Plain English Meaning
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.
Full 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.
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.