Severe thunderstorms have a well-defined structure of updraft and downdraft (with precipitation), situated next to eachother. Cool humid air from the downdraft may flow to the updraft, moistening the air and lowering the cloud base under the updraft. This lowering, the wall cloud, may be enhanced by large-scale rotation of the updraft itself. Well-defined wall clouds usually have the form of a straight wall, more commonly wall clouds look like triangle-shaped sockets of cloud under the thunderstorm base.
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