The first step in ice formation is when the supercooled droplets strike the surface of the aircraft. As the droplet, or portion of it, freezes, it liberates the heat of fusion. The unfrozen drop now begins to evaporate due to its increase in temperature, and in the process, it uses up some of the heat, which, in turn, cools the drop. Due to this cooling process by evaporation, the remainder of the drop is frozen. Icing at 0 C will occur only if the air is not saturated because the nonsaturated condition is favorable for evaporation of part of the drop.
Evaporation cools the drop below freezing, and then ice formation can take place.
There are three intensities of aircraft icing light, moderate, and severe. Immediate diversion is necessary. One hazard results when wet snow is falling during takeoff. This situation can exist when the air temperature at the surface is at or below 0 C. Wet snow sticks tenaciously to aircraft components, and it freezes if the aircraft encounters markedly colder temperatures during takeoff. If not removed before takeoff, frost, sleet, freezing rain, and snow accumulation on parked aircraft become operational hazards.