Precipitation type & freezing rain by temperature profile
Temperature profile decides precipitation type (AWH 14.4):
| Type | Warm layer (above freezing) | Surface layer | Result |
|---|---|---|---|
| Snow | None anywhere | Below freezing, top to bottom | Reaches the ground without melting |
| Ice pellets | Shallow, aloft | DEEP freezing layer | Snow partly melts, then refreezes into pellets (TP 10643) |
| Freezing rain | Deep, aloft | Shallow freezing layer | No time to refreeze into pellets: drops freeze on contact with ground or exposed objects, such as aircraft (AWH 14.4) |
| Rain | Deep, based at the surface | Above freezing | Drops stay liquid |
- Ice pellets and freezing rain share one origin: the depth of the surface cold layer decides which one falls (PHAK ch.12).
- Ice pellets at the surface mean freezing rain above (PHAK ch.12).
Freezing rain and drizzle aloft: classical mechanism is warm air over cold air (inversion).
- Snow falls through the warm layer and melts.
- Drops enter the cold layer below and become supercooled.
- Drops freeze on contact with the aircraft.
- Typically warm fronts and trowals.
- Cold fronts: less common, narrower (steeper slope).
Classical vs non-classical:
| Classical | Non-classical | |
|---|---|---|
| Inversion / warm air aloft | Yes | None |
| Research aircraft hits | n/a | Down to -10 °C, up to 15,000 ft ASL, no warm layer above |
| Escape | Climb above melting layer and above 0 °C: no more supercooled drops form | Climb (still preferred) above the formation region of the drizzle |
| Descending | May not exit: warm-over-cold layer can be wide (AWH 20.3.7) | Can stay in freezing precipitation |
- Non-classical climb caveat: can move aircraft closer to source region (smaller droplets, higher liquid water content), where conventional icing can occur instead.
Trap: "no warm air aloft" is not "no risk"; non-classical climb works by getting above the drizzle formation layer, not by finding above-freezing air. And a shallow warm layer aloft alone does not make ice pellets; the surface cold layer must be deep enough to refreeze the melted drop solid.
References
- TC AIM AIR 2.12.3.5
- AWH 20.3.7 (Icing with Fronts)
- AWH 14.4
- PHAK ch.12 Precipitation
- TP 10643 (Ice Pellets)