Be very, very careful what you put into that head,because you will never, ever get it out. Thomas Cardinal Wolsey (1471-1530) Bad Coriolis Click on the symbol for its explanation.

The water in a sink (or toilet) rotates one way as it drains in the
northern hemisphere and the other way in the southern hemisphere.
Called the Coriolis Effect, it is caused by the rotation of the Earth.

First on this page is a discussion of the issue. Towards the bottom of the page you can see examples of incompetence from PBS,NPR, and Sports Illustrated.

The Coriolis force does influence long-lasting vortices.
 Hurricane Andrew
On the scale of hurricanes and large mid-latitude storms, the Coriolis force causes the air to rotate around a low pressure center in a cyclonic direction. Indeed, the term cyclonic not only means that the fluid (air or water) rotates in the same direction as the underlying Earth, but also that the rotation of the fluid is due to the rotation of the Earth. Thus, the air flowing around a hurricane spins counter-clockwise in the northern hemisphere, and clockwise in the southern hemisphere (as does the Earth, itself). In both hemispheres, this rotation is deemed cyclonic. If the Earth did not rotate, the air would flow directly in towards the low pressure center, but on a spinning Earth, the Coriolis force causes that air to be deviated with the result that it travels around the low pressure center.

In the accompanying picture of the Caribbean, one can see the cyclonically spiraling clouds of Hurricane Andrew (at the mouth of the Mississippi) and of another vortex in the Atlantic.

But, the Coriolis force is very small, indeed.
 water draining in the supposedly wrong way
Compared to the rotations that one usually sees (tires on a travelling automobile, a compact disc playing music, or a draining sink), the rotation of the Earth is very small: only one rotation per day. The water in a sink might make a rotation in a few seconds and so have a rotation rate ten thousand times higher than that of the Earth. It should not be surprising, therefore, to learn that the Coriolis force is orders of magnitude smaller than any of the forces involved in these everyday spinning things. The Coriolis force is so small, that it plays no role in determining the direction of rotation of a draining sink anymore than it does the direction of a spinning CD.

The direction of rotation of a draining sink is determined by the way it was filled, or by vortices introduced while washing. The magnitude of these rotations may be small, but they are nevertheless gargantuan by comparison to the rotation of the Earth. I decided to include a picture of a draining sink, and the first one I tried in my house was found to drain clockwise (the opposite of what the silly assertions would have it do here in the northern hemisphere). This direction was determined entirely by the way the tap filled the sink. The direction of rotation of a draining toilet is determined by the way the water just under the rim is squirted into the bowl when it is flushed.

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 cyclonic.

Why do teachers claim that a draining sink reflects the rotation of the Earth?
A surprisingly large number of my undergraduate students tell me that their high-school teachers told them that sinks drain in opposite directions in the two hemispheres owing to the rotation of the Earth. Why would a teacher offer such garbage to students when it is so easy to check. A trip to the school washroom (let alone the ones at home) will reveal drainage in both directions (which would certainly require the equator to assume a tortuous track through the countryside).

Is knowledge just a bunch of abstractions to be memorized with no recourse to the relevance of everyday experience?

Sigh... I don’t know why teachers do this. I can but assume that those who do so just never feel any need to wash their hands --- or their minds.

Incompetence from those we trust
Incompetence from PBS (USA)
Fakery of the first water (so to speak).
There are charlatans operating at a tourist trap in Nanyuki, Kenya. In this little town, located right on the equator, a local mountebank works for tips as he glibly cons busloads of tourists into believing that the rotation of the Earth causes water draining from a container to spin clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere. (Yes, you read that correctly, the charlatan fakes it backwards. You would think that if he were going to sucker people, he would at least get his directions the same as what really happens in large weather systems.)

