Cloud atlas

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

Stratus (St)

C_L · Low level · Surface–2,000 m (surface–6,500 ft)

A uniform grey layer with a low, often ragged base; fog that does not touch the ground.

Speciesnebulosus fractus

Varietiesopacus translucidus undulatus

Featurespraecipitatio fluctus

When the sun is visible through stratus, its outline is sharp — the opposite of altostratus. It forms in gentle lifting of a moist stable layer, or as advection fog lifting off the surface. Hill fog is simply stratus meeting high ground; the same layer overhead gives long dull spells and, at most, drizzle.

Precipitation: Drizzle, snow grains or ice prisms.

Name: stratus — spread out, layered

Photo: Wikimedia Commons

Back to undulatus

Stratocumulus (Sc)

C_L · Low level · Surface–2,000 m (surface–6,500 ft)

Grey or whitish patches of rounded masses or rolls, elements wider than five degrees.

Speciesstratiformis lenticularis castellanus volutus

Varietiestranslucidus perlucidus opacus duplicatus undulatus radiatus lacunosus

Featuresvirga mamma praecipitatio asperitas fluctus cavum

The most common cloud on Earth and the mainstay of a dull British day. Elements are large and clearly shaded, often arranged in rolls with blue sky between. It forms where shallow convection is capped by an inversion, and typically breaks up as the inversion lifts or the wind eases toward evening.

Precipitation: Light drizzle, rain or snow grains.

Name: stratus + cumulus — a layer of heaps

Photo: Wikimedia Commons

Cumulus (Cu)

C_L · Low level base, with vertical development · Base 600–2,000 m (2,000–6,500 ft); tops to 6,000 m (20,000 ft) when congestus

Detached, sharply outlined heaps with brilliant white tops and flat, dark bases.

Specieshumilis mediocris congestus fractus

Varietiesradiatus

Featurespileus velum virga praecipitatio arcus pannus tuba fluctus

The archetypal convective cloud, produced by thermals rising from surface heating. Depth tells the story: humilis is wider than it is tall and means fair weather, mediocris is roughly as tall as it is wide, and congestus is a towering cauliflower that can produce heavy showers and may glaciate into cumulonimbus.

Precipitation: Showers from congestus; none from humilis.

Name: cumulus — a heap or pile

Photo: Wikimedia Commons

Cumulonimbus (Cb)

C_L · Low level base, growing through all levels · Base 600–2,000 m (2,000–6,500 ft); tops to the tropopause, above 12,000 m (40,000 ft)

A dense, towering storm cloud with a fibrous or striated upper part, often anvil-shaped.

Speciescalvus capillatus

Featuresincus mamma pileus velum virga praecipitatio arcus murus cauda tuba pannus

The only genus that produces thunder, hail and tornadoes. Glaciation of the summit marks the change from cumulus congestus: a cauliflower top becomes fibrous (calvus), then spreads into the classic anvil (capillatus with incus) as it strikes the tropopause. Supplementary features such as arcus, mamma, murus and tuba all belong here.

Precipitation: Heavy showers, hail, thunder and lightning.

Name: cumulus + nimbus — a rain-bearing heap

Photo: Wikimedia Commons