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Q: Drainwater rotation Northern vs Southern hemispheres ( Answered 5 out of 5 stars,   8 Comments )
Subject: Drainwater rotation Northern vs Southern hemispheres
Category: Science > Physics
Asked by: thinkin-ga
List Price: $2.00
Posted: 01 Apr 2004 22:03 PST
Expires: 01 May 2004 23:03 PDT
Question ID: 323911
The water in your sink drains in a clockwise direction. I think it's
because of the rotation of the earth.So logically I conclude that in
the southern hemisphere it must drain the opposite direction.
(settling a bet)
Subject: Re: Drainwater rotation Northern vs Southern hemispheres
Answered By: easterangel-ga on 01 Apr 2004 23:24 PST
Rated:5 out of 5 stars
Hi! Thanks for such an interesting question.

This is what is called by some as an example for the Coriolis force
BUT according to most experts THIS DOES NOT APPLY to the explanation
of the rotation of water draining in your sink.

?Don't believe what you hear about Coriolis making the water in a sink
or toilet rotate one way as it drains in one hemisphere, the other way
in the other hemisphere. The Coriolis force is noticeable only for
large-scale motions such as winds.?

USA Today: ?Understanding the Coriolis force?

?Is it possible to detect the Earth?s rotation in a draining sink??

?Yes, but it is very difficult. Because the Coriolis force is so
small, one must go to extraordinary lengths to detect it. But, it has
been done. You cannot use an ordinary sink for it lacks the requisite
circular symmetry: its oval shape and off-center drain render any
results suspect. Those who have succeeded used a smooth pan of about
one meter in diameter with a very small hole in the center. A stopper
(which could be removed from below so as to not introduce any spurious
motion) blocked the hole while the pan was being filled with water.
The water was then allowed to sit undisturbed for perhaps a week to
let all of the motion die out which was introduced during filling.
Then, the stopper was removed (from below). Because the hole was very
small, the pan drained slowly indeed. This was necessary, because it
takes hours before the tiny Coriolis force could develop sufficient
deviation in the draining water for it to produce a circular flow.
With these procedures, it was found that the rotation was always

?Bad Meteorology? 

?Explanations are funny things. Indeed, what do you mean by suggesting
that the difference in behavior is a result of the difference in the
(underlying) velocity. Let?s start by ignoring the fact that you
thought (incorrectly) that the issue was just the easterly component
of the velocity and just explore the difference between these two
situations: the large scale with a big velocity difference and the
small scale with a small velocity difference. The implication is that
if one were to match the velocity differences on the two scales, then
the Coriolis force, or maybe the displacement, would suddenly be the
same in each case. Alas, it would not be so (unless other things were
also fudged).?

?The traditional response to what you have suggested is that you have
confused two things, a spatial scale and temporal scale. As usually
presented, the Coriolis force (being a force) produces a result (such
as displacement) over time. The spatial scale (how far it is across
something, or how far an object travels) does not even appear in the
equations. There is much to commend this approach. Within this
context, it is not the spatial scale that produces the effect (for a
given force), but how long the event lasts. A sink drains quickly (not
much time for a small force to produce a significant displacement); a
missile or the air in a hurricane takes much longer to traverse its
territory (a much longer time for a force to produce a significant

?Bad Coriolis FAQ?

Ascher Shapiro of MIT demonstrated this in 1962 with the following experiment.

?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.?

?Do bathtubs drain counterclockwise in the Northern Hemisphere?? 

A more technical explanation can be found here.

?In a kitchen sink, of course, speeds and time scales are much smaller
than hours and miles. Water rushing down a drain flows at speeds on
the order of a meter per second in most sinks, which are themselves
less than a meter wide. Qualitatively, there doesn't seem to be much
chance for deflection. Quantitatively, putting these numbers into
Equation 1 results in an estimated change in rotation of only a
fraction of a degree per second, and a very small fraction at
that...less than an arc-second (1/3600th of a degree) per second over
the course of the entire draining of the sink, ignoring additional
effects caused by conservation of angular momentum and the like. Under
extremely controlled conditions, this can cause water to flow out of a
container counter-clockwise in the northern hemisphere and clockwise
in the southern hemisphere, but your kitchen sink is not so
controlled. Things like leftover spin from filling the sink (even when
the water looks still, it's rotating slowly for a long time after it
seems to stop), irregularities in the construction of the basin,
convection currents if the water is warmer or colder than the basin,
and so forth, can affect the direction water goes down the sink. Any
one of these factors is usually more than enough to overwhelm the
small contribution of the Coriolis effect in your kitchen sink or
bathtub. Research in the 1960s showed that if you do carefully
eliminate these factors, the Coriolis effect can be observed.?

"Getting Around The Coriolis Force" 

Search terms used:
water drains clockwise rotation coriolis Shapiro

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thinkin-ga rated this answer:5 out of 5 stars and gave an additional tip of: $2.00

Subject: Re: Drainwater rotation Northern vs Southern hemispheres
From: probonopublico-ga on 01 Apr 2004 22:20 PST

You've won your bet.
Subject: Re: Drainwater rotation Northern vs Southern hemispheres
From: probonopublico-ga on 01 Apr 2004 23:53 PST
Like I said ...

Sorry but you've lost your bet.

(Please remember to double check BEFORE following my advice.)

A very red-faced PB - not a pretty sight.
Subject: Re: Drainwater rotation Northern vs Southern hemispheres
From: probonopublico-ga on 02 Apr 2004 06:28 PST
I now recall who told me that it was so.

It was a Civil Engineer, no less.

I now know why so many of his bridges have collapsed.
Subject: Re: Drainwater rotation Northern vs Southern hemispheres
From: thinkin-ga on 02 Apr 2004 23:21 PST
Hey thanks again for your very detailed answere.
 And if you don't mind could you tell me which briges he
enginered???..... Just kidding!!
Subject: Re: Drainwater rotation Northern vs Southern hemispheres
From: racecar-ga on 03 Apr 2004 00:01 PST

If you are in the northern hemisphere, and the Coriolis force is what
determines the direction of rotation of water in your sink, it would
drain counterclockwise, not clockwise.

Also, there is some relevant discussion in the comments here:
Subject: Re: Drainwater rotation Northern vs Southern hemispheres
From: easterangel-ga on 03 Apr 2004 00:26 PST
Thanks for the 5 stars and for the tip! :)
Subject: Re: Drainwater rotation Northern vs Southern hemispheres
From: turry-ga on 03 May 2004 02:36 PDT
The water will rotate in the same direction in either hemisphere. The
difference is that you are just looking at it in one case from the top
and the other from the bottom !

Subject: Re: Drainwater rotation Northern vs Southern hemispheres
From: probonopublico-ga on 03 May 2004 02:54 PDT
Hi, Turry

Supposing that you are on a boat that sails more or less directly from
the North Pole to the South Pole ...

You will only be precisely on top (or bottom) when you are at one of the poles ...

At all in-between positions, you will be looking at the water at an angle ...

And it must therefore be rotating sideways.

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