Precipitation formation processes
Precipitation: water or ice that falls from a cloud and reaches the ground.
Needs all three (AWH 14.2):
- Water vapour
- Enough lift to condense it into cloud
- Growth process building droplets big enough to fall
Cloud depth (AWH 14.2):
- Rule of thumb: significant precipitation needs cloud at least 4,000 ft thick.
- Aerodrome reports light or greater precipitation: expect thicker cloud.
Growth: condensation particles grow into drops by collision and coalescence (AWH 6.2.7), via one of two processes (AWH 14.3):
| Process | Cloud | Mechanism | Primary in |
|---|---|---|---|
| Collision-coalescence (warm rain) | Warm | Droplets of different fall speeds collide and merge | Warm tropical air masses (freezing level very high) |
| Ice crystal | Colder, ice crystals and water droplets together | Vapour deposits onto crystals; crystals grow at droplets' expense until heavy enough to fall as snow (melts to rain if surface warm) | Mid and high latitudes |
Trap: assuming any cloud can rain; significant precipitation needs cloud at least 4,000 ft thick.
References
- AWH 14.2-14.3