shear, the wind direction

and wind speed at the indicated height and the ending letters “KT” (knots). Height is given in hundreds of feet

(AGL) up to and including 2,000 feet. Wind shear is encoded with the contraction “WS,” followed by a

three−digit height, slant character “/,” and winds at the height indicated in the same format as surface winds. The

wind shear element is omitted if not expected to occur.

WS010/18040KT − “LOW LEVEL WIND SHEAR AT ONE THOUSAND, WIND ONE EIGHT ZERO AT

FOUR ZERO”

d. Probability Forecast. The probability or chance of thunderstorms or other precipitation events occurring,

along with associated weather conditions (wind, visibility, and sky conditions). The PROB30 group is used when

the occurrence of thunderstorms or precipitation is 30−39% and the PROB40 group is used when the occurrence

of thunderstorms or precipitation is 40−49%. This is followed by two four−digit groups separated by a “/”, giving

the beginning date and hour, and the ending date and hour of the time period during which the thunderstorms

or precipitation are expected.

NOTE−

NWS does not use PROB 40 in the TAF . However U.S. Military generated TAFS may include PROB40. PROB30 will not

be shown during the first nine hours of a NWS forecast.

EXAMPLE−

PROB40 2221/2302 1/2SM +TSRA “chance between 2100Z and 0200Z of visibility one−half statute mile in thunderstorms

and heavy rain.”

PROB30 3010/3014 1SM RASN . “chance between 1000Z and 1400Z of visibility one statute mile in mixed rain and

snow.”

e. Forecast Change Indicators. The following change indicators are used when either a rapid, gradual, or

temporary change is expected in some or all of the forecast meteorological conditions. Each change indicator

marks a time group within the TAF report.

1. From (FM) group. The FM group is used when a rapid change, usually occurring in less than one hour,

in prevailing conditions is expected. Typically, a rapid change of prevailing conditions to more or less a

completely new set of prevailing conditions is associated with a synoptic feature passing through the terminal

area (cold or warm frontal passage). Appended to the “FM” indicator is the six−digit date, hour, and minute the

change is expected to begin and continues until the next change group or until the end of the current forecast.

A “FM” group will mark the beginning of a new line in a TAF report (indented 5 spaces). Each “FM” group

contains all the required elements−wind, visibility, weather, and sky condition. Weather will be omitted in “FM”

groups when it is not significant to aviation. FM groups will not include the contraction NSW.

Meteorology 7−1−77

AIM 2/20/25

EXAMPLE−

FM210100 14010KT P6SM SKC − “after 0100Z on the 21st, wind one four zero at one zero, visibility more than six, sky

clear .”

2. Becoming (BECMG) group. The BECMG group is used when a gradual change in conditions is expected

over a longer time period, usually two hours. The time period when the change is expected is two four −digit

groups separated by a “/”, with the beginning date and hour, and ending date and hour of the change period which

follows the BECMG indicator. The gradual change will occur at an unspecified time within this time period. Only

the changing forecast meteorological conditions are included in BECMG groups. The omitted conditions are

carried over from the previous time group.

NOTE−

The NWS does not use BECMG in the TAF .

EXAMPLE−

OVC012 BECMG 0114/0116 BKN020 − “ceiling one thousand two hundred overcast. Then a gradual change to ceiling two

thousand broken between 1400Z on the 1st and 1600Z on the 1st.”

3. Temporary (TEMPO) group. The TEMPO group is used for any conditions in wind, visibility, weather,

or sky condition which are expected to last for generally less than an hour at a time (occasional), and are expected

to occur during less than half the time period. The TEMPO indicator is followed by two four −digit groups

separated by a “/”. The first four digit group gives the beginning date and hour, and the second four digit group

gives the ending date and hour of the time period during which the temporary conditions are expected. Only the

changing forecast meteorological conditions are included in TEMPO groups. The omitted conditions are carried

over from the previous time group.

EXAMPLE−

1. SCT030 TEMPO 0519/0523 BKN030 − “three thousand scattered with occasional ceilings three thousand broken

between 1900Z on the 5th and 2300Z on the 5th.”

2. 4SM HZ TEMPO 1900/1906 2SM BR HZ − “visibility four in haze with occasional visibility two in mist and haze between

0000Z on the 19th and 0600Z on the 19th.”

7−1−78 Meteorology

2/20/25 AIM

Section 2. Barometric Altimeter Errors and Setting

Procedures

7−2−1. General

a. Aircraft altimeters are subject to the following errors and weather factors:

1. Instrument error.

2. Position error from aircraft static pressure systems.

3. Nonstandard atmospheric pressure.

4. Nonstandard temperatures.

b. The standard altimeter 29.92 inches Mercury (“Hg.) setting at the higher altitudes eliminates station

barometer errors, some altimeter instrument errors, and errors caused by altimeter settings derived from different

geographical sources.

