Have you ever noticed that many summertime clouds have flat bottoms and billowy tops? Cumulus clouds form on updrafts — rising columns of air made buoyant by humidity and heat from the ground warmed by sunlight. Cumulus clouds usually become more prolific in the afternoon when the ground gets warm. The bottoms are flat because this is the point at which the rising air is cooled enough for water droplets to form. The air keeps rising into the cloud, and the billowy top is the point at which either the moisture runs out or the air stops rising.
We rarely see cumulus clouds in winter because the cold, snow-covered ground cannot initiate updrafts. Not all summertime clouds are billowy, however. Higher up in the atmosphere, flat-topped clouds made of ice often form at very high altitudes where the rising columns of air do not penetrate.