ce can form whenever flying in visual precipitation such as rain or cloud droplets, and when the temperature is between +2 and -10 degrees C. Even if the outside air temperature is slightly above freezing, aerodynamic cooling can lower the temperature of the aircraft skin to the freezing point, causing ice to form. It is also possible for water droplets to exist in a supercooled state without solidification in temperatures below freezing. When they collide with an aircraft, they can quickly turn to ice.

With a standard (average) temperature lapse rate of 2 degrees C per 1000 ft, the freezing level can be determined by knowing the current temperature and elevation (e.g.—At a field elevation of 1350 ft MSL, the temperature is 8 degrees C. Thus, the freezing level is 4000 ft (8 degrees C divided by 2 degrees C per 1000 ft) above field elevation or 5350 ft. A safe guideline for minimizing icing risk is to avoid the freezing level by +/- 3000 feet.

Cirrus clouds generally do not result in structural icing because they are very high and moisture is already frozen in the form of ice crystals.

Stratus clouds typically produce rime icing, but they are generally not more than 3000 feet thick—an altitude change of +/- 3000 feet may take you out of icing conditions.

Cumulous-cumulonimbus formations have a high moisture content and often cause clear icing, but they have a limited horizontal dimension, so fly around them.