Cumulus clouds commonly indicate fair weather. Cumulus clouds are often seen on bright summer days.
Cumulus clouds are flat at the bottom and billowy above.
They are formed by a mass of unstable air, usually air warmed at the ground that is rising. As the warm air rises from the earth—often the air is rising from a patch of bare ground heated by the sun–it cools and the vapor in the rising air condenses into the visible cumulus cloud.
Think of each cumulus cloud as the top of a column of rising air.
The flat bottom of the cumulus cloud indicates the altitude at which the rising vapor has cooled to the point of condensation. Cumulus clouds commonly appear at 3,000 to 6,000 feet (914-1,828 m) but can form and rise higher.
Low cumulus clouds drift across the sky on the breeze and are often mistaken for sheep or wispy sailing ships by children. These billowy white cumulus clouds are more specifically called cumulus humilis.
Not all cumulus clouds are white and billowy. On hot, muggy summer days, cumulus clouds can become more menacing with yellowish tops and black undersides.
When cumulus clouds are forced high into the sky by very unstable air, they are called cumulus congestus or towering cumulus clouds. These clouds resemble the head of a cauliflower.
Cumulus congestus clouds can extend upward to as high as 39,000 feet (12,000 m).
When cumulus congestus clouds produce rain they are called cumulonimbus clouds. Cumulonimbus clouds often form flat, anvil-like tops. These are known as thunderheads and usually bring rain, thunder, and lightning.