Cumulonimbus (Cb) is a type of cloud that is tall, dense, and involved in thunderstorms and other intense weather. When seen in a storm, it may be called "thunderhead". The clouds can form alone, in clusters, or along a cold front in a squall line, a line of strong winds. Cumulonimbus clouds form from cumulus clouds (namely from cumulus congestus cloud) and can further develop to a supercell, a severe thunderstorm with special features. Cumulonimbus clouds are the source of most tornadoes.