Definition 1 of 2
Definition
A type of cloud characterized by vertical development, with a flat base and a heaped, dome-shaped or cauliflower-like top. Cumulus clouds form in unstable air when rising warm air (a thermal) cools to its dew point and the moisture condenses. They range from small, fair-weather puffs to towering build-ups that can develop into thunderstorms.
Plain English
A puffy, lumpy cloud with a flat bottom and a piled-up, cotton-ball top. It forms when warm air rises and cools enough for its moisture to condense into a visible cloud.
Context Anchor
Seen in aviation weather training, weather briefings, and in-flight cloud identification.
Derivation
From the Latin word 'cumulus,' meaning 'a heap' or 'a pile.' The name fits because these clouds look like heaped or piled-up masses of cotton, which helps distinguish them from the flat, layered look of stratus clouds.
Why Pilots Care
Fair-weather cumulus usually means good flying conditions, but growing cumulus can produce turbulence or develop into thunderstorms.
Analogy
A small cumulus cloud can look like a loose pile of cotton. The important idea is the heaped, built-up shape rather than a smooth layer.
Grounding Statement
Picture a warm patch of air rising from the ground, cooling as it climbs, and turning into a puffy cloud when moisture becomes visible.
Intuition Check
Cumulus does not automatically mean thunderstorm. It means a puffy, heaped cloud type; if it grows large and tall, then it may become more serious weather.
Example Sentence 1
On the climb-out, the pilot noticed scattered cumulus clouds forming over the hills, a sign that thermals were starting to develop.
Example Sentence 2
Light turbulence increased as the aircraft passed beneath building cumulus.