Cumulus

Definition

A genus of detached, generally dense cloud with sharp outlines, developing vertically in the form of rising mounds, domes or towers, with a flat, often dark base and a brilliant white top when lit by the sun.

Field note

Cumulus bases mark the lifting condensation level, so their height is a measurement: base in metres is roughly 125 times the spread between the surface temperature and the dew point, both in °C.

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Cumulus: reading the fair-weather heaps

Cumulus is the first cloud most people learn to name, and the one most often named wrongly. It is a convective cloud: it exists because a parcel of air near the ground got warmer than its surroundings and rose until it hit its condensation level. Everything you can read from a cumulus — base height, edge sharpness, vertical growth rate — comes directly from that single fact.

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Related terms

• Cumulonimbus · Cumulonimbus
• Humilis · humilis
• Mediocris · mediocris
• Congestus · congestus