How a fan rotates?

A tornado's spin originates in wind shear, which is a difference in wind speed or direction at different altitudes. If this wind shear is strong enough air may start to roll in a phenomenon called horizontal vorticity. The updraft of a thunderstorm can then turn this vorticity vertical and take on the rotation to become a mesocyclone. Under the right conditions the mesocyclone can tighten and intensify to produce a smaller, stronger vortex: the tornado itself.