Bruce Perkett, age 14, of Hoyt Lakes, Minnesota, for his question:

Why does draining water swirl in a clockwise direction?

It happens every time, as if by magic. You pull the plug from a tub or basin and the water starts to drain straight down. Then before you know it, the water is swirling down in a clockwise whirlpool. Strange to say, south of the equator, drain water swirls around in an opposite counterclockwise direction. What's even stranger, these happen¬ings occur because the rotating earth is a sphere.

The earth rotates toward the east and because it is a globe, its surface rotates at different speeds.. The equator rotates fastest because it is the largest circle of latitude. North and south of the equator, the surface rotation speed diminishes and dwindles to a stand still at the two opposite poles.

Now imagine what this does to the north and south trade winds that set out from latitudes 30 degrees toward the equator. Where they start, the earth below them is rotating at 403 meters per second. As they proceed, the rotational speed increases to 465 meters per second at the equator. As the ground beneath them moves faster toward the east, trade winds lag behind and slant toward the equator.

Suppose you launch a rocket at a target 1,000 miles to the south. As it travels, the earth's rotation moves the ground beneath it at a faster and faster rate toward the east. If your firing squad is north of the equator, the traveling rocket will veer to the right. South of the equator, the rotation of the round earth veers all objects moving above the surface to the left. The same thing happens to moving surface.. objects that are not affected by friction    such as escaping drain water.

This strange happening was figured out by a French mathematician named Gaspard Coriolis and it is named in his honor. Some people call it the Coriolis force, which seems to suggest a determined push of some kind. Actually no force is involved, and moving objects merely veer or drift off course because the earth rotates beneath them. For this reason it seems more sensible to call the strange event the Coriolis effect. All it does is affect the course of moving objects.

The Coriolis effect veers all winds north of the equator to the right. As hurricane winds blow toward the central eye of rising air, they are twisted toward the right and spiral around in a clockwise direction. The same thing happens to water when it escapes down a drain. In the Northern Hemisphere, the Coriolis effect veers streaming drain water to the right and it spirals down in a clockwise direction.

South of the equator, the twisting effect is reversed. There the round rotating earth veers winds and other friction free moving objects to the left. Winds blowing toward a storm center swerve left and spiral in a counterclockwise direction. Water escaping down a drain veers left and swirls around in a counterclockwise whirlpool.