This man’s nonsense was captured (and endorsed) by Michael Palin in one episode of his BBC TV special, From Pole to Pole, which is often aired on PBS. The presentation went as follows:

faker:
This is the northern hemisphere (gesturing to his left), and this is the southern hemisphere (gesturing to his right). If you drain a sink when you’re on the northern side of the equator, and you watch the water as it drains, you will see that the water always rotates clockwise [sic]. (Shot of a pan with water draining clockwise. Floating match sticks are used to make the motion easier to see.) This phenomenon is caused by the rotation of the Earth. The effect becomes stronger according to how far you move to the north or to the south and becomes weaker according to how close you go towards the line [the equator]. So that’s why we have to give some distance from the equator so that the rotation can be noticeable.
Palin:
This is known as the Coriolis effect and Peter McLeary has given this same lecture every day for the last six years. It’s delivered in the burnt out shell of an old hotel. The equator used to run through the middle of the bar. I bet they were always floating match sticks in the middle of the beer. (The faker has been carrying his pan and water about ten meters to the south of the spot marking the equator, and turns to face the audience.)
faker:
So, this changes to counter-clockwise [sic] indicating that now we are on [sic] the southern hemisphere. (Shot of the water in the pan draining counter-clockwise.)

(Transition to a scene where the faker is placing the water-filled pan directly on the equatorial marker.) So, now we are right on the equator, and as we drain the water, you’ll see there will be no rotation. It just drains straight down. And that’s how we prove that we are right on the equator. (Water draining with no apparent rotation.)
Palin:
It does work.

Sure it does --- in the hands of a mountebank, that is. And now we have Michael Palin acting as a shill for bad science --- and on PBS, no less.

 Huh? A correspondent has written me to say that in the version of the program she saw, that "Mr. Palin proceeded to describe how the demonstration was pure bunk and told how the charlatan pulled off his little deception..." Yet, in my version of the program, Palin unambiguously endorses the demonstration. Could it be that there are two versions out there, the original and an amended one in which Palin attempts to address some of the criticism of the original? Does anyone know? So far, the only readers who have written me on this point are ones who have seen the Palin-endorsing version. There are no further reports of a version where Palin is claimed to have pointed out that the demonstrator had cheated.
But, how is the fraud accomplished? The Coriolis force is so tiny that it cannot cause the rotation in the faker’s draining pan; indeed at only ten meters to either side of the equator, it is so tiny that it could influence neither the carefully performed experiment (described above) nor the large scale motions of weather systems.

So, the faker must be forcing the rotation by other means, and by a sufficiently unobtrusive way that the busloads of tourists do not spot the means. Indeed, a colleague of mine, who witnessed the performance first hand and knew it was a cheat, was not able to spot how the fraud was perpetrated. (It is an interesting sidelight that when back on the bus, he informed his fellow tourists that they had just witnessed fakery --- the Earth did not cause the rotation they had just seen --- there was widespread disappointment. The tourists preferred the fantasy to the reality.)

Do-it-yourself fakery.

There are two clues to the successful fakery of the drainage changing direction at the whim of the mountebank. One is revealed in each of the two images (above) which I captured from the TV program:

Non-circular pan
The non-circular pan allows the faker to easily introduce rotation into the pan after he begins. Let us imagine that the pan is filled very carefully so that there is no rotation initially. Indeed, one wants to be able to show the audience that any rotation introduced while filling has died out. Now what? If the pan were circular, it would be harder to start the water spinning by turning the pan itself, but by having the pan nearly square, the water must turn if the pan does.

Turn to face the audience
But how does one turn the pan without the audience becoming suspicious. Obviously, it must be done in such a way that the audience does not attribute the action to part of the fakery, but to a courtesy which enables them to see better.

The procedure is as follows:

