Figure 1-8.—Cumulus mediocris cloud.extending upwards from the base (fig. 1-9, view A), andalso by their overall size. It is not uncommon to see oneor two cumulus congestus cells in the sky surrounded bycumulus mediocris cells.Rapidly building cumulus congestus cloud cellsmay produce clouds of great vertical extent withrelatively small base areas so that they appear to be inthe form of large towers. The cloud is still classified acumulus congestus cloud, but this appearance iscommonly called towering cumulus (TCU). Toweringcumulus clouds normally do not develop as fasthorizontally as they do vertically. A rule of thumb foridentifying a cumulus congestus cloud as a toweringcumulus is that the height appears to be twice the widthof the base (fig. 1-9, view B).Cumulus congestus cells, and especially toweringcumulus (congestus) clouds may produce light tomoderate showers. Over warm ocean waters, toweringcumulus may produce waterspouts. When a largecumulus congestus cloud begins to produce either awispy cirrus blow-off or a well-defined anvil-shapedtop (the upper portion of the cloud column beginsbulging horizontally) or if lightning is seen or thunder isheard, the cloud is automatically classified a differenttype of cloud: the cumulonimbus.Cumulonimbus (CB)Cumulonimbus clouds are generated from largecumulus congestus clouds. These clouds cells aredistinguished from cumulus congestus by their massiveappearance and extensive vertical development. Thepresence of thunder, lightning, or an anvil topautomatically 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 cirrusblow-off or top of the anvil will be somewhere between25,000 to 45,000 feet in the mid latitudes. Tops of thelarger cumulonimbus cells have been measured in thetropics in excess of 60,000 feet.1-10
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