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.
Plain English
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.
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 leading edge looks remarkably like that shape, which is why early weather observers gave it the name.
Why Pilots Care
An anvil indicates a mature thunderstorm with strong updrafts that can produce severe turbulence, hail, lightning, and wind shear.
Grounding Statement
Picture a tall storm cloud rising until it cannot rise much farther, then flattening and spreading out at the top like smoke hitting a ceiling.
Intuition Check
Anvil does not mean a metal tool here. In aviation weather, it means the flat, spreading top of a thunderstorm cloud.
Example Sentence 1
The pilot noted the anvil pointing east and adjusted the route well to the south to stay clear of the storm.
Example Sentence 2
Anvils from developing storms often signal strong upper-level winds that can affect flight planning.