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Heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers. At least part of its upper portion is usually smooth, or fibrous or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume.
Under the base if this cloud which is often very dark, there are frequently low ragged clouds either merged with it or not, and precipitation sometimes in the form of virga.

When Cumulonimbus covers a large expanse of the sky, it can easily be confused with Nimbostratus, especially when identification is based solely on the appearance of the under surface. In this case, the character of the precipitation may help to distinguish CumYlonimbus from Nimbostratus. If the precipitation is of the showery type, or if it is accompanied by lightning, thunder or hail, the cloud is by convention Cumulonimbus. Certain Cumulonimbus clouds appear nearly identical with Cumulus congestus. The cloud should be called Cumulonimbus as soon as at least a part of its upper portion loses the sharpness of its outlines or presents a fibrous or striated texture. If it is not possible to decide on the basis of the above criteria whether a cloud is a Cumulonimbus or Cumulus, it should by convention be called Cumulonimbus if it is accompanied by lightning, thunder or hail.

Cumulonimbus sometimes develops from Altocumulus castellanus or Stratocumulus castelJafiUS (Cb altocumulogenitus or Cb stratocumulogenitus); in the former case the base of the Cumulonimbus is unusually high. Cumulonimbus may also form as a result of the transformation and development of a portion of Altostratus or Nimbostratus (Cb altostratogenitus or Cb nimbostratogenitus). In the majority of the foregoing cases, the transformation into Cumulonimbus passes through the Cumulus congestus stage. Cumulonimbus most commonly evolves from Cumulus congestus which was formed in the normal manner (Cb cumulogenitus, Cb cumulomutatus).

Cumulonimbus is composed of water droplets and, especially in its upper portion, of ice crystals. It also contains large raindrops and, often, snowflakes, snow pellets, ice pellets or hailstones. The water droplets and raindrops may be substantially supercooled.

The conditions under which Cumulonimbus clouds occur are similar to those which are favourable for the development of Cumulus congestus. The transformation of a Cumulus congestus into a Cumulonimbus is due to the formation of ice particles in its upper part which consequently loses, at least in spots, the sharpness of its outlines or acquires, at least partially, 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 Altostratus or Nimbostratus. Cumulo nimbus may also develop within the general mass of an 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 so as to form a continuous layer partially or totally in contact with the Cumulonimbus base. Cumulonimbus may be described as a “cloud factory”; it may produce more or less 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 subjacent 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 of a vast plume. ‘
Cumulonimbus is rare in polar regions and more frequent in temperate and tropical regions.

Cumulonimbus calvus
Cumulonimbus capillatus

Cumulonimbus does not present any varieties.

One or more of the following supplementary features and accessory clouds may be associated with Cumulonimbus: praecipitatio, virga, pannus, incus, mamma (mamma are observed either on the base of the cloud or, more frequently, on the under surface of the projecting portion of the anvil), pileus, velum, arcus and tuba (rarely).