Cloud features and what they signal
Cloud features to recognise on sight:
| Feature | Look / forms |
|---|---|
| Altostratus | Sun like ground glass, no sharp shadows, no halo |
| Cirrostratus | Halo |
| Mares' tails | Wisps trailed by cirrus ice crystals large enough to fall appreciably |
| Mammatus | Cellular pouches; can form under cumulonimbus anvil |
| Lenticular | Lens-shaped, on mountain-wave crest |
| Scud (stratus fractus / cumulus fractus) | Ragged low-cloud shreds, forming briefly before, during, after precipitation (FAA AWH) |
| Fair-weather cumulus | Fully formed cumulus, clear-cut horizontal base, flattened or slightly rounded top (FAA AWH A) |
Scud forms under:
- Altostratus
- Nimbostratus
- Cumulonimbus (dark base; can form just a few hundred feet above surface) (FAA AWH)
- Precipitating cumulus
Signals:
| Cloud | Signal | Ref |
|---|---|---|
| Altostratus | Light to moderate icing in supercooled water | FAA AWH A.1.3, A.2.2 |
| Cirrostratus | Little if any icing | FAA AWH A.1.3, A.2.2 |
| Anvil cumulonimbus | Squalls and thunder often accompany | FAA AWH |
- Altostratus vs cirrostratus: confusing the two misjudges icing risk.
References
- FAA Aviation Weather Handbook (FAA-H-8083-28) Appendix A