7−2−2. Barometric Pressure Altimeter Errors

a. High Barometric Pressure: Cold, dry air masses may produce barometric pressures in excess of 31.00 “Hg.

Many aircraft altimeters cannot be adjusted above 31.00 “Hg. When an aircraft’s altimeter cannot be set to

pressure settings above 31.00 “Hg, the aircraft’s true altitude will be higher than the indicated altitude on the

barometric altimeter.

b. Low Barometric Pressure: An abnormal low−pressure condition exists when the barometric pressure is less

than 28.00 “Hg. Flight operations are not recommended when an aircraft’s altimeter is unable to be set below

28.00 “Hg. In this situation, the aircraft’s true altitude is lower than the indicated altitude. This situation may be

exacerbated when operating in extremely cold temperatures, which may result in the aircraft’s true altitude being

significantly lower than the indicated altitude.

NOTE−

EXTREME CAUTION SHOULD BE EXERCISED WHEN FLYING IN PROXIMITY TO OBSTRUCTIONS OR TERRAIN

IN LOW PRESSURES AND/OR LOW TEMPERATURES.

7−2−3. Altimeter Errors

a. Manufacturing and installation specifications, along with 14 CFR part 43, Appendix E requirement for

periodic tests and inspections, helps reduce mechanical, elastic, temperature, and installation errors. (See

Instrument Flying Handbook.) Scale error may be observed while performing a ground altimeter check using

the following procedure:

1. Set the current reported airfield altimeter setting on the altimeter setting scale.

2. Read the altitude on the altimeter. The altitude should read the known field elevation if you are located

on the same reference level used to establish the altimeter setting.

3. If the difference from the known field elevation and the altitude read from the altimeter is plus or minus

75 feet or greater, the accuracy of the altimeter is questionable and the problem should be referred to an

appropriately rated repair station for evaluation and possible correction.

b. It is important to set the current altimeter settings for the area of operation when flying at an enroute altitude

that does not require a standard altimeter setting of 29.92 “Hg. If the altimeter is not set to the current altimeter

setting when flying from an area of high pressure into an area of low pressure, the aircraft will be closer to the

surface than the altimeter indicates. An inch Hg. error in the altimeter setting equals 1,000 feet of altitude. For

Barometric Altimeter Errors and Setting Procedures 7−2−1

AIM 2/20/25

example, setting 29.90 “Hg instead of 30.90 “Hg. To quote an old saying: “GOING FROM A HIGH TO A LOW,

LOOK OUT BELOW.”

c. The aircraft cruising altitude or flight level is maintained by referencing the barometric altimeter.

Procedures for setting altimeters during high and low barometric pressure events must be set using the following

procedures:

1. Below 18,000 feet mean sea level (MSL).

(a) Barometric pressure is 31.00 “Hg or less.

(1) Set the altimeter to a current reported altimeter setting from a station along the route and within 100

NM of the aircraft, or;

(2) If there is no station within this area, use the current reported altimeter setting of an appropriate

available station, or;

NOTE−

Air traffic controllers will furnish this information at least once when en route or on an instrument flight plan within their

controlled airspace:

(3) If the aircraft is not equipped with a radio, set the altimeter to the elevation of the departure airport

or use an available appropriate altimeter setting prior to departure.

(b) When the barometric pressure exceeds 31.00 “Hg., a NOTAM will be published to define the affected

geographic area. The NOTAM will also institute the following procedures:

(1) All aircraft: All aircraft will set 31.00 “Hg. for en route operations below 18,000 feet MSL.

Maintain this setting until out of the affected area or until reaching the beginning of the final approach segment

on an instrument approach. Set the current altimeter setting (above 31.00 “Hg.) approaching the final segment,

if possible. If no current altimeter setting is available, or if a setting above 31.00 “Hg. cannot be made on the

aircraft’s altimeter, leave 31.00 “Hg. set in the altimeter and continue the approach.

(2) Set 31.00 “Hg. in the altimeter prior to reaching the lowest of any mandatory/crossing altitudes

or 1,500 feet above ground level (AGL) when on a departure or missed approach.

NOTE−

Air traffic control will issue actual altimeter settings and advise pilots to set 31.00 “Hg. in their altimeters for en route

operations below 18,000 feet MSL in affected areas.

(3) No additional restrictions apply for aircraft operating into an airport that are able to set and measure

altimeter settings above 31.00 “Hg.

