Remote Pilot – Small Unmanned Aircraft Systems Study Guide 15
Chapter 3a:
Aviation Weather Sources
Introduction
In aviation, weather service is a combined effort of the National Weather Service (NWS), Federal
Aviation Administration (FAA), Department of Defense (DOD), other aviation groups, and individuals.
Because of the increasing need for worldwide weather services, foreign weather organizations also
provide vital input. While weather forecasts are not 100 percent accurate, meteorologists, through
careful scientific study and computer modeling, have the ability to predict weather patterns, trends,
and characteristics with increasing accuracy. Through a complex system of weather services,
government agencies, and independent weather observers, pilots and other aviation professionals
receive the benefit of this vast knowledge base in the form of up-to-date weather reports and
forecasts. These reports and forecasts enable pilots to make informed decisions regarding weather and
flight safety before and during a flight.
Surface Aviation Weather Observations
Surface aviation weather observations are a compilation of elements of the current weather at
individual ground stations across the United States. The network is made up of government and
privately contracted facilities that provide continuous up-to-date weather information. Automated
weather sources, such as the Automated Weather Observing Systems (AWOS), Automated Surface
Observing Systems (ASOS), as well as other automated facilities, also play a major role in the gathering
of surface observations.
Surface observations provide local weather conditions and other relevant information for a specific
airport. This information includes the type of report, station identifier, date and time, modifier (as
required), wind, visibility, runway visual range (RVR), weather phenomena, sky condition,
temperature/dew point, altimeter reading, and applicable remarks. The information gathered for the
surface observation may be from a person, an automated station, or an automated station that is
updated or enhanced by a weather observer. In any form, the surface observation provides valuable
information about individual airports around the country. These reports cover a small area and will be
beneficial to the remote pilot.
Aviation Weather Reports
Aviation weather reports are designed to give accurate depictions of current weather conditions. Each
report provides current information that is updated at different times. Some typical reports are
METARs and PIREPs. To view a weather report, go to http://www.aviationweather.gov/.
Aviation Routine Weather Report (METAR)
A METAR is an observation of current surface weather reported in a standard international format.
METARs are issued on a regularly scheduled basis unless significant weather changes have occurred.
A special METAR (SPECI) can be issued at any time between routine METAR reports.
Example: METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB
18/17 A2970 RMK PRESFR
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Remote Pilot – Small Unmanned Aircraft Systems Study Guide 16
A typical METAR report contains the following information in sequential order:
1. Type of report—there are two types of METAR reports. The first is the routine METAR report
that is transmitted on a regular time interval. The second is the aviation selected SPECI. This
is a special report that can be given at any time to update the METAR for rapidly changing
weather conditions, aircraft mishaps, or other critical information.
2. Station identifier—a four-letter code as established by the International Civil Aviation
Organization (ICAO). In the 48 contiguous states, a unique three-letter identifier is preceded
by the letter “K.” For example, Gregg County Airport in Longview, Texas, is identified by the
letters “KGGG,” K being the country designation and GGG being the airport identifier. In
other regions of the world, including Alaska and Hawaii, the first two letters of the four-letter
ICAO identifier indicate the region, country, or state. Alaska identifiers always begin with the
letters “PA” and Hawaii identifiers always begin with the letters “PH.” Station identifiers can
be found by searching various websites such as DUATS and NOAA's Aviation Weather
Aviation Digital Data Services (ADDS).
3. Date and time of report—depicted in a six-digit group (161753Z). The first two digits are the
date. The last four digits are the time of the METAR/SPECI, which is always given in
coordinated universal time (UTC). A “Z” is appended to the end of the time to denote the
time is given in Zulu time (UTC) as opposed to local time.
