Cloud atlas

The ten genera, fifteen species and nine varieties of the WMO International Cloud Atlas, with symbols and abbreviations.

Cirrus (Ci)

C_H · High level · 5,000–13,000 m (16,500–45,000 ft)

Detached white filaments or narrow bands with a fibrous, silky look.

Speciesfibratus uncinus spissatus castellanus floccus

Varietiesintortus radiatus vertebratus duplicatus

Featuresmamma fluctus

Composed entirely of ice crystals, cirrus is transparent enough that it never dims the sun appreciably. Falling crystals sheared by the wind produce the trailing hooks and tails known as fallstreaks. A steadily thickening, invading cirrus sheet is the classic first sign of an approaching warm front.

Precipitation: None reaching the ground.

Name: cirrus — a lock or tuft of hair

Photo: Wikimedia Commons

Back to mamma

Cirrocumulus (Cc)

C_H · High level · 5,000–13,000 m (16,500–45,000 ft)

A thin sheet of tiny granules or ripples, none wider than one degree.

Speciesstratiformis lenticularis castellanus floccus

Varietiesundulatus lacunosus

Featuresvirga mamma cavum

The rarest of the high clouds and the only one that is rippled rather than smooth or fibrous. The apparent width of each element is the diagnostic: if the granules are wider than a finger held at arm's length, it is altocumulus. Extensive rippled sheets give the traditional mackerel sky.

Precipitation: None reaching the ground.

Name: cirrus + cumulus — a heap of filaments

Photo: Wikimedia Commons

Cirrostratus (Cs)

C_H · High level · 5,000–13,000 m (16,500–45,000 ft)

A transparent whitish veil, often covering the whole sky and producing a halo.

Speciesfibratus nebulosus

Varietiesduplicatus undulatus

So thin that shadows still fall on the ground, cirrostratus is best identified by the 22° halo it throws around the sun or moon — refraction through hexagonal ice prisms. A cirrostratus veil that lowers and thickens into altostratus is a dependable warm-front signature, usually with rain 12–24 hours away.

Precipitation: None reaching the ground.

Name: cirrus + stratus — a spread-out veil

Photo: Wikimedia Commons