A principal cloud type (cloud genus), exceptionally dense and vertically developed, occurring either as isolated clouds or as a line or wall of clouds with separated upper portions. These clouds appear as mountains or huge towers, at least a part of the upper portions of which is usually smooth, fibrous, or striated, and almost flattened as it approaches the tropopause. This part often spreads out in the form of an anvil (incus) or vast plume. Under the base of cumulonimbus, which is often very dark, there frequently exist virga, precipitation (praecipitatio), and low, ragged clouds (pannus), either merged with it or not. Its precipitation is often heavy and always of a showery nature. The usual occurrence of lightning and thunder within or from this cloud leads to its popular appellations: thundercloud, thunderhead (the latter usually refers only to the upper portion of the cloud), and thunderstorm. Cumulonimbus is composed of water droplets and ice crystals, the latter almost entirely in its upper portions. It also contains large water drops, snowflakes, snow pellets, and sometimes hail. The liquid water forms may be notably supercooled. Within a cold air mass in polar regions, the fibrous ice crystal structure may extend virtually throughout the cloud mass. Cumulonimbus always evolves from the further development of cumulus congestus, which, in turn, usually has resulted from the growth of cumulus (Cb cumulogenitus). This complete development may initiate also from stratocumulus castellanus (Cb stratocumulogenitus) or from altocumulus castellanus (Cb altocumulogenitus). In the latter case the cumulonimbus base is particularly high. It may also, but infrequently, develop from a portion of altostratus or nimbostratus (Cb altostratogenitus or Cb nimbostratogenitus). The formative process of cumulonimbus starts as a result of convection from the earth's surface or instability in the upper air, or both simultaneously. It therefore has a predominant diurnal cycle similar to that of cumulus. Cumulonimbus is rare over the polar regions, and becomes increasingly frequent with decreasing latitude, and is, in fact, an almost regular climax of the diurnal cloud cycle in the humid areas of the tropical regions and in humid and unstable air masses penetrating the temperate latitudes. Because of its great vertical size and of the magnitude and variety of forces that act within and upon it, cumulonimbus is a vertical cloud factory. In addition to the complex of accessory features it may possess, which includes tornadoes (tuba), it may also be responsible for the formation of nearly all of the other cloud genera. Cumulus congestus always preexists, and therefore is often easily confused with, cumulonimbus. A cloud is called cumulus congestus until its upper portion begins to show the diffuseness or fibrous quality indicative of ice crystal predominance. Only cumulonimbus is accompanied by lightning, thunder, or hail; only cumulus congestus can rival the intensity of its shower-type precipitation. See cloud classification, thunderstorm.