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Characteristics

Cumulus Abbreviation: Cu

Appearance: Heaped

Cloud-base: From 2,000 ft (600 m)

Cloud-top: Up to 70,000 ft (21,000 m) in the tropics

Cause: Warming of land and convection

Weather: Wide range, from fair to stormy, and from dry to torrential rain

Cumulus clouds are probably the most familiar. The Latin root of the word cumulus, meaning ‘heap’, is a good descriptor of the appearance of these clouds, but they are very variable. They range from the benign, fluffy “cotton wool” clouds—fair-weather cumulus, or cumulus humilis—of a fine summer’s day to towering, menacing cumulonimbus clouds, which bring hailstones, thunder, and lightning, and sometimes even spawn tornadoes.

Whatever their scale, all cumulus clouds develop as a result of convection, warm air rising. As it rises, it cools and any water vapour it contains condenses to produce cloud. Depending on local atmospheric conditions, convection may be very limited, producing small clouds, which may later evaporate again. If the warming process continues, bigger clouds will form. In extreme circumstances these may tower to the top of the troposphere, where they flatten out in gleaming white anvils. Such clouds produce storms.

Cumulus and cumulonimbus have relatively low cloud bases, but the suffix “cumulus” is also added to mid- and high-altitude clouds, altocumulus and cirrocumulus respectively.