Cumulonimbus, from the Latin cumulus (“heap”) and nimbus (“rainstorm”, “storm cloud”), is a dense towering vertical cloud associated with thunderstorms and atmospheric instability, forming from water vapor carried by powerful upward air currents. Cumulonimbus are capable of producing lightning and other dangerous severe weather, such as gusts, hail, and occasional tornadoes. Cumulonimbus progress from overdeveloped ‘Cumulus Congestus’ clouds and may further develop as part of a ‘Supercell’.

More info on Cumulonimbus Clouds can be found here