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.

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