On a sunny afternoon, scattered white puffs with flat bases often appear across the sky. These are cumulus clouds. They form when surface heating causes air parcels to rise, cool, and condense at the lifting condensation level.

Fair-weather cumulus remain modest in size and produce little or no rain. When the atmosphere is more unstable, individual clouds can grow taller and evolve into towering cumulus or even cumulonimbus. The flat base marks the altitude where condensation begins; the rounded tops show the rising thermals.

These clouds are useful indicators of the depth of the mixed layer and the potential for later storm development. Photographers prize their bright, sharply defined shapes against blue sky. In the tropics, trade-wind cumulus form in long streets aligned with the wind.

Their presence usually signals daytime heating rather than large-scale forced lift.