The Coriolis Effect
Winds generally blow out from the subtropics towards the equator and subpolar regions, and from the polar regions to the subpolar latitudes. Complicating matters is that the rotation of the Earth causes the winds to rotate as they move (the Coriolis effect). The rotation of the Earth causes an object to deflect towards the right (as viewed by a stationary observer) in the Northern Hemisphere, which results in a clockwise motion, and to deflect towards the left (as viewed by a stationary observer) in the Southern Hemisphere, which results in a counterclockwise motion. These rotations combined with the zonal distribution result in enormous, nearly ocean-scale major cells or gyres of surface winds.
Video: Coriolis Effect (1:00)
The Coriolis force is the deflection of objects observed in a rotating reference frame. If a person is standing in a fixed position in the Northern Hemisphere, an object moving away from him or her will tend to rotate towards the right in the Northern Hemisphere, and the left in the Southern Hemisphere, as a result of the rotation of the earth. This effect is called the Coriolis force. The Coriolis force is very small because the rotation of the earth is very slow, 360 degrees in one day. However, the Coriolis force affects all air masses whether they be giant hurricanes, such as Hurricane Katrina here, with a rotation towards the right and clockwise, because it is in the Northern Hemisphere, cyclones, typhoons, as well as the large ocean gyres.