Cumulonimbus clouds are much larger and more vertically developed than fair weather cumulus. They can exist as individual towers or form a line of towers called a squall line. Fueled by vigorous convective updrafts (sometimes in excess 50 miles/hour), the tops of cumulonimbus clouds can easily reach 39000 feet (12000 meters) or higher.

The lower portion of the cloud consists mostly of water droplets while at the cloud top, where temperatures are well below 0 degrees Celsius, there are primarily ice crystals. Under favorable atmospheric conditions, what may initially appear as harmless fair weather cumulus clouds can quickly develop into large cumulonimbus clouds. Occasionally, these clouds grow into powerful thunderstorms known as supercells.

Supercells are large thunderstorms whose updrafts and downdrafts are so closely in balance that such a storm can have a lifetime of several hours. Fueled by intense updrafts (occasionally reaching 90 m.p.h.), supercells can produce large hail, damaging winds, and tornadoes. Supercells are typically characterized by strong vertical shear and steep lapse rates. Supercells tend to develop during the afternoon and the early hours, when the effects of heating by the sun are strongest.

A line of approaching thunderstorms at sunset might resemble something like the picture above. The sun setting behind this developing cumulonimbus tower clearly reveals the distinct cloud edges that mark the extent of the rising air.