Mesocyclone
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A mesocyclone is an area of vertical atmospheric rotation, typically 2-6 miles across. They are often found in the right-rear flank of supercell thunderstorms, and are visible as a hook echo on Doppler weather radar. The presence of a mesocyclone can only be truly verified by radar, although visual clues such as curverd inflow bands may be present.
Mesocyclones form when strong changes of windspeed and/or direction with height ("wind shear") sets the lower part of the atmosphere spinning horizontally. The updraft of a thunderstorm can then draw this area of spinning air from horizontal to vertical.
The most severe side-effect of the mesocyclone occurs when a downdraft and the flanking line of the mesocyclone collide. The first effects will be a "mammatus", or a bubbly cloud formation protuding down from the cloud base. If the downdraft is strong enough, the flanking line will lower and produce a "wall cloud". These can sometimes form funnel clouds and finally tornadoes.