Recognising clouds

The essentials in two minutes

1. Three keys name the ten cloud genera: the level (the prefix cirro- for high, alto- for middle, nothing for low), the shape (heaps or pebbles: unstable air; sheets or veils: stable air) and rain (nimbo-, -nimbus).
2. Cumulus is the visible part of a thermal: its flat base marks the condensation level, and its shape measures the day’s energy, from humilis to cumulonimbus.
3. A fair-weather cumulus lives 15 to 30 minutes: aim for the one being born (sharp outlines, dark base), not the one that looks beautiful.
4. A congestus before midday, an altocumulus castellanus in the morning: thunderstorms are likely, so fly and walk early.
5. Ice clouds are not dangerous, but they give warning: cirrus spreading in from the west, a halo, then altostratus — rain within 24 to 36 h.
6. Lenticulars, cap cloud, banner cloud: strong wind at altitude, waves and rotors; no flying, even if launch is calm.
7. A sea of clouds (under an inversion: you get out of it from above) and hill fog (on the windward slope: you get out of it by crossing to the other side) have nothing in common.
8. What matters is not the cloud but how it changes: look at the sky every half hour.