A cumulonimbus incus (Latin incus, "anvil") is a cumulonimbus cloud which has reached the level of stratospheric stability and has formed the characteristic flat, anvil-top shape. They can form into supercells resulting into severe storms like tornadoes.

A cumulonimbus incus is a sub-form of cumulonimbus capillatus.

A cumulonimbus incus is a mature thunderstorm cloud and can produce many dangerous elements.

This is a powerful cumulonimbus cloud. If the correct atmospheric conditions are met, they can grow into a supercell storm. This cloud may be a single-cell storm or it may be one cell in a multi-cell storm. They are capable of producing severe storm conditions for a short amount of time.

Anvil Cumulonimbus incus, February 2007

Cumulonimbus incus

Cumulonimbus incus with little precipitation observed

Single-cell Cumulonimbus incus - Mykonos / Greece, June 2009