Cloud atlas

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

Species

Shape and internal structure. A cloud has one species at most.

Species subdivide the genera. They are mutually exclusive: a cloud cannot be both humilis and congestus. Some genera — altostratus and nimbostratus — have no species at all.

fibratus

fibfibrous

Detached filaments or a thin veil of nearly straight strands, with no hooks or tufts at the ends.

Applies toCi Cs

uncinus

unchooked

Comma-shaped filaments ending in a hook or tuft, the head marking where the ice crystals formed.

Applies toCi

spissatus

spithickened

Dense enough to appear greyish against the sun; frequently the remains of a cumulonimbus anvil.

Applies toCi

castellanus

casturreted

Turrets rising from a common base, taller than they are wide — a marker of instability at that level.

Applies toCi Cc Ac Sc

floccus

flotuft of wool

Small ragged cumuliform tufts with a rounded top and often virga trailing beneath.

Applies toCi Cc Ac

stratiformis

strlayered

Spread out in an extensive horizontal sheet or layer.

Applies toCc Ac Sc

nebulosus

nebmisty

A featureless veil or layer showing no distinct detail at all.

Applies toCs St

lenticularis

lenlens-shaped

Smooth almond or lens shapes, usually stationary in the standing wave downwind of hills.

Applies toCc Ac Sc

volutus

volrolled

A long detached horizontal tube that appears to roll slowly about a horizontal axis.

Applies toAc Sc

fractus

frabroken

Ragged, shredded fragments with constantly changing outlines.

Applies toSt Cu

humilis

humlowly

Flattened cumulus of small vertical extent — fair-weather cumulus.

Applies toCu

mediocris

medmoderate

Moderate vertical extent with small sproutings on the top.

Applies toCu

congestus

conpiled up

Markedly sprouting, cauliflower-topped towers of great vertical extent; produces heavy showers.

Applies toCu

calvus

calbald

Cumulonimbus whose upper protuberances have started to lose their sharp outline as they glaciate — but with no cirriform anvil yet.

Applies toCb

capillatus

caphaving hair

Cumulonimbus with a clearly fibrous or striated cirriform upper part, usually an anvil.

Applies toCb