Meteorologists sort clouds into ten genera, each with a Latin name built from a handful of roots: cirro- for what is high and made of ice, alto- for the middle level, strato- for what spreads in a layer, cumulo- for what heaps up, nimbo- for what brings rain. Once you know the roots, a name such as altocumulus or cirrostratus almost reads itself.
This classification, laid down by the World Meteorological Organization in its International Cloud Atlas, rests on two criteria: the height of the cloud base and the cloud's shape. Observers, pilots and forecasters have used it for over a century, and it is still the best way to make sense of the sky.
Three levels, one base height
The high level starts at about 5,000 m and reaches 13,000 m in temperate latitudes. It holds cirrus, cirrocumulus and cirrostratus: thin, white clouds of ice crystals. The middle level runs from about 2,000 to 7,000 m: altocumulus and altostratus belong there, and nimbostratus, the cloud of steady rain, has its top there. The low level goes from the ground up to 2,000 m, with stratus, stratocumulus and cumulus.
Cumulonimbus and nimbostratus spill beyond their level: their bases are low, while their tops reach the middle level for the one and the tropopause for the other. They are therefore classed by their base and recognised by their thickness. These heights are lower near the poles and higher towards the equator, and lower in winter than in summer.
To judge a cloud's level without instruments, look at its detail. At low levels you can make out hollows, shadows and sharp edges. The higher the cloud, the thinner, whiter and smoother it looks, because its elements appear smaller to the eye.
The ten genera in one table
| Genus | Level | Base height | Appearance | Associated weather |
|---|---|---|---|---|
| Cirrus | High level | 5,000 to 13,000 m | white, silky wisps with no shading of their own | fair weather; a warning of change if it thickens into a veil |
| Cirrocumulus | High level | 5,000 to 13,000 m | banks of tiny white grains with no grey side | change possible; short-lived |
| Cirrostratus | High level | 5,000 to 13,000 m | whitish, transparent veil with a halo | often signals rain or snow within the day |
| Altocumulus | Middle level | 2,000 to 7,000 m | white and grey rounded patches or rolls, in banks | often fair; castellanus: thunderstorm risk |
| Altostratus | Middle level | 2,000 to 7,000 m | grey or bluish sheet, sun as if behind frosted glass | rain or snow close, especially if the sky lowers |
| Nimbostratus | Middle level | 0 to 2,000 m | dark grey, very thick layer with a blurred base | steady rain or snow lasting hours |
| Stratocumulus | Low level | 0 to 2,000 m | large grey and white rolls or patches, joined together | grey, dry sky; drizzle or light rain possible |
| Stratus | Low level | 0 to 2,000 m, often close to the ground | uniform grey layer with no features, very low base | low grey sky; drizzle possible |
| Cumulus | Low level | 0 to 2,000 m | white heaps with domed tops, flat greyish base | fair weather; if it grows, showers and storms possible |
| Cumulonimbus | Low level | 0 to 2,000 m (top up to the tropopause) | massive tower, dark base, anvil-shaped top | thunderstorm, violent showers, hail, lightning, gusts |
The ten genera, drawn
How to use this table
Start with the level: look up and ask whether the cloud is high and thin, middle and dappled, or low and bulky. Then look at the shape: filaments, grains, a continuous veil, rounded heaps or a uniform grey layer. The name follows from the two. A thin veil of ice crystals at high level is cirrostratus; a thick grey layer that hides the sun and gives steady rain is nimbostratus.
Often several genera are stacked in the same sky. Observers then record each layer separately, from lowest to highest, with the share of sky it covers in eighths (oktas). The "today" block on each cloud page follows the same idea: it gives the forecast average cover at each level for the country's largest city.
Sources
- World Meteorological Organization (WMO), International Cloud Atlas, WMO-No. 407: definitions of the genera, levels and base heights for temperate regions. https://cloudatlas.wmo.int (copyright WMO; no text or image from the Atlas is reproduced here).
- Drawings and text: original to this site.