Cumulonimbus clouds are responsible for most thunderstorms that produce rain, thunder, lightning, and if conditions are right, tornadoes. Thunderstorms form as the moisture in updrafts—rapidly rising, warm air—condenses into raindrops or hail. Rain or hail starts to fall when the updrafts no longer have the energy to carry the heavy precipitation. Thunderstorms can last from one to several hours.
Courtesy of UCAR Digital Image Library