The Coriolis force is an apparent force exerted on moving air parcels that are not situated on the equator. It is apparent in that it does not physically affect the motion of air parcels; however from a fixed location on the earth's surface, the Coriolis force appears to divert the motion of air parcels.

In the Northern Hemisphere, the effect of the Coriolis force is to divert air parcels to the right of their direction of movement. In the Southern Hemisphere, this apparent diversion is to the left. The amount of diversion depends on latitude.

The Coriolis effect is only important on time scales more than a few hours. For mesoscale atmospheric motions that last much less than this, like thunderstorms, the Coriolis effect is not important. For synoptic scale low pressure systems, air drawn toward the low center is deflected to the right in the Northern

Hemisphere. The net effect of the Coriolis force is counterclockwise circulation around the low center.

The Coriolis effect also influences oceanic motions. Surface ocean currents driven by the wind do not flow parallel to the wind direction. They flow at a large angle to the right of the wind direction in the Northern Hemisphere. The total transport of water within the layer affected by the wind is displaced about 45 degrees from the mean wind direction. For the U.S. West Coast, the climatological northerly surface winds in the summer lead to upwelling since the water transport is diverted to the right of the wind in the Northern Hemisphere.

To learn more about the Coriolis effect, visit an excellent discription and discussion on the University of Illinois Department of Atmospheric Sciences' Weather World 2010 Web site.