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A Straight Dope Classic from Cecil's storehouse of human knowledge
15-Apr-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. --Victor C., Chicago
Dear Victor:
Lucky you found me then. Given the limitations of a once-a-week column, it is maybe an
exaggeration to describe myself as an everlasting fountain of knowledge. But I am
definitely an unstoppable leak.
The erroneous bit of folk wisdom you refer to says that 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 "fact" is the Coriolis effect, which
has to do with the effect of the earth's rotation on moving objects.
Well, 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 which have to do mostly 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, now, that 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 read comic books or whatever scientists do while they wait for their experiments
to percolate. 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, if Shapiro pulled the plug after waiting a full 24 hours, the draining
water spiraled counterclockwise, indicating that 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 are a consummate slob, you do not wait 24 hours (or even 4-5
hours) to drain your bathtub. Hence the influence of the Coriolis effect may 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.
--CECIL ADAMS
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