Clouds can be classified according to their usual altitudes. The altitudes listed here apply to the temperate zone. In polar regions, these clouds may be found in lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.
Clouds with vertical development include cumulus and cumulonimbus clouds.
CUMULUS (Cu)
On a sunny day, it is common to see billowy cloud puffs that resemble cotton balls in the sky. These are cumulus clouds. Cumulus clouds grow in a vertical direction from the base (bottom) up. In Latin, cumulus means "heaps or piles." It is easy to see by looking at these clouds how they got their name.
Small cumulus clouds are associated with fair weather. The base of the cloud is often no more than 3,000 feet above the ground, but they vary in height.
Cumulus clouds can grow vertically throughout the day. The top of a cumulus cloud can easily reach 20,000 feet into the troposphere. Small cumulus clouds usually do not produce rain. Under certain atmospheric conditions, these clouds can develop into rain-producing cumulonimbus clouds, and large cumulus clouds can themselves sometimes create rain.
CUMULONIMBUS (Cb)
The word nimbus is Latin for rain. As the name suggests, cumulonimbus is a rain-producing cloud. These clouds, often associated with thunderstorms, are sometimes referred to as thunderheads.
It is not uncommon in the American Midwest for the top of a cumulonimbus to reach 40,000 feet into the troposphere. On rare occasions, a cumulonimbus cloud can reach levels of 60,000 feet. Because its flat top resembles a blacksmith's anvil, it is known as an anvil top.
As a cumulonimbus cloud grows, reaching higher into the troposphere, it encounters colder air. Some of the water droplets that make up the cloud begin to freeze, turning into ice. The ice and water droplets collide with one another causing friction. It has been theorized that friction causes electricity within the cloud, which creates lightning.
