Occurrences of Icing
The atmospheric distribution of icing depends on temperature and cloud structures,
which vary with altitude, synoptic situation, season, location, and terrain.
Icing and Temperature
Aircraft icing generally occurs between the freezing level and -22 degrees Celsius/-7
degrees Fahrenheit. Icing can also occur in the upper parts of cumulonimbus clouds.
The type and amount of ice varies with each type of cloud.
Stable air masses often produce stratiform clouds with extensive areas of continuous
icing conditions. Icing typically occurs in layers of 3000 to 4000 feet and is found at
elevations where the temperatures range from -1 deg C to -15 deg C. It normally forms
as rime ice type icing. High-level stratiform clouds contain mostly ice crystals and
produce little to no icing.
The zone of icing is smaller horizontally but greater vertically than in stratiform clouds.
Expect clear icing at flight altitudes where the temperatures vary from 0 deg C to 8 deg
C, mixed icing from -9 deg C to -15 deg C, and rime ice from -15 deg C to -22 deg C.
Clear and mixed icing will extend to greater vertical levels in the updraft and in the
Icing in Relation to Fronts
Fronts provide the lifting mechanism to form clouds and therefore concentrated areas
for icing. All types and intensities may be encountered and are dependent on the
instability aloft, speed, and slope of the front. Overrunning warm fronts and shallow cold
fronts are extremely hazardous as they may generate large areas of freezing
rain/drizzle. Severe clear icing is often associated with this situation.
Icing in Relation to Terrain
Icing is more likely and more severe in clouds over mountainous regions than over other
terrain. The strong upslope flow on the windward side of a range may lift large water
droplets upwards to 5,000 feet into subfreezing layers above a peak. If a frontal system
moves across a mountain range, the normal frontal lift combines with the orographic
effects to create extremely hazardous icing zones.