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wall cloud
A wall cloud (or pedestal cloud) is a large, localized, persistent, and often abrupt lowering of cloud that develops beneath the surrounding base of a cumulonimbus cloud and from which sometimes forms tornadoes.〔(【引用サイトリンク】title=Definition of Wall Cloud )〕 It is typically beneath the rain-free base (RFB) portion of a thunderstorm, and indicates the area of the strongest updraft within a storm. Rotating wall clouds are an indication of a mesocyclone in a thunderstorm; most strong tornadoes form from these. Many wall clouds do not rotate, however.
== Genesis ==
Wall clouds are formed by a process known as entrainment, when an inflow of warm, moist air rises and converges, overpowering wet, rain-cooled air from the normally downwind downdraft. As the warm air continues to entrain the cooler air, the air temperature drops and the dew point increases (thus the dew point depression decreases). As this air continues to rise, it becomes more saturated with moisture, which results in additional cloud condensation, sometimes in the form of a wall cloud. Wall clouds may form as a descending of the cloud base or may form as rising scud comes together and connects to the storm's cloud base.
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抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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