Mesocyclone

Storm relative motion of a tornado-producing mesocyclone over Greensburg, Kansas on May 4 2007. The storm was producing an EF5 tornado at the time of the image.

A mesocyclone is a vortex of air within a convective storm. [1] It is air that rises and rotates around a vertical axis, usually in the same direction as low pressure systems in a given hemisphere. They are most often cyclonic, that is, associated with a localized low-pressure region within a severe thunderstorm. Such thunderstorms can feature strong surface winds and severe hail. Mesocyclones often occur together with updrafts in supercells, within which tornadoes may form at the interchange with certain downdrafts.

Mesocyclones are localized, approximately 2 km (1.2 mi) to 10 km (6.2 mi) in diameter within strong thunderstorms. [1] Thunderstorms containing persistent mesocyclones are supercell thunderstorms. Mesocyclones occur on the " mesoscale" from a few kilometers to hundreds of kilometers. Doppler radar is used to identify mesocyclones. A mesovortex is a similar but typically smaller and weaker rotational feature associated with squall lines.

Formation

Mesocyclones form when strong changes of wind speed and/or direction with height (" wind shear") sets parts of the lower part of the atmosphere spinning in invisible tube-like rolls. The convective updraft of a thunderstorm then draw up this spinning air, tilting the rolls' orientation upward (from parallel to the ground to perpendicular) and causing the entire updraft to rotate as a vertical column. [2]

As the updraft rotates and ingests cooler moister air from the forward flank downdraft (FFD), it may form a wall cloud, a spinning layer of clouds lowered from ambient storm cloud base under the mid-level mesocyclone. The wall cloud tends to form closer to the center of the mesocyclone. As it descends, a funnel cloud may form near its center. This is the first visible stage of tornadogenesis.

Wind shear (red) sets air spinning (green). 
The updraft (blue) 'tips' the spinning air upright. 
The updraft then starts rotating. 
Other Languages
català: Mesocicló
čeština: Mezocyklóna
dansk: Mesocyklon
Deutsch: Mesozyklone
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italiano: Mesociclone
norsk: Mesosyklon
norsk nynorsk: Mesosyklon
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Simple English: Mesocyclone
slovenčina: Mezocyklóna
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