Cloud atlas

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

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Altocumulus (Ac)

C_M · Medium level · 2,000–7,000 m (6,500–23,000 ft)

White or grey patches of rounded masses or rolls, often partly shaded.

Speciesstratiformis lenticularis castellanus floccus volutus

Varietiestranslucidus perlucidus opacus duplicatus undulatus radiatus lacunosus

Featuresvirga mamma asperitas fluctus cavum

The shading is what separates altocumulus from cirrocumulus: the elements are large enough, and contain enough supercooled water, to have darker undersides. Turreted altocumulus castellanus on a summer morning indicates mid-level instability and often precedes thunderstorms later in the day.

Precipitation: Virga only; rarely reaches the ground.

Name: altum + cumulus — a heap at height

Photo: Wikimedia Commons

Altostratus (As)

C_M · Medium level · 2,000–7,000 m (6,500–23,000 ft)

A greyish striated or uniform sheet through which the sun shows weakly.

Varietiestranslucidus opacus duplicatus undulatus radiatus

Featuresvirga praecipitatio pannus mamma

The 'watery sun' cloud: the disc is visible as though through ground glass, and — crucially — it never produces a halo, which distinguishes it from cirrostratus. Shadows are lost. A mixed layer of ice, snow and supercooled droplets, it commonly thickens downward into nimbostratus as a front closes in.

Precipitation: Light continuous rain or snow, often as virga.

Name: altum + stratus — a layer at height

Photo: Wikimedia Commons

Nimbostratus (Ns)

C_M · Medium level, but usually spanning all levels · Base often below 2,000 m (6,500 ft); body extends above 6,000 m (20,000 ft)

A dark grey, featureless layer giving continuous rain or snow.

Featurespraecipitatio virga pannus

Thick enough to blot the sun out completely, nimbostratus has a diffuse base frequently obscured by ragged pannus scudding beneath it. It is the only genus defined partly by what it does rather than how it looks: continuous, widespread precipitation of the kind that follows a warm front for hours at a time.

Precipitation: Continuous rain, snow or ice pellets.

Name: nimbus + stratus — a rain-bearing layer

Photo: Wikimedia Commons