Anvil-shaped cloud is a special cloud species exclusive to cumulonimbus. When the cumulonimbus cloud develops to the top of the troposphere, the top part of the cloud will spread in all directions due to divergent winds. The shape may also lean to one direction due to the shear effect. The top of the cloud is flat and thinner just like an "anvil". The anvil-shaped cloud is composed of ice crystals. After the cumulonimbus dissipates, the anvil-shaped cloud will transform cirrus cloud.