Cloud types — WMO classification
WMO classification of clouds by altitude layer.
The WMO classifies clouds by height, state of aggregation, and shape in the International Cloud Atlas (standard since 1956).
Source: WMO International Cloud Atlas (online, updated 2017); AMS Glossary.
Classification by height (levels)
Clouds are classified according to their height (levels). A differentiation is made between low clouds, medium clouds, high clouds and clouds of great vertical extent:
| Level | Range (mid latitudes) | Cloud types |
|---|---|---|
| Low | 0 — 2 km (~up to 6500 ft) | Sc, St, Cu |
| Medium | 2 — 7 km (6500 — 23 000 ft) | Ac, As |
| High | 5 — 13 km (16 000 — 43 000 ft) | Ci, Cc, Cs |
| Great vertical extent | surface to tropopause | Cb, Ns |
Low-level clouds
Low cloud levels are Sc, St, Cu:
- Stratus (St): grey uniform layer — fog-like.
- Stratocumulus (Sc): grey/white bands/patches with light/dark parts.
- Cumulus (Cu): white cotton-ball clouds with flat bottoms.
Greatest vertical extent
Cumulonimbus (Cb) and Nimbostratus (Ns) clouds can achieve the greatest vertical extent, far exceeding the typical heights of stratus, altocumulus, altostratus, stratocumulus, or cumulus:
- Cb: from a few hundred metres above the ground up to 12-15 km (tropopause).
- Ns: dense, thick, raining layer — usually 1-6 km tall.
Cumuliform — convective only
Cc, Ac, Cu and Cb are clouds generated exclusively by convection, as they are cumuliform types formed through localised vertical air ascent:
- Cc: high small clouds.
- Ac: medium small clouds.
- Cu: low cotton-ball clouds.
- Cb: thunderstorm clouds.
Classification by state of aggregation
Clouds are classified according to the state of aggregation of their particles. A differentiation is made between water, ice and mixed clouds:
| Type | Temperature range | Cloud types |
|---|---|---|
| Water clouds | T > 0 °C | Cu, Sc, low St |
| Mixed clouds | 0 °C to -40 °C | Ac, As, Ns, Cb |
| Ice clouds | T < -40 °C | Ci, Cs, Cc |
Cloud sequence at a warm front
When approaching a warm front, pilots typically encounter clouds in the sequence cirrus (Ci), cirrostratus (Cs), altostratus (As), nimbostratus (Ns), and stratus (St) due to the gradual ascent of warm air over cooler air:
- Starts with thin Ci high (500-800 km ahead of the front).
- Then Cs (halos around the sun).
- Then As (sun visible as disc).
- Then Ns (rain).
- Finally St (low on the ground).
Warning: altocumulus castellanus
There are typical clouds suggesting upcoming thunderstorms called altocumulus castellanus — turret- or pinnacle-shaped clouds:
- Medium height with small vertical turrets.
- Indicate unstable middle layer → thunderstorm potential in the following hours.
Cap clouds and lenticularis (orographic)
- Cap cloud: cloud directly on top of a mountain.
- Lenticularis: lens-shaped cloud on the lee side of a mountain → Foehn indicator (see local winds).
Clouds visible on satellite
Clouds can be recognised on satellite images — on both visible and IR satellite imagery. Detail in the met-products lesson.
Cloud-coverage terms
"Cloud coverage" means high or medium layers of clouds blocking thermals as well as almost overcast sky with Cu or Sc.
International Cloud Atlas* (cloudatlas.wmo.int, 2017+); AMS Glossary; ICAO Annex 3; FAA-H-8083-25B PHAK Ch. 12.*