Chapter 24, Observations 24-1
24 Observations
24.1 Introduction
The first of five types of aviation weather information discussed in this handbook is observations.
Observations are weather data collected automatically by sensor(s), manually by trained weather observers,
or by a combination of both, and are the basic information upon which forecasts and advisories are made
in support of a wide -range of weather-sensitive activities within the public and private sectors, including
aviation.
For this handbook, observations include the following:
• Surface observations,
• Trained weather observers,
• Aircraft observations,
• Radar observations,
• Satellite observations,
• Upper air observations (e.g., weather balloon), and
• Aviation weather cameras.
Chapter 24, Observations 24-2
24.2 Surface Observations
Surface weather observations are fundamental t o all meteorological services. Observations are the basic
information upon which forecasts and warnings are made in sup port of a wide range of weather -sensitive
activities within the public and private sectors, including aviation.
There are three general types of surface weather observations:
• Manual.
• Automated.
• Augmented.
24.2.1 Manual Observation
Manual surface weather observations are made by a human weather observer who is certified by the FAA.
While manual observations were the primary type prior to the mid -1990s, automated and augmented
observations make up the vast majority of today’s surface observations in the United States.
24.2.2 Automated Observation
Automated observations are derived from instruments and algorithms without human input or oversight. In
the United States, there are two main kinds of automated observing systems: ASOS and AWOS. Automated
observations contain “AUTO” in the report unless they are augmented by a human weather observer.
24.2.3 Augmented Observation
At select airports in the United States, the automated observing system will have input and oversight by
human weather observers or tower controllers certified in weather observing. These are referred to as
augmented observations. Human observers report weather elements that are beyond the capabilities of the
automated system and/or are deemed operationally significant. The weather elements observed and reported
by the human observer vary, depending on the airport. “AUTO” is not used in augmented reports.
24.2.4 Recency of Observed Elements at Automated Stations
For those elements that the human observer evaluates using spatial averaging techniques (e.g., sky cover
and visibility), the automated station substitutes time averaging of sensor data. Therefore, in an automated
observation, sky condition is an evaluatio n of sensor data gathered during the 30 -minute period ending at
the actual time of the observation. All other elements are based on sensor data that is within 10 minutes or
less of the actual time of the observation.
24.3 Automated Surface Observing System (ASOS) and Automated Weather Observing
System (AWOS)
24.3.1 Automated Surface Observing System (ASOS)
The ASOS program is a joint effort of the NWS, the FAA, and the DOD. ASOS serves as the nation ’s
primary surface weather observing network.
ASOS detects significant changes, disseminating hourly and special observations. Additionally, ASOS
routinely and automatically provides computer -generated voice observations directly to aircraft in the
vicinity of airports, using FAA ground -to-air radio. These messages are also available via a telephone.
ASOS observes, formats, archives, and transmits observations automatically. ASOS transmits a special
Chapter 24, Observations 24-3
report when conditions exceed preselected weather element thresholds (e.g., the visibility decreases to less
than three miles).
All ASOS locations prepare and disseminate METAR s/SPECIs in accordance with the format shown in
Section 24.4.3.
24.3.1.1 ASOS One-Minute Observations (OMO)
In addition to the ASOS METARs/SPECIs are ASOS OMOs, which are updated once a minute. OMOs can
be in various formats and are sometimes encoded in the METAR format.
ASOS broadcasts can be different from the METAR seen on the internet or FIS-B, since the broadcast is
the OMO ASOS data.
The OMOs are not “instant weather”; rather, the clouds and visibility are time averaged (30 minutes for
clouds, 10 minutes for visibility). The averaging algorithms are designed to report deteriorating conditions
much quicker than improving conditions.
24.3.1.2 ASOS Reporting
ASOS reports the following basic weather elements:
• Sky condition: cloud height and amount (e.g., clear, few, scattered, broken, overcast) up to 12,000 ft
(future upgrade may raise height limit).
• Visibility (to at least 10 SM).
• Basic present weather information: type and intensity for rain ( RA), snow ( SN), freezing rain
(FZRA), and unknown precipitation (UP).
• Thunderstorms on site (TS) or in the vicinity (VCTS).
• Obstructions to vision: fog (FG), mist (BR), and haze (HZ).
Note: FG is reported when visibility is less than 5/8 SM. Freezing fog ( FZFG) is reported when
temperature is below 0 °C. BR or HZ is reported for visibilities from 5/8 SM to less than 7 SM,
depending on the difference between the temperature and dewpoint. If the difference is 4°F (about
2°C) or less, then BR is reported; otherwise, HZ is reported.