Find the materials
You want two plastic containers: one which is to be the pan to be drained, and the other is a bucket for the storage of water. The pan should be non-circular. I found a quasi-square one (made by Sterilite) for \$1.28 in the local Wal-Mart. It is the three-cup size (.7 liter). It works well. Drill a hole (say, 1/4 inch or 3/8 inch) through the center bottom. You don’t need a special stopper for the hole: your finger placed underneath works just fine. The other container can be any small plastic bucket with the only restriction that your pan should be able to sit on top of it so that the bucket can catch the water as it drains.
Fill the pan
Choose the spot that you wish to claim is the equator (the center of your classroom will do). Fill the pan from the bucket there and let the motion from filling die out. (You can insert and then gently remove a vertical object, such as index card, into the water to dampen the motion.) You can then show that there is little or no circulation by floating a match stick (or sprinkling pepper) on the water.
If you are going to the north side of the equator, stand in front of the pan facing south. Pick it up, turn around by turning to your left, walk to the north end of the room, turn around by turning to your left, and face the audience. As you have a non-circular pan, you have now introduced counter-clockwise rotation (which is cyclonic in the northern hemisphere) into the water.
The coup-de-grace
Add a tracer (such as matches or pepper). Remove the stopper (your finger) and let the water drain. Lie through your teeth by claiming that it is the rotation of the Earth here in the north which is causing the water to circulate the way it does.
The other hemisphere
Go back to the equator but this time stand on the south side looking north. After filling the pan, turn around by turning to your right, walk south, again turn around by turning to your right to face your audience (cyclonic rotation in the southern hemisphere is clockwise), and remember to complete the demonstration by lying again.
The equator
This is the hardest part of the fakery, because it is actually very difficult indeed to eliminate all rotation from a pan of water. It really should sit for a very long time (and your finger might get tired). If the pan is fairly deep, and the hole small, it takes a moment or two before the rotation is apparent. This seems to be the dodge followed by the Kenyan faker.
What can I say, it all worked the very first time I tried it. If any of you can devise any improvements in the procedure, please send me a note.

Incompetence from Sports Illustrated
One does not normally turn to Sports Illustrated for insight into the natural world, unless, possibly that bit of the natural world which sometimes wears swimming suits. However, one does not expect incompetence from the magazine either. Yet, the special Winter 1998 swimsuit issue offers much more than its standard set of salacious images, it offers geographic and scientific swill. In an article by Jamie Malanowski, entitled, Zero Latitude, (starting on page 16) we learn:
Say you’re vacationing at a nice hotel in Costa Rica [sic]. Feeling restless, you go for a walk, heading in a generally southern direction. After a few days and a few beverages of your choice, you stop at an appropriate facility and do what you do whenever nature calls. When you flush, you note that the water whirls down the bowl in a counterclockwise direction. Now resume your walk. Go far enough south, and the next time you hit the head, the water will spin clockwise down the drain. And then it hits you: Somewhere back there you crossed the equator!
Apart from the fact that the equator is one very long (and difficult) walk from Costa Rica, any difference in the behavior of toilets on the journey is the result of happenstance and not the crossing of the line.

Incompetence from NPR (USA)
On October 16, 1996 the NPR program, Rewind, blotted its escutcheon with their  nonsensical discussion  of the Coriolis force. The librarian who prepared it supported her position with references to the book, Rainbows, Curveballs and other Wonders of the Natural World Explained, by Ira Flatow, host of NPR’s Talk of the Nation - Science Friday and with a previously aired PBS (cum BBC) program called, Pole to Pole (or, Full Circle). You can read about the silliness in Pole to Pole, above. I have yet to see just what it is that Flatow had to say. Note: The program has since retracted its mistake.

Incompetence from the author of a standard undergraduate physics textbook.
A physics student from Nottingham University, in the U.K., wrote to tell me that the physics textbook they are assigned in one of his courses states:
"...on a smaller scale, the coriolis effect causes water draining out a bathtub to rotate counter clockwise in the northern hemisphere..."
Sigh, this mind-numbing example of scientific incompetence is offered by author, Paul A. Tipler, on page 128 of his book, Physics for Engineers and Scientists, 4th Edition. One wonders if Tipler gets the relative magnitude of the other forces in nature wrong, or if he reserves this privilege for the Coriolis force. In the U.S.A., the book's publisher is W.H.Freeman, and in the U.K. it is Worth Publishers. SHAME ON BOTH PUBLISHERS.

But, competence from teachers

So what do you tell your students.
The direction of rotation in draining sinks and toilets is NOT determined by the rotation of the Earth, but by rotation that was introduced earlier when it was being filled or subsequently being disturbed (say by washing). The rotation of the Earth does influence the direction of rotation of large weather systems and large vortices in the oceans, for these are very long-lived phenomena and so allow the very weak Coriolis force to produce a significant effect, with time.