The Bathtub Vortex
Many people recall hearing in their childhood that water draining from a basin will swirl in different directions on opposite sides of the equator. Not everyone considers this to be a fact or remembers which way the water is supposed to swirl or why.
There are two underlying reasons why water should swirl in opposite directions in the northern and southern hemispheres, and both result from rotation of the earth. Looking downward in the northern hemisphere, the ground is rotating counterclockwise as the earth turns beneath our feet (in the southern hemisphere, it rotates clockwise). Everything at rest around us, including water in a basin, rotates at the same time. The water therefore possesses a small measure of angular momentum (which merely means that it would attempt to keep rotating if the earth stopped). If the water is drained through a hole with a diameter smaller than that of the container, the angular momentum must be taken up in a smaller and smaller cross-section. When this happens, the rotation increases in the same manner that a figure skater's rate of spin increases when she pulls her arms to her sides.
The second factor contributing to a preferred direction of rotation is the Coriolis effect, which diverts moving objects (such as projectiles, airplanes, or, in the present case, water) to the right in the northern hemisphere and to the left in the southern hemisphere. As long as the water in the basin is at rest, the Coriolis effect plays no role, but, in the northern hemisphere when a plug is pulled and the water begins to move toward the drain, it is deflected ever so slightly to the right. The Coriolis effect would be quite noticeable (and frustrating) to two people playing a game of catch on a rotating turntable such as a carousel.
If one person throws a ball directly toward someone else also standing on the turntable, he would probably be quite astonished to see that he has thrown a curve that even Sandy Koufax would be proud of, and that he has missed his catcher entirely. Actually, to people observing from the ground, the ball has traveled in a straight line (neglecting drop), and it only appears to curve to someone standing on the turntable. Although the Coriolis effect is not a "force" at all, as it is sometimes called, it must be dealt with when one is working with a rotating system such as a carousel or the earth. What of the water in the basin? As it moves toward the outlet, is deflected slightly to the right (in the northern hemisphere) on all sides before it reaches the drain, thus setting up a counterclockwise vortex.
In reality these effects are almost vanishingly small at the scale with which we are dealing. Anyone who has tried knows that water can easily be made to swirl in either direction in a drain if it is prodded a little first by stirring. Scientists in both hemispheres have taken the matter seriously and conducted tests under carefully controlled laboratory conditions. They found, as expected, that any small amount of rotation initially introduced (such as from a slightly angled spigot) persisted for many hours, and that the water would always swirl out the way it swirled in. If the water was allowed to settle for several days, however, experiments at the Massachusetts Institute of Technology showed that a vortex consistently developed in a counterclockwise direction (as it "should" in the northern hemisphere) and workers in Australia observed a clockwise rotation.
Because the magnitude of both underlying causes increases poleward from the equator (where they are zero), residents at the higher latitudes find themselves in an enhanced position to perform their own experiments. But don't be surprised if the next time you take a bath the water flows out clockwise, unless you have taken the hours required to still the water completely.
