Cumulonimbus (Cb) is a towering vertical cloud (family D2) that is very tall, dense, and involved in thunderstorms and other inclement weather. Cumulonimbus originates from Latin: Cumulus "heap" and nimbus "cloud". It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold front in a squall line. They can create lightning and other dangerous severe weather. Cumulonimbus clouds form from cumulus clouds (namely from cumulus congestus) and can further develop into a supercell, a severe thunderstorm with special features.

The anvil is the flat layer at the top. The downdraft is the rainy area to the right. Cumulonimbus clouds usually form from cumulus coop at a much lower height, thus making them, like cumulus clouds, grow vertically instead of horizontally, thus giving the cumulonimbus its mushroom shape. The base of a cumulonimbus can be several miles across, and it can be tall enough to occupy middle as well as low altitudes; though formed at an altitude of about 500 to 13,000 feet (150 to 3,960 metres), its peak can reach up to 75,000 feet (23,000 metres) in extreme cases. Typically, it peaks at a much lower height, usually up to 20,000 feet (6,090 meters).^{[verification needed]} Well-developed cumulonimbus clouds are also characterized by a flat, anvil-like top (anvil dome), caused by straight line winds at the higher altitudes which shear off the top of the cloud, as well as by an inversion over the thunderstorm caused by rising temperatures at the equilibrium level near the tropopause. This anvil shape can precede the main cloud structure (vertical updraft towers) for many miles, causing anvil lightning. Sometimes rising air parcels surpass the equilibrium level due to momentum and form an overshooting top culminating at the maximum parcel level. This is the largest of clouds usually extending through all three cloud regions when vertically developed. All other clouds are dwarfed even compared to small cumulonimbus clouds.