4. Modifier—denotes that the METAR/SPECI came from an automated source or that the report
was corrected. If the notation “AUTO” is listed in the METAR/SPECI, the report came from an
automated source. It also lists “AO1” (for no precipitation discriminator) or “AO2” (with
precipitation discriminator) in the “Remarks” section to indicate the type of precipitation
sensors employed at the automated station. When the modifier “COR” is used, it identifies a
corrected report sent out to replace an earlier report that contained an error (for example:
METAR KGGG 161753Z COR).
5. Wind—reported with five digits (14021KT) unless the speed is greater than 99 knots, in which
case the wind is reported with six digits. The first three digits indicate the direction the true
wind is blowing from in tens of degrees. If the wind is variable, it is reported as “VRB.” The
last two digits indicate the speed of the wind in knots unless the wind is greater than 99
knots, in which case it is indicated by three digits. If the winds are gusting, the letter “G”
follows the wind speed (G26KT). After the letter “G,” the peak gust recorded is provided. If
the wind direction varies more than 60° and the wind speed is greater than six knots, a
separate group of numbers, separated by a “V,” will indicate the extremes of the wind
directions.
6. Visibility—the prevailing visibility (¾ SM) is reported in statute miles as denoted by the letters
“SM.” It is reported in both miles and fractions of miles. At times, runway visual range (RVR)
is reported following the prevailing visibility. RVR is the distance a pilot can see down the
runway in a moving aircraft. When RVR is reported, it is shown with an R, then the runway
number followed by a slant, then the visual range in feet. For example, when the RVR is
reported as R17L/1400FT, it translates to a visual range of 1,400 feet on runway 17 left.
7. Weather—can be broken down into two different categories: qualifiers and weather
phenomenon (+TSRA BR). First, the qualifiers of intensity, proximity, and the descriptor of the
weather are given. The intensity may be light (–), moderate ( ), or heavy (+). Proximity only
depicts weather phenomena that are in the airport vicinity. The notation “VC” indicates a
specific weather phenomenon is in the vicinity of five to ten miles from the airport.
Descriptors are used to describe certain types of precipitation and obscurations. Weather
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Remote Pilot – Small Unmanned Aircraft Systems Study Guide 17
phenomena may be reported as being precipitation, obscurations, and other phenomena,
such as squalls or funnel clouds. Descriptions of weather phenomena as they begin or end
and hailstone size are also listed in the “Remarks” sections of the report. [Figure 3-1]
Figure 3-1. Descriptors and weather phenomena used in a typical METAR.
8. Sky condition—always reported in the sequence of amount, height, and type or indefinite
ceiling/height (vertical visibility) (BKN008 OVC012CB, VV003). The heights of the cloud bases
are reported with a three-digit number in hundreds of feet AGL. Clouds above 12,000 feet are
not detected or reported by an automated station. The types of clouds, specifically towering
cumulus (TCU) or cumulonimbus (CB) clouds, are reported with their height. Contractions are
used to describe the amount of cloud coverage and obscuring phenomena. The amount of
sky coverage is reported in eighths of the sky from horizon to horizon. [Figure 3-2]
Figure 3-2. Reportable contractions for sky condition.
9. Temperature and dew point—the air temperature and dew point are always given in degrees
Celsius (C) or (18/17). Temperatures below 0 °C are preceded by the letter “M” to indicate
minus.
10. Altimeter setting—reported as inches of mercury ("Hg) in a four-digit number group (A2970).
It is always preceded by the letter “A.” Rising or falling pressure may also be denoted in the
“Remarks” sections as “PRESRR” or “PRESFR,” respectively.
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Remote Pilot – Small Unmanned Aircraft Systems Study Guide 18
11. Zulu time—a term used in aviation for UTC, which places the entire world on one time
standard.
12. Remarks—the remarks section always begins with the letters “RMK.” Comments may or may
not appear in this section of the METAR. The information contained in this section may
include wind data, variable visibility, beginning and ending times of particular phenomenon,
pressure information, and various other information deemed necessary. An example of a
remark regarding weather phenomenon that does not fit in any other category would be:
OCNL LTGICCG. This translates as occasional lightning in the clouds and from cloud to ground.
