Durbanville clouds Cumulonimbus incus, also known as an anvil cloud. These clouds are typically associated with thunderstorms and are characterized by their flat, anvil-like top. The flat top forms when the rising air in the thunderstorm reaches the tropopause, the boundary between the lower atmosphere and the stratosphere, where it can no longer rise and spreads out horizontally. Anvil clouds are often composed of ice particles, which can lead to the formation of hail. They can also produce heavy rain, snow, and even tornadoes.