A mesocyclone is storm-scale region of rotation (vortex), typically around 2 to 6 mi (3.2 to 9.7 km) in diameter, within a thunderstorm. In the northern hemisphere it is particularly found in the right rear flank of a supercell or often on the eastern, or front, flank of an HP storm. The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it.[1]

Supercell diagram with the mesocyclone rotation in red.

Mesocyclones are detectable on Doppler weather radar as a rotation signature which meets specific criteria for magnitude, vertical depth, and duration. On US NEXRAD radars displays, they will be highlighted by a yellow solid circle on the Doppler velocity products but other countries may have other conventions. Therefore, a mesocyclone should not be considered a visually-observable phenomenon; although visual evidence of rotation, such as curved inflow bands, may imply the presence of a mesocyclone.