Cumulonimbus incus

A cumulonimbus incus (Latin incus, "anvil") also known as an anvil cloud is a cumulonimbus cloud which has reached the level of stratospheric stability and has formed the characteristic flat, anvil-top shape. A cumulonimbus incus means that the thunderstorm is in its mature stage, succeeding the preceding cumulonimbus calvus stage. Cumulonimbus capillatus is a sub-form of cumulonimbus incus.