Description & Characteristics. Translated from latin meaning anvil, the ‘incus’ cloud feature is found only in the cumulonimbus cloud type. No other cloud type besides a cumulonimbus cloud can be paired with an incus cloud feature. The proper cloud classification abbreviation for this cloud type and feature is ‘Cb inc’. Cumulonimbus incus cloud formations are more popularly known as anvil clouds and are generally found in strong thunderstorms. They’re often said to look as if a bomb went off because of their mushroom cloud appearance.

To visualize how an anvil cloud is formed, imagine boiling a pot of water. When you take the top off of the pot, steam rises up, and then spreads out as it hits the ceiling. Apply this to a cumulonimbus cloud. The powerful rising updraft hits the tropopause, a layer of air above the troposphere where the air temperature stops decreasing with altitude. The tropopause effectively acts as a ceiling: the rising updraft of the cloud has nowhere else to go and creates the anvil cloud as it spreads out across the sky. The larger the anvil cloud, the stronger the thunderstorm.

As a cloudspotter, note that the incus cloud feature is often paired with the capillatus cloud species. You can also expect to see the mammatus clouds (mamma cloud feature) beneath the anvil cloud. When a cumulonimbus incus cloud begins to dissipate, many times the anvil is the last part of the cloud to disappear. When this happens, the cloud that’s left over is classified as cirrus spissatus cumulonimbogenitus, which describes a thick cirrus cloud that’s formed from a cumulonimbus on its way out.