From National Weather Service: Most tornadoes are spawned from supercell thunderstorms. Supercell thunderstorms are characterized by a persistent rotating updraft and form in environments of strong vertical wind shear. Wind shear is the change in wind speed and/or direction with height. The updraft lifts the rotating column of air created by the speed shear. This provides two different rotations to the supercell; cyclonic or counter clockwise rotation and an anti-cyclonic of clockwise rotation. The directional shear amplifies the cyclonic rotation and diminishes the anti-cyclonic rotation. All that remains is the cyclonic rotation called a mesocyclone. By definition a supercell is a rotating thunderstorm. When viewed from the top, the counter-clockwise rotation of the mesocyclone gives the