Finally, Thomas Humphrey, a senior scientist at the San Francisco Exploratorium, discusses in more detail the reasons why we do not see the Coriolis effect at work in the bathroom:
"There is an African country near the equator where entrepreneurs have set up two toilets, one just north of the equator, the other just south of it. For a fee, they will allegedly demonstrate that the toilets flush in opposite directions. It is only for show, however; there is no real effect. Yes, there is such a thing as the Coriolis effect, but it is not enough to dominate the flushing of a toilet--and the effect is weakest at the equator.
"The telling comparison is between the magnitude of the Coriolis effect and the initial amount of angular momentum in the water--that is, how much is it spinning anyway, regardless of the earth's rotation. Coriolis acceleration at mid-latitudes is about one ten-millionth the acceleration of gravity. Because it is a very small acceleration, it needs a very long distance for it to produce an appreciable curvature--and hence directionality--to the motion. A toilet or sink is just not large enough. The Coriolis effect influences because wind velocities may be hundreds of times greater than the motions in a sink and because the distances involved are far larger than the tiny draining diameter in a sink or toilet.
"It is impossible to find a cup full of water that does not have some average net motion; it will always be going one way or the other, and that little amount of angular motion is enough to swamp the Coriolis effect. The net motion in the water becomes much more pronounced as the water is forced to move in toward the center of evacuation, causing the normally invisible flows in the water to become visible as the water nears the drain. The ultimate direction of that flow is random--it can go one way one time, the other way the next.
"If you run an experiment in your sink--fill the sink, then pull out the stopper--the water will almost always go down the same way, making you wonder if this is really a random effect. But you will find that the faucet is almost always off center or that there is some other asymmetry in the sink. As a result, filling the sink consistently gives it some net rotation in the same direction, which you see as the normal direction of evacuation. Toilets will always drain and fill the same way, for the same reason.