Myth 3: Does water swirl counter-clockwise in the Southern Hemisphere?

by Stephanie Chasteen on April 6, 2008

The answer: yes and no. When applied to toilets and sinks, this is one of those “too good to be true” science factoids, I’m afraid. But it does apply in some situations.

The myth goes that if you flush a toilet in Australia the water swirls down the drain the opposite way than in the northern hemisphere, due the Coriolis effect (an apparent force which describes how objects veer to the left or right when traveling on something that’s rotating — see the link above for a good visualization of this).

If there were no other forces on that water in the sink or toilet, that would be true. The Coriolis effect does actually make hurricanes rotate the opposite direction in the two hemispheres. But for toilets and sinks it’s another story. The toilet myth is easy to dispell — just peek around the rim of the toilet and you’ll see that the water is jetted into the bowl at an angle, which determines the direction the water swirls. Sinks, however, are a little more tricky.

I’ve heard of charlatans who hang around the equator in Kenya, carrying basins of water. They’ll stand on the southern side of the equator with the basin, pull a plug at the bottom, and show that it swirls out counter-clockwise. Then they’ll walk to the northern side of the equator, fill the basin and pull the plug, and it swirls out clockwise. Irrefutable proof? Be careful! You have to know all the initial conditions in any experiment, and in this one, there is one that is hidden from you. The huckster just has to add a slight rotation to the water before letting it out (for example, pour the water in at a very slight angle to give it an initial rotation, and it will “remember” that rotation as it swirls out of the basin. In fact, you can swirl the water in the basin, then walk away from it for several hours, and it will still “remember” that rotation when you pull the plug! Plus, the charlatans got it backwards — water should actually swirl counter-clockwise in the northern hemisphere if the Coriolis effect were at play! (See Alistair Fraser’s website for a great explanation of how you can re-create this fakery for a fun party trick!)

Even if you don’t give the water an initial swirl, tiny pits and imperfections in the basin can give the water a rotation — which may be clockwise or counterclockwise, but doesn’t depend on which hemisphere you’re in!

Now, all that said, the Coriolis effect does play a role. It’s just really tiny relative to all these other effects. It’s about 10 millions the size of the effect of gravity. So, if you have a perfect basin, with completely still water, then the Coriolis force will make the water swirl opposite directions in the two hemispheres. This was demonstrated by Ascher Shapiro, a researcher at MIT in 1962. You can see the Straight Dope talk about this topic, or detailed information from Alistair Fraser.

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{ 13 comments… read them below or add one }

avatar John W Ryan September 10, 2008 at 1:11 am

Kenya is not alone in this demonstration of swirl. Colombia Museum also does a demo.

My view is simply that a static bowl of water is travelling at two speeds. the water clostest to the equator is moving faster than the on the opposite side.
clockwise in Australia and anti in the UK.

i

avatar Matt September 10, 2008 at 1:08 pm

I suspect that imperfections in the bowl, and the initial direction of the water play a much more important part in the direction of the water than any initial speed from the Coriolis effect.

avatar sciencegeekgirl September 10, 2008 at 1:23 pm

What Matt says is what my blog post is trying to convey. I’m not quite clear on what point John is making above. One thing for certain — a “static” bowl of water is practically a physical impossibility, as is a perfect bowl of water. There will be some movement to begin with and, as Matt says, there are imperfections in the bowl that play a huge role.

avatar travelingBeauty February 26, 2009 at 5:36 am

I am from Canada. And there, water always, ALWAYS swirls clockwise. Even if storms seem to do the opposite, all sinks, toilets, showers, and any other drains I have ever used have gone clockwise.
I am currently living in Australia, and while the toilets don’t swirl at all – everything is sucked quickly down the drain – the sinks definitely swirl counter-clockwise, or anti-clockwise, every time. I notice, because I’m so used to it going clockwise that it looks really strange. The first time, my stomach actually flipped.

avatar sciencegeekgirl February 28, 2009 at 6:47 am

Not sure what to say to that, travelingBeauty. It defies what the experiments have told us and what I understand from the post. Do you see this in all the sinks there, even after the water has sat for a long time? What about a basin? Remember that water has a long “memory” of which way it was rotating when it filled a basin, and will swirl out in the same way.

avatar Linda Aukschun April 17, 2009 at 12:08 am

Thanks. I’m watching the Simpsons episode about this (again) and I Googled and got you. Good answer.

avatar Andy September 8, 2010 at 1:57 am

My dad lived in Austrailia for 2 years and says that it does. Are you sure?

avatar sciencegeekgirl September 8, 2010 at 3:26 am

Read the other links and make up your own mind based on the evidence and explanations!

The way to find out experimentally is to take a bowl with a plug in the bottom and let it out that way. Otherwise, you don’t know if it’s just that the toilet manufacturers tend to make toilets that swirl the water counter-clockwise down there!

avatar Trimac20 April 21, 2012 at 1:34 pm

When I first saw that Simpsons episode the first thing I did was try it. Yes, the toilet water does swirl counter-clockwise here, as does the water going down the sink. Unfortunately I did not remember to do that when I visited the US or anywhere else in the Northern Hemisphere. I live in Oz, in case you haven’t already guessed.

avatar Dan C May 4, 2012 at 12:11 am

I live in Los Angeles. I’m sitting in Sydney right now and of course the addition of jets facing a certain direction will affect it, but that’s not really the science experiment, correct?

Filled up the bathtub and sink in my hotel 3 times each and let them drain. The water doesn’t spin as quickly as in the States, but it does most certainly, definitely, without a doubt go down my bathtub drain and sink drain counterclockwise – i.e. the opposite direction from the northern hemisphere.

There’s no mind to be made up here. Anyone trying to experiment using a force flush device like a toilet needs to learn how to identify what the goal and inputs of a science experiment are.

avatar Jen August 4, 2012 at 10:32 pm

Hello there, I have a question about the Coriolis effect and my bath tub. Its a square tub with bumps on the bottom that lead towards the drain.
Every night after my daughter has a bath we watch the water drain out and we’ve noticed something very strange. It will start draining clockwise (we live in Australia), then it will choke a bit and after its done choking it will finish draining anti-clockwise!
Why????

avatar PK December 31, 2012 at 6:34 am

OK, contrary to what some ppl call them, this huckster and his friends are showing Coriolis Effect at the Equator in Kenya
These hucksters keep things simple:
As they do in economics, they hold things the same:
the same water, the same bowl and the same sticks.
Furthermore, they show the water under the hole.
No toilet.
http://vimeo.com/55947506

avatar nbd March 15, 2014 at 10:30 pm

it is caused by the affect but through memory not the direct corriolis affect maybe

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