Precipitation includes all forms of moisture that fall to the earth s surface. Snow, drizzle,
and rain are the most common forms of precipitation that cause a restriction to visibility
(see Table 1-11).
Usually the most effective in reducing visibility. Heavy snow and
blowing snow frequently reduce surface visibility to near zero.
Rarely reduces surface visibility to below 1 mile. Has a tendency to
wash dust, smoke, and fog out of the air.
Often accompanied by fog and results in lower visibility than rain.
Table 1-11 -- Common forms of precipitation
A severe weather hazard to flying is airframe icing. Having a thorough understanding of
when and how ice forms on aircraft will enable you to lend invaluable assistance to
Icing interferes with the performance of aircraft by increasing drag and weight while
decreasing lift, changing the airfoil shape of the wings and tail. There are four types of
airframe ice: rime, clear (glaze), mixed, and frost. Icing conditions encountered in flight
are a combination of rime and clear ice, with the characteristics of one or the other
being dominant. Frost usually forms on aircraft on the ground. For ice to form on aircraft
in flight, two conditions are necessary: the aircraft must be flying through rain, drizzle, or
cloud droplets, and at the time the water droplets strike the aircraft, their temperature
and the temperature of the surface of the aircraft must be 32° F or colder. Heaviest
airframe icing generally occurs within the temperature range of 15°F to 32°F provided
moisture is available. Engine system icing reduces the effective power of aircraft
engines. Not all aircraft have de-icing capability, while others have limited de-icing
capability and must avoid icing conditions at all times.
Clear ice, sometimes referred to as glaze ice, is considered the most serious of the four
types. It is clear, dense, solid, and adheres firmly to the structure upon which it forms.
The water droplets strike the aircraft in such rapid succession that none have a chance
to freeze before the next strikes in the same place, and it is more difficult to remove with