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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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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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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.
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Stability
Stability of an air mass determines its typical weather characteristics. When one type of air mass
overlies another, conditions change with height. Characteristics typical of an unstable and a stable
air mass are as follows:
Unstable Air Stable Air
Cumuliform clouds Stratiform clouds and fog
Showery precipitation Continuous precipitation
Rough air (turbulence) Smooth air
Good visibility (except in blowing obstructions) Fair to poor visibility in haze and smoke
Fronts
As air masses move out of their source regions, they come in contact with other air masses of different
properties. The zone between two different air masses is a frontal zone or front. Across this zone,
temperature, humidity and wind often change rapidly over short distances.
Mountain Flying
When planning a flight over mountainous terrain, gather as much preflight information as possible on
cloud reports, wind direction, wind speed, and stability of air. Satellites often help locate mountain
waves. Adequate information may not always be available, so remain alert for signposts in the sky.
Wind at mountain top level in excess of 25 knots suggests some turbulence. Wind in excess of 40 knots
across a mountain barrier dictates caution. Stratified clouds mean stable air. Standing lenticular and/or
rotor clouds suggest a mountain wave; expect turbulence many miles to the lee of mountains and
relative smooth flight on the windward side. Convective clouds on the windward side of mountains
mean unstable air; expect turbulence in close proximity to and on either side of the mountain.
Structural Icing
Two conditions are necessary for structural icing in flight:
1. The aircraft must be flying through visible water such as rain or cloud droplets
2. The temperature at the point where the moisture strikes the aircraft must be 0° C or colder.
Aerodynamic cooling can lower temperature of an airfoil to 0° C even though the ambient temperature
is a few degrees warmer.
Thunderstorm Life Cycle
A thunderstorm cell during its life cycle progresses through three stages-( 1) the cumulus, (2) the
mature, and (3) the dissipating. It is virtually impossible to visually detect the transition from one stage
to another; the transition is subtle and by no means abrupt. Furthermore, a thunderstorm may be a
cluster of cells in different stages of the lifecycle.
The Cumulus Stage
Although most cumulus clouds do not grow into thunderstorms, every thunderstorm begins as a
cumulus. The key feature of the cumulus stage is an updraft as illustrated in figure 3-4. The updraft
varies in strength and extends from very near the surface to the cloud top. Growth rate of the cloud
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Remote Pilot – Small Unmanned Aircraft Systems Study Guide 27
may exceed 3,000 feet per minute, so it is inadvisable to operate a small UA in an area of rapidly
building cumulus clouds. Early during the cumulus stage, water droplets are quite small but grow to
raindrop size as the cloud grows. The upwelling air carries the liquid water above the freezing level
creating an icing hazard. As the raindrops grow still heavier, they fall. The cold rain drags air with it
creating a cold downdraft coexisting with the updraft; the cell has reached the mature stage.
The Mature Stage
Precipitation beginning to fall from the cloud base is your signal that a downdraft has developed and
a cell has entered the mature stage. Cold rain in the downdraft retards compressional heating, and
the downdraft remains cooler than surrounding air. Therefore, its downward speed is accelerated
and may exceed 2,500 feet per minute. The down rushing air spreads outward at the surface as
shown in figure 3-4 producing strong, gusty surface winds, a sharp temperature drop, and a rapid
rise in pressure. The surface wind surge is a "plow wind" and its leading edge is the "first gust."
Meanwhile, updrafts reach a maximum with speeds possibly exceeding 6,000 feet per minute.
Updrafts and downdrafts in close proximity create strong vertical shear and a very turbulent
environment. All thunderstorm hazards reach their greatest intensity during the mature stage.
The Dissipating Stage
Downdrafts characterize the dissipating stage of the thunderstorm cell as shown in figure 3-4 and
the storm dies rapidly. When rain has ended and downdrafts have abated, the dissipating stage 'is
complete. When all cells of the thunderstorm have completed this stage, only harmless cloud
remnants remain.
Figure 3-4. Life cycle of a thunderstorm.
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Ceiling
For aviation purposes, a ceiling is the lowest layer of clouds reported as being broken or overcast, or
the vertical visibility into an obscuration like fog or haze. Clouds are reported as broken when five-
eighths to seven-eighths of the sky is covered with clouds. Overcast means the entire sky is covered
with clouds. Current ceiling information is reported by the aviation routine weather report (METAR)
and automated weather stations of various types.
Visibility
Closely related to cloud cover and reported ceilings is visibility information. Visibility refers to the
greatest horizontal distance at which prominent objects can be viewed with the naked eye. Current
visibility is also reported in METAR and other aviation weather reports, as well as by automated
weather systems. Visibility information, as predicted by meteorologists, is available for a pilot during a
preflight weather briefing.