Definition
A cumulonimbus cloud — a tall, dense, vertically developed cloud capable of producing thunder, lightning, heavy rain, hail, strong updrafts and downdrafts, severe turbulence, and icing. Thunderclouds form when warm, moist, unstable air rises rapidly and condenses, often building from a few thousand feet up to the tropopause.
Plain English
The big, towering storm cloud that produces thunder and lightning. Inside it, the air is violent — strong rising and falling currents, heavy rain, possibly hail, and ice. Aircraft are not designed to fly through one.
Context Anchor
Pilots encounter this term in weather briefings, forecasts, and in-flight weather decisions when thunderstorms may be near the route or airport.
Derivation
From 'thunder' (the sound produced by lightning's rapid heating of air) and 'cloud.' The aviation-technical name for the same thing is 'cumulonimbus,' from Latin 'cumulus' (a heap or pile) and 'nimbus' (a rain cloud) — literally a heaped-up rain cloud.
Why Pilots Care
Thunderclouds produce severe turbulence, icing, lightning strikes, and wind shear that can exceed aircraft structural limits.
Grounding Statement
If you see a tall, dark cloud building upward and hear thunder from it, that cloud is not just rain—it is a sign of powerful moving air inside and around it.
Intuition Check
Do not assume a thundercloud is just any dark rain cloud. In aviation, the key warning is lightning and thunder, which point to a much more dangerous storm cloud.
Example Sentence 1
The briefer pointed out a line of thunderclouds developing along the route, so the pilot filed a deviation to the south.
Example Sentence 2
Preflight briefing warned of scattered thunderclouds forming along the afternoon route.