Cumulonimbus clouds (Cb) 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
knots), the tops of cumulonimbus clouds can easily reach 39,000
feet (12,000 meters) or higher. Photograph by: NOAA Lower levels
of cumulonimbus clouds consist mostly of water droplets while at
higher elevations, where temperatures are well below 0 degrees
Celsius, ice crystals dominate. Under favorable atmospheric
conditions, harmless fair weather cumulus clouds can quickly
develop into large cumulonimbus clouds associated with powerful
thunderstorms known as supercells. Photograph by: Knupp
Photograph by: Holle These storms tend to develop during the
afternoon and early evening when the effects of heating by the
sun are strongest. For more information about supercells and
other types of severe weather phenomena, visit the Severe Storm
Spotters Guide.