Cloud Type and Icing
Air can rise because of many factors, including convection, orographic lifting, or lifting along a front. As the air rises, it expands and cools adiabatically. If a parcel of air reaches its saturation point, the moisture within the parcel will condense and the resulting drops form a cloud. Cloud water drops are generally very small, averaging 20 micrometres in diameter, and are of such small mass that they can be held aloft by small air currents within clouds.
In some cases, rising moist air can result in clouds with substantial liquid water content and large drops. The greater the liquid water content, the more rapid the icing and the larger the drops, the greater the extent of icing. Tops of clouds often contain the most liquid water and largest drops, because the drops that reach the tops have undergone the most lifting.
Several types of clouds and the hazardous aircraft icing conditions that may be associated with them are discussed below.
Stratus Clouds
Stratus clouds, sometimes called layer clouds, form a stratified layer that may cover a wide area. The lifting processes that form them are generally quite weak, and so they rarely have exceptionally high liquid water content. Icing layers in stratus clouds with a vertical thickness in excess of 3,000 feet are rare, so a change of altitude of a few thousand feet may take the aircraft out of icing.
Cumulus Clouds
Cumulus clouds, which often form because of vigorous convection, can have high liquid water content. If an aircraft traverses them, icing can develop rapidly. Because of the vertical development of cumulus clouds, icing conditions can be found in layers thousands of feet in depth, but with a much smaller horizontal development than in stratus clouds.