Automated stations also use the remarks section to indicate the equipment needs
maintenance.
Example: METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB
18/17 A2970 RMK PRESFR
Explanation: Routine METAR for Gregg County Airport for the 16th day of the month at 1753Z
automated source. Winds are 140 at 21 knots gusting to 26. Visibility is ¾ statute
mile. Thunderstorms with heavy rain and mist. Ceiling is broken at 800 feet, overcast
at 1,200 feet with cumulonimbus clouds. Temperature 18 °C and dew point 17 °C.
Barometric pressure is 29.70 "Hg and falling rapidly.
Aviation Forecasts
Observed weather condition reports are often used in the creation of forecasts for the same area. A
variety of different forecast products are produced and designed to be used in the preflight planning
stage. The printed forecasts that pilots need to be familiar with are the terminal aerodrome forecast
(TAF), aviation area forecast (FA), inflight weather advisories (Significant Meteorological Information
(SIGMET), Airman’s Meteorological Information (AIRMET)), and the winds and temperatures aloft
forecast (FB).
Terminal Aerodrome Forecasts (TAF)
A TAF is a report established for the five statute mile radius around an airport. TAF reports are
usually given for larger airports. Each TAF is valid for a 24 or 30-hour time period and is updated four
times a day at 0000Z, 0600Z, 1200Z, and 1800Z. The TAF utilizes the same descriptors and
abbreviations as used in the METAR report. These weather reports can be beneficial to the remote
pilot for flight planning purposes. The TAF includes the following information in sequential order:
1. Type of report—a TAF can be either a routine forecast (TAF) or an amended forecast (TAF
AMD).
2. ICAO station identifier—the station identifier is the same as that used in a METAR.
3. Date and time of origin—time and date (081125Z) of TAF origination is given in the six-
number code with the first two being the date, the last four being the time. Time is always
given in UTC as denoted by the Z following the time block.
4. Valid period dates and times—The TAF valid period (0812/0912) follows the date/time of
forecast origin group. Scheduled 24 and 30 hour TAFs are issued four times per day, at 0000,
0600, 1200, and 1800Z. The first two digits (08) are the day of the month for the start of the
TAF. The next two digits (12) are the starting hour (UTC). 09 is the day of the month for the
end of the TAF, and the last two digits (12) are the ending hour (UTC) of the valid period. A
forecast period that begins at midnight UTC is annotated as 00. If the end time of a valid
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Remote Pilot – Small Unmanned Aircraft Systems Study Guide 19
period is at midnight UTC, it is annotated as 24. For example, a 00Z TAF issued on the 9th of
the month and valid for 24 hours would have a valid period of 0900/0924.
5. Forecast wind—the wind direction and speed forecast are coded in a five-digit number group.
An example would be 15011KT. The first three digits indicate the direction of the wind in
reference to true north. The last two digits state the wind speed in knots appended with
“KT.” Like the METAR, winds greater than 99 knots are given in three digits.
6. Forecast visibility—given in statute miles and may be in whole numbers or fractions. If the
forecast is greater than six miles, it is coded as “P6SM.”
7. Forecast significant weather—weather phenomena are coded in the TAF reports in the same
format as the METAR.
8. Forecast sky condition—given in the same format as the METAR. Only CB clouds are forecast
in this portion of the TAF report as opposed to CBs and towering cumulus in the METAR.
9. Forecast change group—for any significant weather change forecast to occur during the TAF
time period, the expected conditions and time period are included in this group. This
information may be shown as from (FM), and temporary (TEMPO). “FM” is used when a rapid
and significant change, usually within an hour, is expected. “TEMPO” is used for temporary
fluctuations of weather, expected to last less than 1 hour.
10. PROB30—a given percentage that describes the probability of thunderstorms and
precipitation occurring in the coming hours. This forecast is not used for the first 6 hours of
the 24-hour forecast.
