Cumulonimbus (Cb) is a type of cloud that is tall, dense, and involved in thunderstorm and other intense weather. It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold front in a squall line. They create lightning through the heart of the cloud. Cumulonimbus clouds form from cumulus clouds (namely from cumulus congestus) and can further develop into a supercell, a severe thunderstorm with special features.

As the earth’s surface heats unevenly (air over a parking lot will end up hotter than an adjacent grass field), the warmest pockets of air begin to accelerate upward as they end up less dense than the surrounding (relatively cooler) warm air.