Start with the silhouette: Cumulonimbus is the towering, dense cloud with a dark base and an upper part that often flattens into an anvil. The tower can begin as Cumulus congestus. As warm, moist air rises, condensation into droplets and ice releases heat and keeps the updraft buoyant. When the plume reaches a stable layer near the tropopause, the top spreads sideways. A typical base sits below about 6,500 ft; strong mature storms can reach 40,000–60,000 ft. In the mature stage, rising and sinking air coexist, bringing heavy rain, lightning, gusts and hail, with tornadoes possible in some storms. Use the top and the precipitation as stage clues, then check official warnings for local risk.
Cumulonimbus has two WMO species: calvus, with a still-puffy top, and capillatus, with fibrous ice aloft. Incus names the anvil; mamma names the rounded pouches beneath it. Cumulus congestus is tall and cauliflower-like while it is still building. Nimbostratus spreads as a gray rain layer. Read the WMO International Cloud Atlas, NOAA NSSL, NWS storm life-cycle guide, and Met Office cloud guide. Photos: Peter Broster's four incus clouds, Janne Naukkarinen's Cumulonimbus calvus, AborontsOgley009293's mammatus photo, and NOAA/NWS's towering cumulus example. The Nimbostratus comparison uses the WMO cloud illustration.
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