Icing in Clouds

Cloud Types and Icing Risk

Cumuliform clouds (such as cumulus and cumulonimbus) are associated with the most severe aircraft icing. Their strong vertical currents suspend larger and more numerous supercooled water droplets, leading to rapid ice accumulation. Stratiform clouds (like stratus and nimbostratus) generally contain smaller droplets and produce lighter icing, but this can intensify over mountains due to orographic lifting, which increases droplet size and concentration.

Temperature and Icing Severity

Icing is most hazardous between 0°C and -12°C, where supercooled liquid water is abundant. Below -20°C, icing risk drops sharply as clouds transition to mostly ice crystals. At temperatures below -40°C, icing is extremely rare because only ice crystals remain.

Supercooled Water and Ice Crystal Icing

Supercooled water exists in clouds, rain, and drizzle when liquid droplets remain unfrozen below 0°C. When aircraft fly through these droplets, ice forms on the airframe. In high-level clouds like cirrus, only ice crystals are present, so conventional icing is not a concern. However, ice crystal icing can occur at high altitudes, especially near and downwind of large cumulonimbus clouds, even in clear air.

Orographic Effects

When air is forced to rise over mountains, orographic clouds form with higher concentrations of supercooled droplets. This increases icing intensity, particularly on windward slopes.

Icing Types and Hazards

• Clear ice: Forms from large supercooled droplets, common in cumulonimbus clouds.
• Rime ice: Forms from small droplets at lower temperatures, typical in stratiform or high-level clouds.
• Mixed ice: A combination of clear and rime ice, often found in nimbostratus.
• Ice crystal icing: Occurs at high altitude from ice crystals, affecting sensors and engines even outside visible cloud.

Icing Avoidance

Effective avoidance starts with pre-flight planning: analyzing weather reports, cloud base and top heights, freezing levels, and considering alternate routes. In flight, pilots should be prepared to change altitude, reroute, or turn back if icing is encountered.