text: It is the Coriolis effect.  When fluids move at a large scale in a rotating frame of reference, they deflect.  The deflection is always to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere.  Air always flows away from high pressure and towards lower pressure.  If the air is moving away in all directions and deflecting to the right (Northern Hemisphere), it rotates clockwise.  If the air is moving away in all directions and deflecting to the left (Southern Hemisphere), it rotates counterclockwise.  The Coriolis effect applies only to large scales, like storm systems.  It does not affect the direction that tubs, sinks, or toilets drain.