Plain English Meaning

The flat top that spreads out from a tall thunderstorm cloud when it can't grow any higher. It looks like a wide, flat shelf at the top of the cloud, and it usually points in the direction the storm is heading.

Full Definition

The flat, spreading top of a mature cumulonimbus cloud, formed when the rising column of a thunderstorm reaches the stable air of the tropopause and can no longer climb, causing the cloud to spread horizontally — typically downwind — into a wide, flattened shape resembling a blacksmith's anvil. The presence of an anvil indicates a fully developed thunderstorm and points in the direction the storm is moving.

Context Anchor

Seen in thunderstorm descriptions, weather briefings, and visual scanning when identifying large, dangerous storm clouds.

Derivation

Named after the blacksmith's anvil — a heavy iron block with a flat top and a horn extending from one side. The cloud's flat, spreading top with a looks remarkably like that shape, which is why early observers gave it the name.