Definition
A tall, dense, vertically developed cloud associated with thunderstorms, characterized by strong updrafts and downdrafts, heavy precipitation, lightning, and often hail, severe turbulence, and icing. It typically extends from a low base to very high altitudes, sometimes reaching the tropopause, with an anvil-shaped top when fully mature.
Plain English
A thunderstorm cloud. It builds upward into a towering shape, often topped with a flat anvil, and contains the most violent weather a pilot can encounter.
Context Anchor
Pilots encounter cumulonimbus in weather briefings, forecasts, radar displays, and visually in flight as large, towering storm clouds, often with dark bases and sometimes a spreading, flat top.
Derivation
From Latin cumulus meaning 'heap' or 'pile,' and nimbus meaning 'rain cloud.' So the name literally describes a heaped-up rain cloud — which is exactly what a thunderstorm cloud looks like from a distance.
Why Pilots Care
Cumulonimbus clouds signal severe hazards including extreme turbulence, icing, lightning strikes, and possible hail that can damage the aircraft or force emergency deviations.
Grounding Statement
Picture a cloud growing upward like a dark tower: the taller and more active it becomes, the more likely it is to contain strong storm forces inside and around it.
Intuition Check
Do not think of cumulonimbus as just a big puffy cloud. It is a thunderstorm cloud, and the danger can extend outside the visible cloud itself.
Example Sentence 1
The pilot deviated 25 miles off course to stay clear of a line of cumulonimbus clouds building along the route.
Example Sentence 2
Cumulonimbus clouds forming along the front forced the flight to climb or reroute for safety.