Click here for picture
Click here for picture
Fair weather cumulus are fueled by buoyant bubbles of air, or thermals, that rise upwards from the earth's surface. As they rise, the air within the thermal cools and the water vapor condenses, forming cloud droplets. The height above the ground where this process takes place is known as the lifting condensation level, and the location of this level is easily identified by the distinct flat cloud bases commonly associated with fair weather cumulus.
Since air is primarily rising within the clouds themselves, there must be "subsidence", or downward motion of air between the clouds. Subsiding air slowly descends to replace the warmer air that has ascended to higher levels from below. A valuable rule of thumb to remember is that: if clouds appear, they are most likely to develop where there is some type of rising motion. In contrast, sinking air, or downward motions, inhibit cloud development, and as a result, clear skies are common in areas of subsidence.
Clouds in their early stages of development have sharply defined edges, while in older clouds, their bases and edges are more ragged in appearance. This reveals the process of cloud erosion. The evaporation of cloud edges cools the air surrounding the cloud, making it heavier and also resulting in sinking motion. This downward motion inhibits further convection and the growth of additional thermals from below, which is why fair weather cumulus typically have expanses of clear sky between them. Without a continued supply of rising air into the cloud, the erosion process will continue, eventually resulting in the cloud's eventual dissipation.