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Ocean Surface Topography from Space
OVERVIEW
Movement of the Earth through Space
Coriolis force

The 24-hour-per-day spin of the Earth accounts for the Coriolis "force." The Earth is 40,000 kilometers (24,900 miles) around at its widest part, the equator. Because it spins on its axis once in 24 hours, a point on the Earth's equator is traveling about 1,700 km per hour (1,000 miles per hour) relative to its axis. But the closer you get to the poles, the smaller the track a point takes in its daily rotation. At 60° North or South latitude, the track is only half the distance that it is at the equator, and so a point travels only half as fast. Air (or water) moving from high latitudes to low then tends to lag, and a person on the surface would feel a wind blowing out of the east. On the other hand, air moving from low latitudes to high is deflected westwards. This also means that moving air or water is deflected to the right in the northern hemisphere, and to the left in the southern hemisphere.

Because of this Coriolis force, air rushing into a region of low pressure will turn into a counterclockwise cyclone in the northern hemisphere and a clockwise one in the southern hemisphere. (Contrary to popular belief, the Coriolis force does not determine the direction water swirls down a drain--the scale is just too small.) Air streaming out of high-pressure systems will also be deflected into clockwise anticyclones in the north and counterclockwise ones in the south.

Ocean currents circulate in gyres, huge clockwise-running currents in the North Pacific and North Atlantic, and counterclockwise-running currents in the South Pacific, South Atlantic, and Indian Oceans. The direction of flow in smaller eddies is also determined by the Coriolis force.


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