(4) Flight operations are restricted to VFR weather conditions to and from an airport that is unable to

accurately measure barometric pressures above 31.00 “Hg. These airports will report the barometric pressure as

“missing” or “in excess of 31.00 “Hg.”.

(5) VFR aircraft. VFR operating aircraft have no additional restrictions. Pilots must use caution when

flight planning and operating in these conditions.

(6) IFR aircraft: IFR aircraft unable to set an altimeter setting above 31.00 “Hg. should apply the

following:

[a] The suitability of departure alternate airports, destination airports, and destination alternate

airports will be determined by increasing the published ceiling and visibility requirements when unable to set

the aircraft altimeter above 31.00 “Hg. Any reported or forecast altimeter setting over 31.00 “Hg. will be rounded

up to the next tenth to calculate the required increases. The ceiling will be increased by 100 feet and the visibility

by 1/4 statute mile for each 1/10 “Hg. over 31.00 “Hg. Use these adjusted values in accordance with operating

regulations and operations specifications.

EXAMPLE−

Destination airport altimeter is 31.21 “Hg. The planned approach is an instrument landing system (ILS) with a decision altitude (DA)

7−2−2 Barometric Altimeter Errors and Setting Procedures

AIM2/20/258/7/25 AIM

200 feet and visibility 1/2 mile (200−1/2). Subtract 31.00 “Hg. from 31.21 “Hg. to get .21 “Hg. .21 “Hg rounds up to .30 “Hg. Calculate

the increased requirement: 100 feet per 1/10 equates to a 300 feet increase for .30 “Hg. 1/4 statute mile per 1/10 equates to a 3/4 statute

mile increase for .30 “Hg. The destination weather requirement is determined by adding the 300−3/4 increase to 200−1/2. The destination

weather requirement is now 500−1 ¼..

[b] 31.00 “Hg. will remain set during the complete instrument approach. The aircraft has arrived at

the DA or minimum descent altitude (MDA) when the published DA or MDA is displayed on the barometric

altimeter.

NOTE−

The aircraft will be approximately 300 feet higher than the indicated barometric altitude using this method.

[c] These restrictions do not apply to authorized Category II/III ILS operations and certificate

holders using approved atmospheric pressure at aerodrome elevation (QFE) altimetry systems.

(7) Air Traffic Organization (ATO) Service Center Directors, in their area of jurisdiction, may

authorize temporary waivers to high barometric pressure flying NOTAM requirements to permit emergency

supply, transport, or medical services, per 14 CFR § 91.144 (b). ATO Service Center contact information can be

found at: https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/mission_support/sc.

2. At or above 18,000 feet MSL. All operators will set 29.92 “Hg. (standard setting) in the barometric

altimeter. The lowest usable flight level is determined by the atmospheric pressure in the area of operation as

shown in TBL 7−2−1. Air Traffic Control (ATC) will assign this flight level.

TBL 7−2−1

Lowest Usable Flight Level

Altimeter Setting Lowest Usable Flight

(Current Reported) Level

29.92 or higher 180

29.91 to 28.92 190

28.91 to 27.92 200

3. When the minimum altitude per 14 CFR section 91.159 and 14 CFR section 91.177 is above 18,000 feet

MSL, the lowest usable flight level must be the flight level equivalent of the minimum altitude plus the number

of feet specified in TBL 7−2−2. ATC will accomplish this calculation.

TBL 7−2−2

Lowest Flight Level Correction Factor

Altimeter Factor Correction Factor

29.92 or higher None

29.91 to 29.42 500 feet

29.41 to 28.92 1000 feet

28.91 to 28.42 1500 feet

28.41 to 27.92 2000 feet

27.91 to 27.42 2500 feet

EXAMPLE−

The minimum safe altitude of a route is 19,000 feet MSL and the altimeter setting is reported between 29.92 and 29.43 “Hg,

the lowest usable flight level will be 195, which is the flight level equivalent of 19,500 feet MSL (minimum altitude

(TBL 7−2−1) plus 500 feet).

Barometric Altimeter Errors and Setting Procedures 7−2−3

2/20/25 AIM

Section 3. Cold Temperature Barometric Altimeter

Errors, Setting Procedures and Cold Temperature

Airports (CTA)

7−3−1. Effect of Cold Temperature on Barometric Altimeters

a. Temperature has an effect on the accuracy of barometric altimeters, indicated altitude, and true altitude. The

standard temperature at sea level is 15 degrees Celsius (59 degrees Fahrenheit). The temperature gradient from

sea level is minus 2 degrees Celsius (3.6 degrees Fahrenheit) per 1,000 feet. For example, at 5000 feet above sea

level, the ambient temperature on a standard day would be 5 degrees Celsius. When the ambient (at altitude)

temperature is colder than standard, the aircraft’s true altitude is lower than the indicated barometric altitude.

