These types of clouds are most often detached clouds, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes or towers, having their bases located in the low level etage. The sunlit parts of these clouds are mostly brilliant white. Their base is relatively dark and nearly horizontal. Since these clouds may grow from regions above freezing to levels well below freezing, they may be composed solely of water droplets, supercooled water droplets, ice crystals, snowflakes, ice pellets or a combination of all. These clouds form in an unstable environment which allows air parcels to continue to rise once started either through forced lifting or heating and are sometimes referred to as "clouds of ascending air currents.".

Cumulus take on a variety of shapes. This image, of cumulus humilis, (of small size) is characterized by only a small vertical extent and may appear flattened. It never produces precipitation. They are sometimes called fair-weather cumulus. Cumulus mediocris clouds are of moderate vertical extent, with small protuberances and sproutings at their tops. They generally give no precipitation.
When the cloud has grown to resemble a cauliflower with strong sproutings and generally sharp outlines and often great vertical extent, a cumulus congestus cloud has developed. Cumulus congestus often release abundant precipitation in the form of showers. If a cumulus congestus cloud resembles a narrow, very high tower, it often is called a towering cumulus, although a more proper designation would be cumulus castellanus.
The tops of these towers occasionally are formed of small "puffs" which may detach themselves successively from the main body of the cloud. They are then carried away by the wind and disintegrate more or less rapidly, occasionally producing virga. Cumulus congestus often develops into cumulonimbus type clouds.

Cumulonimbus clouds, with the anvil tops as seen in the distance in this image, are heavy and dense clouds with considerable vertical extent, in the form of a mountain or huge towers. The clouds to the front are cumulus and towering cumulus. At least part of its upper portion is usually smooth, or fibrous or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume as shown. The top may reach to the tropopause. Under the base of this cloud, which is often very dark, there are frequently low ragged clouds either merged with it or not, and precipitaion of the showery form, often very intense.

When cumulonimbus covers a large expanse of the sky, it can easily be confused with nimbostratus, especially when identification is based solely on the appearance of the undersurface. In this case, the character of the precipitation may help to distinguish cumulonimbus, which produces a showery type and is often accompanied by lightning and thunder, from nimbostratus, which produces a steady, continuous precipitation.

Certain cumulonimbus clouds appear identical to cumulus congestus. The cloud should be called cumulonimbus as soon as at least a part of its upper portion loses the sharpness of its outlines or presents a fibrous or striated texture. If it is accompanied by lightning, thunder or hail, it is a cumulonimbus. Cumulonimbus is composed of water droplets, supercooled droplets and ice crystals, snowflakes, snow pellets, ice pellets or hail stones.

Cumulonimbus may be described as a "cloud factory," and it may produce more or less thick patches or sheets of cirrus spissatus, altocumulus, altostratus or stratocumulus by the spreading out of its upper portions and by the dissipation of its subjacent parts. The spreading of the highest part usually leads to the formation of an anvil.

Copyright © 1996-2007 Texas A&M University, Texas A&M Meteorology Department and Marion Alcorn.