Clouds
Condensed water, classified by height and shape.
A cloud is water droplets or ice crystals suspended in air. It forms when rising air cools to its dew point and vapour condenses onto microscopic condensation nuclei — dust, salt, pollen. Without those nuclei, air can become supersaturated without forming cloud at all.
The ten types
| Level | Types | Typically means |
|---|---|---|
| High, above 6 km | Cirrus, cirrostratus, cirrocumulus | Ice crystals; cirrus often precedes a warm front |
| Middle, 2–6 km | Altostratus, altocumulus | Thickening altostratus signals rain within hours |
| Low, below 2 km | Stratus, stratocumulus, nimbostratus | Nimbostratus is steady prolonged rain |
| Vertical | Cumulus, cumulonimbus | Fair-weather cumulus; cumulonimbus means thunderstorms |
Reading the names
The Latin is systematic and worth learning once: cirro- high, alto- middle, strato- layered, cumulo- heaped, nimbo- raining. So cumulonimbus is a heaped raining cloud, and altostratus a mid-level layer.
Cumulonimbus
The only cloud that produces lightning, hail and tornadoes. It can extend from near the ground to the tropopause, where the flattened anvil top forms because the temperature inversion stops further rise.