A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Do bathtubs drain counterclockwise in the Northern Hemisphere?

April 15, 1983

Dear Cecil:

My sources tell me that water, when it spirals out of a drain, flows in one direction only in the Northern Hemisphere and in the opposite direction in the Southern Hemisphere. Is this so? Why? Is the spiral especially pronounced at the poles? Subdued at the equator? If I carried a drain across the equator would the spiral reverse directions? How do drains work in outer space? I know this probably seems like a lot of questions, but I have an unquenchable thirst for learning.

Cecil replies:

Lucky you found me then. Given the limitations of a once-a-week column, it'd be an exaggeration to describe myself as a fountain of knowledge. But I'm definitely a persistent leak.

The erroneous bit of folk wisdom you refer to says water always drains in a clockwise direction in the Southern Hemisphere and in a counterclockwise direction in the Northern Hemisphere. The supposed reason for this is the Coriolis effect, which has to do with the effect of the earth's rotation on moving objects.

Now, there is such a thing as the Coriolis effect. It explains why macroevents such as hurricanes rotate in a clockwise direction in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. However, when you get down to itty-bitty phenomena such as the water draining out of your bathtub, the Coriolis effect is insignificant, amounting to roughly three ten-millionths of the force of gravity (in Boston, at least, which is where they happened to do the measuring).

The boring truth is that water drains every which way no matter what hemisphere you're in, for reasons having mostly to do with the shape of the drain, the way you poured in the water in the first place, and so on.

All this was demonstrated way back in 1962 by one Ascher Shapiro, a researcher at the Massachusetts Institute of Technology. Shapiro filled a circular tank six feet in diameter and six inches high in such a way that the water swirled in a clockwise direction. (Remember, Coriolis forces in the Northern Hemisphere act in a counterclockwise direction.)

Shapiro then covered the tank with a plastic sheet, kept the temperature constant, and sat down to wait. When he pulled the plug after an hour or two, the water went down the drain clockwise, presumably because it still retained some clockwise motion from filling.

On the other hand, when Shapiro pulled the plug after a full 24 hours, the draining water spiraled counterclockwise, indicating the motion from filling had subsided enough for the Coriolis effect to take over. When the plug was pulled after four to five hours, the water started draining clockwise, then gradually slowed down and finally started swirling in the opposite direction.

Needless to say, unless you're a consummate slob, you don't wait 24 hours (or even 4-5 hours) to drain your bathtub. The influence of the Coriolis effect may thus be safely described as slight.

But I'm sure the myth of the bathtub spirals will endure. Shapiro did his work in 1962 and I proclaimed it to the world in 1983. Yet next to the mystery of where all the baby pigeons are, this remains the commonest question I get.

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