Example: TAF KPIR 111130Z 1112/1212 TEMPO 1112/1114 5SM BR FM1500 16015G25KT P6SM
SCT040 BKN250 FM120000 14012KT P6SM BKN080 OVC150 PROB30 1200/1204 3SM
TSRA BKN030CB FM120400 1408KT P6SM SCT040 OVC080 TEMPO 1204/1208 3SM
TSRA OVC030CB
Explanation: Routine TAF for Pierre, South Dakota…on the 11th day of the month, at 1130Z…valid
for 24 hours from 1200Z on the 11th to 1200Z on the 12th…wind from 150° at 12
knots… visibility greater than 6 SM…broken clouds at 9,000 feet… temporarily,
between 1200Z and 1400Z, visibility 5 SM in mist…from 1500Z winds from 160° at 15
knots, gusting to 25 knots visibility greater than 6 SM…clouds scattered at 4,000 feet
and broken at 25,000 feet…from 0000Z wind from 140° at 12 knots…visibility greater
than 6 SM…clouds broken at 8,000 feet, overcast at 15,000 feet…between 0000Z and
0400Z, there is 30 percent probability of visibility 3 SM…thunderstorm with moderate
rain showers…clouds broken at 3,000 feet with cumulonimbus clouds…from
0400Z…winds from 140° at 8 knots…visibility greater than 6 miles…clouds at 4,000
scattered and overcast at 8,000… temporarily between 0400Z and 0800Z…visibility 3
miles… thunderstorms with moderate rain showers…clouds overcast at 3,000 feet
with cumulonimbus clouds…end of report (=).
Convective Significant Meteorological Information (WST)
Convective SIGMETs are issued for severe thunderstorms with surface winds greater than 50 knots, hail
at the surface greater than or equal to ¾ inch in diameter, or tornadoes. They are also issued to advise
pilots of embedded thunderstorms, lines of thunderstorms, or thunderstorms with heavy or greater
precipitation that affect 40 percent or more of a 3,000 square mile or greater region. A remote pilot
will find these weather alerts helpful for flight planning.
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Remote Pilot – Small Unmanned Aircraft Systems Study Guide 21
Chapter 3b:
Effects of Weather on Small Unmanned Aircraft Performance
Introduction
This chapter discusses the factors that affect aircraft performance, which include the aircraft weight,
atmospheric conditions, runway environment, and the fundamental physical laws governing the forces
acting on an aircraft.
Since the characteristics of the atmosphere have a major effect on performance, it is necessary to
review two dominant factors—pressure and temperature.
Density Altitude
The more appropriate term for correlating aerodynamic performance in the nonstandard atmosphere
is density altitude—the altitude in the standard atmosphere corresponding to a particular value of air
density.
As the density of the air increases (lower density altitude), aircraft performance increases. Conversely,
as air density decreases (higher density altitude), aircraft performance decreases. A decrease in air
density means a high density altitude; an increase in air density means a lower density altitude. Density
altitude has a direct effect on aircraft performance.
Air density is affected by changes in altitude, temperature, and humidity. High density altitude refers to
thin air while low density altitude refers to dense air. The conditions that result in a high density
altitude are high elevations, low atmospheric pressures, high temperatures, high humidity, or some
combination of these factors. Lower elevations, high atmospheric pressure, low temperatures, and low
humidity are more indicative of low density altitude.
Effects of Pressure on Density
Since air is a gas, it can be compressed or expanded. When air is compressed, a greater amount of
air can occupy a given volume. Conversely, when pressure on a given volume of air is decreased, the
air expands and occupies a greater space. That is, the original column of air at a lower pressure
contains a smaller mass of air. In other words, the density is decreased. In fact, density is directly
proportional to pressure. If the pressure is doubled, the density is doubled, and if the pressure is
lowered, so is the density. This statement is true only at a constant temperature.
