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A mesocyclone is a vortex of air, approximately 2 to 10 km in diameter (the mesoscale of meteorology), within a convective storm.^[1] That is, 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 storms can feature strong surface winds and severe hail. Mesocyclones often occur together with updrafts in supercells, where tornadoes may form.

Mesocyclones are normally relatively localized: they lie between the synoptic scale (hundreds of kilometers) and small scale (hundreds of meters).

A mesocyclone from the Greensburg, Kansas tornado indicated on Doppler weather radar.

The best way to detect and verify the presence of a mesocyclone is by Doppler weather radar. Thus the word mesocyclone is associated with weather radar terminology. Mesocyclones are most often identified in the right-rear flank of supercell thunderstorms and squall lines, and may be distinguished by a hook echo rotation signature on a Doppler weather radar map.

Visual cues such as a rotating wall cloud or tornado may also hint at the presence of a mesocyclone. This is why the term has entered into wider usage in connection with rotating features in severe storms.

Wind shear (red) sets air spinning (green).

The updraft (blue) 'tips' the spinning air upright.

The updraft then starts rotating.

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 in invisible tube-like rolls. The convective updraft of a thunderstorm is then thought to 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.

As the updraft rotates, it may form a wall cloud, a spinning layer of clouds descending from the mesocyclone. The wall cloud tends to form closer to the center of the mesocyclone. As it descends, a funnel-shaped cloud may form at its center. This is the first stage of tornado formation.

The presence of a mesocyclone is believed to be a key factor in the formation of the strong tornadoes associated with severe thunderstorms. Tornadoes typically form at the peak of thunderstorm intensity as the storm begins to weaken. This is because the momentum and vacuum built up as large masses of air rise into the upper atmosphere cause a siphoning effect nearer to the ground. As the updraft is restricted, the entire thunderstorm is fed by smaller pockets of remaining warm air at the ground. The back-pressure created as the warm air runs out sucks the base of the thunderstorm towards the ground (i.e. a wall cloud). Once the warm air at the ground is nearly depleted, the entire top of the thunderstorm and the large wall cloud siphons air from a 1 mile or less diameter region at the ground, forming a tornado. If a moderate supply of warm air is available ahead of the storm, the storm may be tornadic for some time. If the warm air runs out, then the storms essentially chokes itself off and gradually dies.

• Supercell

• Electronic Journal of Severe Storms Meteorology