In an unstable and moist airmass, clouds appears to be piled up and are called "Cumulo" clouds which is Latin for "heap". When the cloudbase is (relative) low, two genera of clouds are derived: the Cumulus & Cumulonimbus. The distinction between these two is sometimes hard to see on its shape only, but some rules are applied: when at least some protuberances of the upper part begin to lose their cumuliform outlines, but in which still no cirriform parts can be distinguished, the cloud is called Cumulonimbus calvus. Usually this upper part evolves further into an anvil with smooth, fibrous or striated appearance, an typical character of a Cumulonimbus cloud which produces showers.

Taken at the end of August, showing two genera of clouds: on the left side a Cumulonimbus calvus praecipitatio, and on the right side Cumulus congestus. Comparing them on a time scale, the left cloud was a bit older than the right one, but at the end the Cumulus evolved also to a Cumulonimbus.