• Pressure: sea level pressure and altimeter setting.
• Temperature: ambient temperature and dewpoint temperature.
• Wind: direction from which the wind is blowing, speed, and character (e.g., gusts, squalls).
Note: National network distribution (e.g., FSS, internet, and FIS-B) of wind direction is in true
degrees, while local dissemination (e.g., radio and telephone) is in magnetic degrees.
• Precipitation: accumulation.
• Selected significant remarks including variable cloud height, variable visibility, precipitation
beginning/ending times, rapid pressure changes, pressure change tendency, wind shift, and peak
wind, and may include density altitude.
24.3.2 Automated Weather Observing System (AWOS)
AWOS is a system similar to ASOS. Generally, AWOS does not report all the elements that ASOS reports
and may not have the same level of backup sensors or maintenance response levels. Regardless, AWOS
Chapter 24, Observations 24-4
provides pilots with the necessary weather information to conduct 14 CFR part 91 flight operations as well
as others, depending on their operations specifications (OpSpecs).
AWOS automatically provides computer-generated voice observations directly to aircraft in the vicinity of
airports, using FAA ground-to-air radio. AWOS reports are also available via a telephone.
AWOS may be located on airports, at or near ground -based or roof top-based heliports, as well as on
offshore platforms and drill ships.
AWOS are either Federal or non-Federal. Federal AWOS units are owned, operated, and maintained by the
FAA. Non-Federal AWOS are owned, operated, and maintained by the site owner.
AWOS generate s a METAR at 20 -minute intervals and do es not report SPECIs. AWOS also provide s
OMOs available by phone or radio.
The OMOs are not “instant weather;” rather, the clouds and visibility are time averaged (30 minutes for
clouds, 10 minutes for visibility). The averaging algorithms are designed to report deteriorating conditions
much quicker than improving conditions. For example, if dense fog had been reported and then suddenly
dissipated, it might take up to 10 minutes for the OMOs to report VFR conditions.
There are six types of AWOS systems:
• AWOS-A: The AWOS-A system measures and reports altimeter only.
• AWOS-AV: The AWOS-AV consists of an AWOS-A with a visibility sensor.
• AWOS-1: The AWOS-1 system measures and reports wind data (e.g., speed, direction, and gusts;
temperature; dewpoint; altimeter; and density altitude).
• AWOS-2: The AWOS-2 system measures and reports all of the parameters of an AWOS-1 system
plus visibility.
• AWOS-3: The AWOS-3 system measures and reports all of the parameters of an AWOS-2 system
plus precipitation accumulation (rain gauge) and cloud height. AWOS-3 can have optional sensors
such as precipitation type/intensity (present weather, P) and/or thunderstorm/lightning (T). The
addition of an optional sensors will change the designation to AWOS-3P or AWOS-3PT.
• AWOS-4: The AWOS -4 system measures and reports all of the AWOS -3PT parameters plus
freezing rain.
Depending on the type of AWOS unit, the following parameters may be measured:
• Altimeter.
• Wind speed.
• Wind direction (from which the wind is blowing).
Note: National network distribution (e.g., FSS, internet, and FIS-B) of wind direction is in true
degrees, while local dissemination (e.g., radio and telephone) is in magnetic degrees.
• Gusts.
• Temperature.
• Dewpoint.
• Density altitude.
• Visibility.
• Precipitation accumulation.
Chapter 24, Observations 24-5
• Cloud height.
• Precipitation type.
• Precipitation intensity.
• Present weather.
• Thunderstorm/lightning.
• Freezing rain.
• Runway surface condition.
24.4 Aviation Routine Weather Report (METAR) and Aviation Selected Special
Weather Report (SPECI)
The METAR and SPECI are the code form used for aviation surface observations (reports) to satisfy World
Meteorological Organization (WMO) and ICAO instructions for reporting surface meteorological data.
Although the METAR and SPECI code is used worldwide, there are some code differences among
countries. Each country is allowed to make modifications to the code for use in their particular country, as
long as they notify ICAO. These sections will focus on the METAR/SPECI code as used in the
United States.
Traditionally, it was critical for pilots to know how to decode the METAR and SPECI. The majority of
current preflight briefing services, including many weather applications, provide the decoded report in a
plain language format in addition to the coded re port. Table 24-1 provides an example of a decoded
METAR. While the decoded version has been welcomed by many pilots, it is still important for all pilots
to know how to decode the METAR/SPECI in case the source does not provide a decoded version.