Effects of Temperature on Density
Increasing the temperature of a substance decreases its density. Conversely, decreasing the
temperature increases the density. Thus, the density of air varies inversely with temperature. This
statement is true only at a constant pressure.
In the atmosphere, both temperature and pressure decrease with altitude and have conflicting
effects upon density. However, the fairly rapid drop in pressure as altitude is increased usually has
the dominant effect. Hence, pilots can expect the density to decrease with altitude.
Effects of Humidity (Moisture) on Density
The preceding paragraphs are based on the presupposition of perfectly dry air. In reality, it is never
completely dry. The small amount of water vapor suspended in the atmosphere may be negligible
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Remote Pilot – Small Unmanned Aircraft Systems Study Guide 22
under certain conditions, but in other conditions humidity may become an important factor in the
performance of an aircraft. Water vapor is lighter than air; consequently, moist air is lighter than dry
air. Therefore, as the water content of the air increases, the air becomes less dense, increasing
density altitude and decreasing performance. It is lightest or least dense when, in a given set of
conditions, it contains the maximum amount of water vapor.
Humidity, also called relative humidity, refers to the amount of water vapor contained in the
atmosphere and is expressed as a percentage of the maximum amount of water vapor the air can
hold. This amount varies with the temperature; warm air can hold more water vapor, while colder
air can hold less. Perfectly dry air that contains no water vapor has a relative humidity of zero
percent, while saturated air that cannot hold any more water vapor has a relative humidity of 100
percent. Humidity alone is usually not considered an essential factor in calculating density altitude
and aircraft performance; however, it does contribute.
Performance
Performance is a term used to describe the ability of an aircraft to accomplish certain things that make
it useful for certain purposes.
The primary factors most affected by performance are the takeoff and landing distance, rate of climb,
ceiling, payload, range, speed, maneuverability, stability, and fuel economy.
Climb Performance Factors
Since weight, altitude and configuration changes affect excess thrust and power, they also affect
climb performance. Climb performance is directly dependent upon the ability to produce either
excess thrust or excess power.
Weight has a very pronounced effect on aircraft performance. If weight is added to an aircraft, it
must fly at a higher angle of attack (AOA) to maintain a given altitude and speed. This increases the
induced drag of the wings, as well as the parasite drag of the aircraft. Increased drag means that
additional thrust is needed to overcome it, which in turn means that less reserve thrust is available
for climbing. Aircraft designers go to great lengths to minimize the weight, since it has such a
marked effect on the factors pertaining to performance.
A change in an aircraft’s weight produces a twofold effect on climb performance. An increase in
altitude also increases the power required and decreases the power available. Therefore, the climb
performance of an aircraft diminishes with altitude.
Measurement of Atmosphere Pressure
To provide a common reference, the International Standard Atmosphere (ISA) has been established.
These standard conditions are the basis for most aircraft performance data. Standard sea level
pressure is defined as 29.92 "Hg and a standard temperature of 59 °F (15 °C). Atmospheric pressure is
also reported in millibars (mb), with 1 "Hg equal to approximately 34 mb. Standard sea level pressure is
1,013.2 mb. Typical mb pressure readings range from 950.0 to 1,040.0 mb. Surface charts, high and low
pressure centers, and hurricane data are reported using mb.
Since weather stations are located around the globe, all local barometric pressure readings are
converted to a sea level pressure to provide a standard for records and reports. To achieve this, each
station converts its barometric pressure by adding approximately 1 "Hg for every 1,000 feet of
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Remote Pilot – Small Unmanned Aircraft Systems Study Guide 23
elevation. For example, a station at 5,000 feet above sea level, with a reading of 24.92 "Hg, reports a
sea level pressure reading of 29.92 "Hg.
By tracking barometric pressure trends across a large area, weather forecasters can more accurately
predict movement of pressure systems and the associated weather. For example, tracking a pattern of
rising pressure at a single weather station generally indicates the approach of fair weather. Conversely,
decreasing or rapidly falling pressure usually indicates approaching bad weather and, possibly, severe
storms.
