Mesocyclone
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Definition
A mesocyclone is an area of vertically oriented atmospheric rotation (usually but not always cyclonic) that is most often associated with a localized low pressure region within a severe thunderstorm. The phenomenon is normally relatively localized in nature lying between the synoptic scale (hundreds of kilometers) and small scale (hundreds of meters).
Identification
The word mesocyclone is, strictly speaking, only associated with weather radar terminology. This is because the presence of a mesocyclone can only be truly verified by Doppler weather radar. Mesocyclones are most often identified in the right-rear flank of supercell thunderstorms and squall lines, and may be distinguished by a hook echo or gate-to-gate rotation signature on Doppler weather radar.
Visual cues such as a rotating wall cloud or tornado may also hint at the presence of a mesocyclone. This explains why the term has entered into wider usage in connection with rotating features in severe storms.
Formation
Mesocyclones are believed to form when strong changes of wind speed and/or direction with height ("wind shear") sets parts of the lower part of the atmosphere spinning horizontally in invisible tube like rolls. The convective updraft of a thunderstorm is then thought to draw in this spinning air changing its orientation from horizontal to vertical and causing the entire updraft to rotate.
The presence of a mesocyclone is believed to be a key factor in the formation of the strong tornados associated with severe thunderstorms. A mesocyclone is best seen by radar and is the begining of a tornado.