This view of Earth's horizon as the sunsets over the Pacific Ocean was taken by an Expedition 7 crew member onboard the International Space Station (ISS). Anvil tops of thunderclouds are also visible. 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 can form alone, in clusters, or along cold front squall lines. These clouds are capable of producing lightning and other dangerous severe weather, such as tornadoes. Cumulonimbus progress from overdeveloped cumulus congestus clouds and may further develop as part of a supercell. Cumulonimbus is abbreviated Cb and are designated in the D2 family.