Pockets of warm rising air, called thermals, create the characteristic cauliflower look of cumulus clouds. Each cauliflower floret in a cloud is formed by individual thermal updrafts that rise until they reach air that is either the same temperature or warmer. This air acts as a barrier and forces the updrafts outward and downward (creating downdrafts). The cloud dissipates when the updrafts stop circulating. Cumulus clouds are usually fair weather clouds. However, rapidly growing cumulus clouds can sometimes turn into cumulonimbus, or thunderstorms, that can produce rain, thunder, lightning, and, if conditions are right, tornadoes.
Courtesy of University Corporation for Atmospheric Research