Cumulonimbus (CB)

Figure 1-8.—Cumulus mediocris cloud. extending upwards from the base (fig. 1-9, view A), and also by their overall size. It is not uncommon to see one or two cumulus congestus cells in the sky surrounded by cumulus mediocris cells. Rapidly building cumulus congestus cloud cells may produce clouds of great vertical extent with relatively small base areas so that they appear to be in the form of large towers. The cloud is still classified a cumulus congestus cloud, but this appearance is commonly called towering cumulus (TCU). Towering cumulus clouds normally do not develop as fast horizontally as they do vertically. A rule of thumb for identifying a cumulus congestus cloud as a towering cumulus is that the height appears to be twice the width of the base (fig. 1-9, view B). Cumulus congestus cells, and especially towering cumulus (congestus) clouds may produce light to moderate showers. Over warm ocean waters, towering cumulus may produce waterspouts. When a large cumulus congestus cloud begins to produce either a wispy cirrus blow-off or a well-defined anvil-shaped top (the upper portion of the cloud column begins bulging horizontally) or if lightning is seen or thunder is heard, the cloud is automatically classified a different type of cloud: the cumulonimbus. Cumulonimbus (CB) Cumulonimbus clouds are generated from large cumulus congestus clouds. These clouds cells are distinguished from cumulus congestus by their massive appearance and extensive vertical development. The presence of thunder, lightning, or an anvil top automatically classifies the cloud a cumulonimbus. Although cumulonimbus may develop cirrus blow- off in the polar regions or during the winter in the mid- latitudes at 20,000 feet, most commonly the cirrus blow-off or top of the anvil will be somewhere between 25,000 to 45,000 feet in the mid latitudes. Tops of the larger cumulonimbus cells have been measured in the tropics in excess of 60,000 feet. 1-10