Wikipedia:POTD column/May 19, 2006

Cumulus clouds are characterized by dense individual elements in the form of puffs, mounds or towers, with flat bases and tops that often resemble cauliflower. They are formed due to convection. Buoyant, upward air currents, known as thermals rise to a height at which the moisture in the air can condense. Because of this, they "grow" vertically instead of horizontally. Though most common in warm, summer weather, cumulus clouds can be formed at any time of year.

Photo credit: Fir0002
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