Cumulonimbus are heavy, dense clouds in the form of a mountain or huge towers, such as these in Pace del Mela, Messina, Italy. At least part of the upper portion is usually smooth, or fibrous or striated and nearly always flattened. The cloud base is often very dark. In this image, the upper portions have a cirriform appearance identifying the species as capillatus and an anvil is in the early stages of development which is the supplementary feature incus.

Photograph: Fabrizio Micalizzi/WMO