A mesocyclone is a vortex of air, approximately 2 to 10 km in diameter (the mesoscale of meteorology), within a convective storm. 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).