Table 24-1. An Example of a Decoded METAR for an Aviation Weather Website
+ KPNS (Pensacola Intl, FL, US)
Text: KPNS 030053Z 36005KT 10SM CLR 30/23 A2992 RMK AO2 SLP132
Temperature: 30.0 °C (86 °F)
Dewpoint: 23.3 °C (74 °F) [RH = 67%]
Pressure
(altimeter): 29.92 inHg (1013.3 mb) [Sea level pressure: 1013.2 mb]
Winds: from the N (360 degrees) at 6 mph (5 knots; 2.6 m/s)
Visibility: 10 SM or more (16+ km)
Ceiling: at least 12,000 ft AGL
Clouds: sky clear below 12,000 ft AGL
24.4.1 Aviation Routine Weather Report (METAR)
A METAR includes the airport identifier, time of observation, wind, visibility, Runway Visual Range
(RVR), present weather phenomena, sky conditions, temperature, dewpoint, and altimeter setting.
Excluding the airport identifier and the time of observation , this information is collectively referred to as
the “body” of the report. Coded and/or plain language information elaborating on data in the body may be
Chapter 24, Observations 24-6
appended to the end of the METAR as “remarks.” The contents of the remarks section vary with manual,
automated, and augmented surface observations. At some designated stations, the METAR may be abridged
to include only a few of the mentioned elements. METARs are sometimes referred to as “hourly” reports
since they are routinely produced near the top of the hour.
24.4.2 Aviation Selected Special Weather Report (SPECI)
A SPECI is an unscheduled report taken when any of the criteria given in Table 24-2 are observed during
the period between hourly reports. SPECIs contain all data elements found in a METAR. All SPECIs are
issued as soon as possible when relevant criteria are observed.
Whenever SPECI criteria are met at the time of the routine METAR, a METAR is issued.
Table 24-2. SPECI Criteria
1 Wind Shift Wind direction changes by 45° or more, in less than 15 minutes, and the wind
speed is 10 knots or more throughout the wind shift.
2 Visibility
Visibility as reported in the body of the report decreases to less than, or if below,
increases to equal or exceed:
• 3 miles.
• 2 miles.
• 1 mile.
• ½ mile.
• ¼ mile.
• The lowest standard IAP minimum as published in the U.S. Terminal
Procedures, if not listed above.
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.
4 Tornado, Funnel
Cloud, 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 active and 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 Squalls When a squall occurs.
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.
Chapter 24, Observations 24-7
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.
24.4.3 METAR/SPECI Format
A U.S. METAR/SPECI has two major sections: the body (consisting of a maximum of 11 groups) and the
remarks (consisting of two categories). When an element does not occur, or cannot be observed, the
corresponding group is omitted from that particular report. See Figure 24-1 for the format.
Figure 24-1. METAR/SPECI Coding Format
Chapter 24, Observations 24-8
24.4.3.1 Type of Report
METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB
18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132
The type of report, METAR or SPECI, precedes the body of all reports, but may not be shown or displayed
on all aviation weather websites.
24.4.3.2 Station Identifier
METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB
18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132
The station identifier, in ICAO format, is included in all reports to identify the station to which the coded
report applies.
The ICAO airport code is a four -letter alphanumeric code designating each airport around the world. The
ICAO codes are used for flight planning by pilots and airline operation departments. These codes are not
the same as the International Air Transport Ass ociation (IATA) codes encountered by the general public
used for reservations, baggage handling, and in airline timetables.
Unlike the IATA codes, the ICAO codes have a regional structure. The first letter identifies the region and
country (see Figure 24-2). In some regions, the second letter identifies the country. ICAO station identifiers
in Alaska begin with “PA,” Hawaii begins with “PH,” Guam begins with “PG,” and Puerto Rico begins
with “TJ.” For example, the San Juan , Puerto Rico, IATA identifier “SJU” becomes the ICAO identifier
“TJSJ.” The remaining letters are used to identify each airport.
In the CONUS, ICAO station identifiers are coded K, followed by the three -letter IATA identifier. For
example, the Seattle, WA, IATA identifier “SEA” becomes the ICAO identifier “KSEA.”
ICAO station identifiers in Alaska, Hawaii, and Guam begin with the continent code P.
For a list of all U.S. identifiers, refer to FAA Order JO 7350.9, Location Identifiers. For a complete
worldwide listing, refer to ICAO Doc 7910, Location Indicators. Both are available online.