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Science File: An exploration of issues and trends affecting
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I Didn't Know That

July 16, 1998

Q: Does the Coriolis effect really cause water to swirl around drains in different directions in the Northern and Southern hemispheres?

A: Yes, but only if the body of water is large enough--say a lake or reservoir--for the Coriolis force to overcome random water currents, according to astronomer Neil de Grasse Tyson of the Hayden Planetarium. The Coriolis force is an inertial force caused by the Earth's rotation; it deflects a moving body to the right in the Northern Hemisphere and to the left in the Southern. For toilets, sinks or even swimming pools, random currents are stronger than the Coriolis force and water is no more likely to swirl in one direction than another.

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