Definition
The flat, spreading top of a mature cumulonimbus (thunderstorm) cloud, formed when the rising column of air reaches the stable layer at the tropopause and can no longer climb, causing the cloud to spread horizontally into a broad, anvil-shaped sheet of ice crystals. The anvil typically points downwind and indicates a fully developed thunderstorm.
Plain English
The wide, flat top of a thunderstorm cloud. When a thunderstorm grows tall enough that it can't rise any higher, the top spreads out sideways and looks like a blacksmith's anvil.
Context Anchor
Seen during weather planning, in-flight weather avoidance, and visual scanning for thunderstorms.
Derivation
Named for its shape, which resembles a blacksmith's anvil — a flat-topped iron block with one end pointing out. The visual likeness is the whole reason for the name.
Why Pilots Care
Signals a mature thunderstorm with risks of severe turbulence, lightning, hail, and icing that pilots must avoid.
Grounding Statement
Picture a tall column of cloud growing straight up until it hits an invisible ceiling, then mushrooming sideways into a wide, flat top that points downwind.
Intuition Check
An anvil cloud is not just any cloud with a flat top. In aviation weather, it usually points to a thunderstorm strong enough to be avoided by a wide margin.
Example Sentence 1
The pilot spotted an anvil cloud about thirty miles ahead and requested a deviation around the storm.
Example Sentence 2
Anvil clouds forming on radar confirmed the storm had reached its mature stage with strong vertical development.