Plain English Meaning

The wide, flat top of a big thunderstorm cloud. The storm pushes upward until it hits a layer of air it can't punch through, then spreads sideways into a shape that looks like an anvil.

Full Definition

The flat, spreading top of a mature thunderstorm cloud (cumulonimbus), formed when the rising column of air reaches the stable layer at the top of the troposphere and can no longer climb. The cloud spreads horizontally, usually downwind, producing a wide, flat top with a sloping leading edge that resembles a blacksmith's anvil.

Context Anchor

Seen when identifying thunderstorms visually from the ground or from the cockpit, especially when judging whether a storm is tall and well developed.

Derivation

Named after the blacksmith's anvil — a heavy iron block with a flat top and one rounded, overhanging end. The cloud takes on the same outline: a wide, flat top with one side spreading further downwind. Naming it after a familiar object makes the shape easy to recognise from the cockpit or the ground.