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AVIATION WEATHER
CHAPTER TWO
repetition of this process increases the size of the hailstone. It does not lead to the formation of
structural ice, but it can cause structural damage to aircraft.
6.
Ice pellets or sleet Small translucent and irregularly shaped particles of ice. They form
when rain falls through air with temperatures below freezing. They usually bounce when hitting
hard ground and make a noise on impact. Ice pellets do not produce structural icing unless
mixed with super-cooled water.
7.
Snow White or translucent ice crystals, usually of branched hexagonal or star-like form
that connect to one another forming snowflakes. When condensation takes place at temperatures
below freezing, water vapor changes directly into minute ice crystals. A number of these crystals
unite to form a single snowflake. Partially melted, or "wet" snow, can lead to structural icing.
8.
Snow grains Very small white, opaque grains of ice. When the grains hit the ground,
they do not bounce or shatter. They usually fall in small quantities from stratus-type clouds,
never as showers.
Precipitation, depending on the type and intensity, affects aviation in many ways:
Visibility in light rain or drizzle is somewhat restricted. In heavy rain or drizzle, it may drop to a
few hundred feet. Rain or drizzle streaming across a windscreen further restricts forward
visibility. Snow can greatly reduce visibility and can lead to a total lack of forward vision.
Very heavy rain falling on a runway may cause hydroplaning. During hydroplaning, the tires are
completely separated from the runway surface by a thin film of water. Tire traction becomes
negligible and the wheels may stop rotating. The tires now provide no braking capability and do
not contribute to directional control of the aircraft. Loss of control may result.
If there is enough wet snow on the runway, it tends to pile up ahead of the tires during takeoff.
This creates sufficient friction to keep the aircraft from reaching rotation speed and becoming
airborne.
Heavy rain ingested into the engines of a jet or turboprop aircraft in flight can cause power loss
or even flameout.
Hail can cause serious damage to any aircraft, but so can rain if it is penetrated at very high
speed.
Clouds
Clouds may be defined as the visible manifestation of weather. With some knowledge of the
weather conditions that cause clouds to develop, a pilot can get an excellent picture of the
weather environment and can make a reasonable forecast of the weather conditions to follow.
The most important element in the formation of clouds is water vapor.
Atmospheric Mechanics of Winds, Clouds and Moisture, and Atmospheric Stability
2-15


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