The conditions under which Cumulonimbus clouds occur are similar to those that are favourable for the development of Cumulus congestus. The transformation of Cumulus congestus into Cumulonimbus is due to the formation of ice particles in its upper part. The presence of ice particles is evident when some or all of the upper part loses the sharpness of its outlines or acquires a fibrous or striated texture.
Cumulonimbus clouds may appear either as isolated clouds or in the form of a continuous line of clouds resembling a very extensive wall.
In certain cases, the upper portion of Cumulonimbus clouds may be merged with Cumulus congestus or Nimbostratus. Cumulonimbus may also develop within the general mass of Altostratus or Nimbostratus.
Low, ragged accessory clouds (pannus) often develop under Cumulonimbus; these clouds are at first separated from one another, but they may later merge to form a continuous layer that is partially or totally in contact with the Cumulonimbus base.
Cumulonimbus may be described as a “cloud factory”; it can produce thick patches or sheets of Cirrus spissatus, Altocumulus, Altostratus or Stratocumulus by the spreading out of its upper portions and by the dissipation of the lower parts. The spreading of the highest part usually leads to the formation of an anvil; if the wind increases strongly with altitude, the cloud top spreads only downwind, assuming the shape of a half anvil or, in some cases, a vast plume.
Cumulonimbus is rare in polar regions and more frequent in temperate and tropical regions.
Cumulonimbus may develop from convection initiated by heat from forest fires, wildfires or volcanic eruption activity. Cumulonimbus that is clearly observed to have originated as a consequence of localized natural heat sources will be classified by any appropriate species, variety and supplementary feature, followed by flammagenitus.