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Characteristics

Cumulus humilis Abbreviation: Cu hum

Appearance: Shallow and flattish

Cloud-base: 1,000–5,000 ft (300–1,500 m)

Cause: Warming of land and convection

Composition: Water droplets, sometimes supercooled

Weather: Fine

These “cotton-wool” clouds of fair days may be widely spread out or almost touching each other. Often occurring with only light breezes, they have flattish bases, all at the same height. They look mid-grey from beneath although their tops can be dazzlingly white if there are no higher clouds to obscure the Sun.

As with other cumulus clouds, they form where rising air currents caused by a warming land cause water vapour in moist air to condense as tiny water droplets. Fair-weather cumulus does not contain ice crystals. These clouds often form just inland of coastlines, where summer sea breezes carry moist air over warm land. The bottom of the clouds shows the condensation level in the atmosphere. Although fair-weather cumulus clouds never produce rain or snow, they can be the precursors of thicker cloud and rain.

The rising currents within these clouds are generally fairly light. If the air is stable, they remain shallow, sometimes pancake-flat, and may evaporate within a few minutes. However, fair-weather cumulus can also indicate unstable conditions, in which case they are likely to grow bigger and taller—and could develop later into rain-bearing forms of cumulus.