Effect of Obstructions on Wind
Another atmospheric hazard exists that can create problems for pilots. Obstructions on the ground
affect the flow of wind and can be an unseen danger. Ground topography and large buildings can break
up the flow of the wind and create wind gusts that change rapidly in direction and speed. These
obstructions range from man-made structures, like hangars, to large natural obstructions, such as
mountains, bluffs, or canyons.
The intensity of the turbulence associated with ground obstructions depends on the size of the
obstacle and the primary velocity of the wind. This can affect the performance of any aircraft and can
present a very serious hazard.
This same condition is even more noticeable when flying in mountainous regions. [Figure 3-3] While
the wind flows smoothly up the windward side of the mountain and the upward currents help to carry
an aircraft over the peak of the mountain, the wind on the leeward side does not act in a similar
manner. As the air flows down the leeward side of the mountain, the air follows the contour of the
terrain and is increasingly turbulent. This tends to push an aircraft into the side of a mountain. The
stronger the wind, the greater the downward pressure and turbulence become.
Figure 3-3. Turbulence in mountainous regions.
Low-Level Wind Shear
Wind shear is a sudden, drastic change in wind speed and/or direction over a very small area. Wind
shear can subject an aircraft to violent updrafts and downdrafts, as well as abrupt changes to the
horizontal movement of the aircraft. While wind shear can occur at any altitude, low-level wind shear
is especially hazardous due to the proximity of an aircraft to the ground. Low-level wind shear is
commonly associated with passing frontal systems, thunderstorms, temperature inversions, and strong
upper level winds (greater than 25 knots).
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Remote Pilot – Small Unmanned Aircraft Systems Study Guide 24
Wind shear is dangerous to an aircraft. It can rapidly change the performance of the aircraft and
disrupt the normal flight attitude. For example, a tailwind quickly changing to a headwind causes an
increase in airspeed and performance. Conversely, a headwind changing to a tailwind causes a
decrease in airspeed and performance. In either case, a pilot must be prepared to react immediately to
these changes to maintain control of the aircraft.
The most severe type of low-level wind shear, a microburst, is associated with convective precipitation
into dry air at cloud base. Microburst activity may be indicated by an intense rain shaft at the surface
but virga at cloud base and a ring of blowing dust is often the only visible clue. A typical microburst has
a horizontal diameter of 1–2 miles and a nominal depth of 1,000 feet. The lifespan of a microburst is
about 5–15 minutes during which time it can produce downdrafts of up to 6,000 feet per minute (fpm)
and headwind losses of 30–90 knots, seriously degrading performance. It can also produce strong
turbulence and hazardous wind direction changes. During an inadvertent microburst encounter, the
small UA may first experience a performance-increasing headwind, followed by performance-
decreasing downdrafts, followed by a rapidly increasing tailwind. This can result in terrain impact or
flight dangerously close to the ground. An encounter during approach involves the same sequence of
wind changes and could force the small UA to the ground short of the intended landing area.
It is important to remember that wind shear can affect any flight at any altitude. While wind shear may
be reported, it often remains undetected and is a silent danger to aviation. Always be alert to the
possibility of wind shear, especially when flying in and around thunderstorms and frontal systems.
Atmospheric Stability
The stability of the atmosphere depends on its ability to resist vertical motion. A stable atmosphere
makes vertical movement difficult, and small vertical disturbances dampen out and disappear. In an
unstable atmosphere, small vertical air movements tend to become larger, resulting in turbulent
airflow and convective activity. Instability can lead to significant turbulence, extensive vertical clouds,
and severe weather.
The combination of moisture and temperature determine the stability of the air and the resulting
weather. Cool, dry air is very stable and resists vertical movement, which leads to good and generally
clear weather. The greatest instability occurs when the air is moist and warm, as it is in the tropical
regions in the summer. Typically, thunderstorms appear on a daily basis in these regions due to the
instability of the surrounding air.