When the ambient temperature is warmer than the standard day, the aircraft’s true altitude is higher than the

indicated barometric altitude.

b. TBL 7−3−1 indicates how much error may exist when operating in non−standard cold temperatures. To use

the table, find the reported temperature in the left column, and read across the top row to locate the height above

the airport (subtract the airport elevation from the flight altitude). Find the intersection of the temperature row

and height above airport column. This number represents how far the aircraft may be below the indicated altitude

due to possible cold temperature induced error.

TBL 7−3−1

ICAO Cold Temperature Error Table

7−3−2. Pre− Flight Planning for Cold Temperature Altimeter Errors

Flight planning into a CTA may be accomplished prior to flight. Use the predicted coldest temperature for plus

or minus 1 hour of the estimated time of arrival and compare against the CTA published temperature. If the

predicted temperature is at or below CTA temperature, calculate an altitude correction using TBL 7−3−1. This

correction may be used at the CTA if the actual arrival temperature is the same as the temperature used to calculate

the altitude correction during preflight planning.

7−3−3. Effects of Cold Temperature on Baro −Vertical Navigation (VNAV) Vertical Guidance

Non−standard temperatures can result in a change to effective vertical paths and actual descent rates when using

aircraft baro−VNA V equipment for vertical guidance on final approach segments. A lower than standard

temperature will result in a shallower descent angle and reduced descent rate. Conversely, a higher than standard

temperature will result in a steeper angle and incr eased descent rate. Pilots should consider potential

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consequences of these effects on approach minima, power settings, sight picture, visual cues, etc., especially for

high−altitude or terrain−challenged locations and during low−visibility conditions.

REFERENCE−

AIM, Para 5−4−5, Instrument Approach Procedure (IAP) Charts.

a. Uncompensated Baro −VNA V note on 14 CFR part 97 IAPs. The area navigation (RNA V) global

positioning system (GPS) and RNA V required navigation performance (RNP) notes, “For uncompensated

Baro−VNA V systems, lateral navigation (LNAV)/VNA V NA below –XX°C (−XX°F) or above XX°C (XXX°F)”

and “For uncompensated Baro −VNA V systems, procedure NA below –XX °C (−XX°F) or above XX °C

(XXX°F)” apply to baro−VNA V equipped aircraft. These temperatures and how they are used are independent

of the temperature and procedures applied for a Cold Temperature Airport.

1. The uncompensated baro−VNA V chart note and temperature range on an RNA V (GPS) approach is

applicable to the LNA V/VNA V line of minima. Baro −VNA V equipped aircraft without a temperature

compensating system may not use the RNA V (GPS) approach LNA V/VNA V line of minima when the actual

temperature is above or below the charted temperature range.

2. The uncompensated baro−VNA V chart note and temperature range on an RNA V (RNP) approach applies

to the entire procedure. For aircraft without a baro−VNA V and temperature compensating system, the RNA V

(RNP) approach is not authorized when the actual temperature is above or below the charted uncompensated

baro−VNA V temperature range.

b. Baro−VNA V temperature range versus CTA temperature: The baro−VNA V and CTA temperatures are

independent and do not follow the same correction or reporting procedures. However, there are times when both

procedures, each according to its associated temperature, should be accomplished on the approach.

c. Operating and ATC reporting procedures.

1. Do not use the CTA operating or reporting procedure found in this section, 7−3−4a thru 7−3−5e when

complying with the baro−VNA V temperature note on an RNA V (GPS) approach. Correction is not required nor

expected to be applied to procedure altitudes or VNA V paths outside of the final approach segment.

2. Operators must advise ATC when making temperature corrections on RNP authorization required (AR)

approaches while adhering to baro−VNA V temperature note.

3. Reporting altitude corrections is required when complying with CTAs in conjunction with the

baro−VNA V temperature note. The CTA altitude corrections will be reported in this situation. No altitude

correction reporting is required in the final segment.

NOTE−

When executing an approach with vertical guidance at a CTA (i.e., ILS, localizer performance with vertical guidance (LPV),

LNAV/VNAV), pilots are reminded to intersect the glideslope/glidepath at the corrected intermediate altitude (if applicable)

and follow the published glideslope/glidepath to the corrected minima. The ILS glideslope and WAAS generated glidepath

are unaffected by cold temperatures and provide vertical guidance to the corrected DA. Begin descent on the ILS glideslope

or WAAS generated glidepath when directed by aircraft instrumentation. Temperature affects the precise final approach fix

(PFAF) true altitude where a baro−VNAV generated glidepath begins. The PF AF altitude must be corrected when below the

CTA temperature restriction for the intermediate segment or outside of the baro−VNAV temperature restriction when using

the LNAV/VNAV line of minima to the corrected DA.

