Why do tornadoes always spin in the same direction?
In probably 95 percent of tornado occurrences the wind circulation is counterclockwise, but not all of the time. In the majority of tornadoes, air spirals into a tornado in a large counterclockwise circulation. The direction of tornadic air motion is dominated by the Coriolis effect, which dictates that motions in the Northern Hemisphere always experience a deflection to the right of the intended path. The Coriolis deflection always manifests itself in wind motions hundreds of miles long, but the deflection diminishes practically to zero in movements of a few feet. The horizontal dimension of tornadic circulations, a few miles, is large enough for Coriolis to establish counterclockwise rotation most, but not all of the time.