Motion | Pages 32-33
Spinning Storms and Worlds

The Coriolis effect is an apparent deflection in the paths of moving objects caused by Earth's faster speed near the equator than near the poles. In the solar system, the greatest expression of this effect occurs in planetary atmospheres. On Earth, for example, the air around us moves freely above the ground. Low-pressure centers draw air in; high-pressure centers push air away. As illustrated here, Earth's differential rotation bends the paths of moving air just enough to create a consistent circulating pattern.

Unlike spinning bicycle wheels and other kinds of rotation we see all around us, Earth's rotational rate is very small--only one rotation a day. Water swirling down a drain, by contrast, may take only a few seconds to make one rotation, a very fast rate. Thus, contrary to popular belief, the Coriolis effect doesn't influence the direction of draining water one way or the other.

A draining tub

The Coriolis Effect

Because Earth rotates faster near the equator than near the poles (indicated by the varying lengths of the yellow arrows), moving air drawn to low-pressure areas at midlatitudes travels east either faster or slower than the low itself. As the low draws air in (white arrows), the difference in speeds causes the air to curve--counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere (purple arrows).

On Earth

As shown in this photograph of the 1997 Pacific storm named Hurricane Linda, air moving from all directions toward a low-pressure area rotates counterclockwise in the Northern Hemisphere. By contrast, air being pushed away from high-pressure areas circulates clockwise north of the equator and counterclockwise south of the equator.


In the Solar System

Jupiter's Great Red Spot, a huge storm bigger than Earth itself, exemplifies the Coriolis effect at work on other planets. Observed for more than 300 years, the Great Red Spot rotates counterclockwise in Jupiter's Southern Hemisphere--a dead giveaway that it is a high-pressure system.

Not Coriolis

The direction of rotation of a draining tub is determined by the way it was filled or by ripples introduced while washing. The magnitude of these rotations may be small, but at the scale of your bathtub they easily overpower any effect of Earth's rotation.