Inversion
As air rises and expands in the atmosphere, the temperature decreases. There is an atmospheric
anomaly that can occur; however, that changes this typical pattern of atmospheric behavior. When
the temperature of the air rises with altitude, a temperature inversion exists. Inversion layers are
commonly shallow layers of smooth, stable air close to the ground. The temperature of the air
increases with altitude to a certain point, which is the top of the inversion. The air at the top of the
layer acts as a lid, keeping weather and pollutants trapped below. If the relative humidity of the air
is high, it can contribute to the formation of clouds, fog, haze, or smoke resulting in diminished
visibility in the inversion layer.
Surface-based temperature inversions occur on clear, cool nights when the air close to the ground is
cooled by the lowering temperature of the ground. The air within a few hundred feet of the surface
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Remote Pilot – Small Unmanned Aircraft Systems Study Guide 25
becomes cooler than the air above it. Frontal inversions occur when warm air spreads over a layer of
cooler air, or cooler air is forced under a layer of warmer air.
Temperature/Dew Point Relationship
The relationship between dew point and temperature defines the concept of relative humidity. The
dew point, given in degrees, is the temperature at which the air can hold no more moisture. When the
temperature of the air is reduced to the dew point, the air is completely saturated and moisture begins
to condense out of the air in the form of fog, dew, frost, clouds, rain, or snow.
Methods by Which Air Reaches the Saturation Point
If air reaches the saturation point while temperature and dew point are close together, it is highly
likely that fog, low clouds, and precipitation will form. There are four methods by which air can
reach the saturation point. First, when warm air moves over a cold surface, the air temperature
drops and reaches the saturation point. Second, the saturation point may be reached when cold air
and warm air mix. Third, when air cools at night through contact with the cooler ground, air reaches
its saturation point. The fourth method occurs when air is lifted or is forced upward in the
atmosphere.
Dew and Frost
On cool, clear, calm nights, the temperature of the ground and objects on the surface can cause
temperatures of the surrounding air to drop below the dew point. When this occurs, the moisture in
the air condenses and deposits itself on the ground, buildings, and other objects like cars and
aircraft. This moisture is known as dew and sometimes can be seen on grass and other objects in the
morning. If the temperature is below freezing, the moisture is deposited in the form of frost. While
dew poses no threat to a small UA, frost poses a definite flight safety hazard. Frost disrupts the flow
of air over the wing and can drastically reduce the production of lift. It also increases drag, which
when combined with lowered lift production, can adversely affect the ability to take off. A small UA
must be thoroughly cleaned and free of frost prior to beginning a flight.
Clouds
To pilots, the cumulonimbus cloud is perhaps the most dangerous cloud type. It appears individually or
in groups and is known as either an air mass or orographic thunderstorm. Heating of the air near the
Earth’s surface creates an air mass thunderstorm; the upslope motion of air in the mountainous
regions causes orographic thunderstorms. Cumulonimbus clouds that form in a continuous line are
nonfrontal bands of thunderstorms or squall lines.
Since rising air currents cause cumulonimbus clouds, they are extremely turbulent and pose a
significant hazard to flight safety. For example, if a small UA enters a thunderstorm, the small UA could
experience updrafts and downdrafts that exceed 3,000 fpm. In addition, thunderstorms can produce
large hailstones, damaging lightning, tornadoes, and large quantities of water, all of which are
potentially hazardous to an aircraft.
Standing Lenticular Altocumulus Clouds.
Standing lenticular altocumulus clouds are formed on the crests of waves created by barriers in the
wind flow. The clouds show little movement, hence the name standing. Wind, however, can be
quite strong blowing through such clouds. They are characterized by their smooth, polished edges.
The presence of these clouds is a good indication of very strong turbulence and should be avoided.