It is caused, not by the gravitational field of the Earth, but by what is called the coriolis force due to the Earth's rotation, and the desire for matter in motion to move in the straightest possible lines in inertial space. As seen from your reference frame on the rotating Earth, air masses that follow the general convection patterns near the equator are deflected to the west. In the northern hemisphere below latitudes of 30 degrees, the air tries to move towards the equator along the surface, and is deflected to the west. In the southern hemisphere, air also tries to move towards the equator at latitudes less than -30 degrees, and is also deflected to the west, to form the 'Equatorial Trade Winds'.
Hurricanes also rotate in opposite directions because of the coriolis effect.
Dr. Sten Odenwald Return to Ask the Astronomer .