Heavy and dense cloud with considerable vertical extent in the form of a mountain or huge tower. At least part of its upper portion is usually fibrous or striated, often appearing as an anvil or vast plume. This appearance is due to the formation of ice particles in its upper part.
The base of the cloud appears dark and stormy. Low ragged clouds are frequently observed below the base and generally other varieties of low cloud, (Cu) and (Sc) are joined to or in close proximity to the Cumulonimbus. Lightning and thunder are characteristic of Cumulonimbus.
Cumulonimbus calvus is a species in which the sproutings of the upper part, whether partially or wholly, are more or less indistinct and flattened and have the appearance of a whitish mass without sharp outlines. No fibrous or striated parts are visible. This species is the transition between Towering Cumulus and the Cb species capillatus.
Cumulonimbus capillatus is characterised by an upper portion having cirriform parts of clearly fibrous or striated structure, a plume or a vast more or less disorderly mass of hair.
When in the shape of an anvil, the term incus is appended to the name as a supplementary feature, eg. Cumulonimbus capillatus incus.
Cumulonimbus does not present any varieties.
Distinguishing Cb from other genera
Cumulonimbus differs from Nimbostratus in that the precipitation is in the form of showers which can include hail, and may be accompanied by lightning and thunder. Also, Nimbostratus usually covers the sky for extended periods whereas Cumulonimbus is rarely extensive enough to cover the whole sky for very long.
Cumulonimbus is distinguished from Cumulus and Towering Cumulus in that the upper portion, at least in part, does not have clearly defined edges. It mostly appears fibrous or striated, frequently like an anvil or a vast plume. Lightning, thunder and hail only occur with Cumulonimbus.
Precipitation associated with Cumulonimbus is showers of rain, small hail, hail, snow or snow pellets, often heavy in nature.