7−3−4. Cold Temperature Airports (CTA)

a. General: The FAA has determined that operating in cold temperatures has placed some 14 CFR part 97

instrument approach procedures in the United States National Airspace System at risk for loss of required

obstacle clearance (ROC). An airport that is determined to be at risk will have an ICON and temperature

published on the instrument approach procedure (IAP) in the terminal procedures publication (TPP).

b. CTA identification in TPP: A CTA is identified by a “snowflake” icon (

) and temperature limit, in

Celsius, on U.S. Government approach charts.

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c. A current list of CTAs is located at: https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/

dtpp/search/. Airports are listed by ICAO code, Airport Name, Temperature in Celsius, and affected segment(s).

d. Airport Criteria. The CTA risk analysis is performed on airports that have at least one runway of 2500 ft.

Pilots operating into an airport with a runway length less than 2500 ft may make a cold temperature altitude

correction in cold temperature conditions, if desired. Comply with operating and reporting procedures for CTAs.

e. ATC Reporting Requirements. Pilots must advise ATC with the corrected altitude when applying an altitude

correction on any approach segment with the exception of the final segment.

f. Methods to apply correction: The FAA recommends operators/pilots use either the All Segments Method

or the Individual Segments Method when making corrections at CTAs.

7−3−5. Cold Temperature Airport Procedures

a. PILOTS MUST NOT MAKE AN ALTIMETER CHANGE to accomplish an altitude correction. Pilots

must ensure that the altimeter is set to the current altimeter setting provided by ATC in accordance with 14 CFR

§91.121.

b. Actions on when and where to make corrections: Pilots will make an altitude correction to the published,

“at”, “at or above”, and “at or below” altitudes on all designated segment(s) to all runways for all published

instrument approach procedures when the reported airport temperature is at or below the published CTA

temperature on the approach plate. A pilot may request an altitude correction (if desired) on any approach at any

United States airport when extreme cold temperature is encountered. Pilots making a correction must comply

with ATC reporting requirements.

c. Correctable altitudes: ATC does not apply a cold temperature correction to their Minimum Vectoring

Altitude (MVA) or Minimum IFR Altitude (MIA) charts. Pilots must request approval from ATC to apply a cold

temperature correction to any ATC assigned altitude. Pilots must not correct altitudes published on Standard

Instrument Departures (SIDs), Obstacle Departure Pr ocedures (ODPs), and Standard Terminal Arrivals

(STARs).

d. Use of corrected MDA/DA: Pilots will use the corrected MDA or DA as the minimum altitude for an

approach. Pilots must meet the requirements in 14 CFR part 91.175 in order to operate below the corrected MDA

or DA. Pilots must see and avoid obstacles when descending below the minimum altitude on the approach.

NOTE−

The corrected DA or MDA does not affect the visibility minima published for the approach. With the application of a cold

temperature correction to the DA or MDA, the airplane should be in a position on the glideslope/glidepath or at the published

missed approach point to identify the runway environment.

e. Acceptable use of the table for manual CTA altitude correction (see TBL 7−3−1): Pilots may calculate a

correction with a visual interpolation of the chart when using reported temperature and height above airport. This

calculated altitude correction may then be rounded to the nearest whole hundred or rounded up. For example,

a correction of 130 ft from the chart may be rounded to 100 ft or 200 ft. A correction of 280 ft will be rounded

up to 300 ft. This rounded correction will be added to the appropriate altitudes for the “Individual” or “All”

segment method. The correction calculated from the table for the MDA or DA may be used as is or rounded up,

but never rounded down. This number will be added to the MDA, DA, and all step-down fix altitudes inside of

the FAF/PFAF.

1. No extrapolation above the 5000 ft column is required. Pilots may use the 5000 ft “height above airport

in feet” column for calculating corrections when the calculated altitude is greater than 5000 ft above reporting

station elevation. Pilots must add the correction(s) from the table to the affected segment altitude(s) and fly at

the new corrected altitude. Do not round down when using the 5000 ft column for calculated height above airport

values greater than 5000 ft. Pilots may extrapolate above the 5000 ft column to apply a correction if desired.

2. These techniques have been adopted to minimize pilot distraction by limiting the number of entries into

the table when making corrections. Although not all altitudes on the approach will be corrected back to standard

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