Cumulonimbus clouds form when warm, moisture-laden air travels upward and is cooled, condensing into a tall body of water droplets. Cumulonimbus clouds can reach up to 10 kilometers in height and take on a flat-topped appearance due to the strong winds at high altitudes. Cumulonimbus clouds are both tall, like regular cumulus clouds, and produce precipitation like other nimbus clouds.
Cumulonimbus clouds are behind some of the most severe weather that occurs on Earth. They are also frequent sources of lightning, as they are highly energetic. While these clouds are capable of forming supercells and causing flash floods and other catastrophes, they rarely do so for long. These clouds require a large amount of energy to sustain, and it is expended in causing severe weather.
Cumulonimbus clouds form relatively close to the ground, at least at their bottom side, because of the large amount of heat required to form them, and the nature of updrafts. An updraft begins when heat radiates from the ground into the air, warming it and causing it to rise. This movement of air, because of thermal differences, is not only the cause of cumulonimbus and other cumulus clouds, it is ultimately the cause of most weather on Earth.