Icing distribution by cloud, altitude, and terrain
Icing clusters by altitude, cloud type, front, terrain. Any season, anywhere in the country.
By cloud type:
| Stratiform | Cumuliform | |
|---|---|---|
| Layer shape | Thin; climb or descent of a few thousand feet can clear it, even in cloud | Horizontally smaller, vertically greater |
| Intensity | Trace to light | Trace in small cumulus up to severe in towering cumulus or cumulonimbus |
| Worst zone | Upper portion of cloud | Upper portion, where updraft concentrates supercooled drops (AWH 20.3.6) |
| Ice type | Rime and mixed (rime: TC AIM AIR 2.12.3.1; mixed: AWH 20.3.5) | Usually clear (TC AIM AIR 2.12.3.1), or mixed with rime in upper levels (AWH 20.3.6) |
| Main hazard | Great horizontal extent, not intensity (AWH 20.3.5) | Rapid severity in upper portion |
Fronts (AWH 20.3.7):
- Most icing reports occur near a front, above or below the frontal surface.
| Position | Air stability | Icing |
|---|---|---|
| Above front, lifted warm air | Unstable | Sporadic |
| Above front, lifted warm air | Stable | Continuous over extended area |
| Warm air over cold | - | Freezing rain icing can be severe over a large area |
Mountains (AWH 20.3.8):
- Icing more likely and more severe over high terrain.
- Windward slope upward motion supports large supercooled droplets above freezing level.
- Worst zone: above crests and windward ridges.
- Once ice builds, descent below freezing level or climb clear of ridge may be impossible.
Trap: treating "cloud" as one icing risk. Stratiform: thin, low-intensity layer, often escapable by climb or descent. Cumuliform: tall, uneven layer, can turn severe fast in its upper portion.
References
- AWH 20.3.5 (Icing in Stratiform Clouds)
- AWH 20.3.6 (Icing in Cumuliform Clouds)
- AWH 20.3.7 (Icing with Fronts)
- AWH 20.3.8 (Icing with Mountains)
- TC AIM AIR 2.12.3.1 (Types of Ice)