Cumulonimbus (from the Latin cumulus ("heap") and nimbus ("cloud") is a dense towering vertical cloud associated with thunderstorms and atmospheric instability, forming from water vapour carried by powerful upward air currents. Cumulonimbus may form alone, in clusters, or along cold front squall lines. They may produce lightning and other dangerous severe weather, such as gusts and hail. Cumulonimbus progress from over-developed cumulus congestus clouds, and may further develop as part of a supercell. Cumulonimbus is abbreviated Cb, and are designated in the D2 family.