Science Questions

Why does water go the opposite way in the Southern Hemisphere?

Sun, 14th Jun 2009

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Mike Butcher, Galston asked:

When the plug is taken out of the sink full of water, the water goes down the plug hole and the water goes down empty on a clockwise direction. I e-mailed a friend of mine in Australia and asked him to check and he tells me that the water goes on a clockwise direction. Can you explain why?


Dave -   Okay, this is an effect, which theoretically would work in certain circumstances.  It definitely works with big weather systems or low pressure areas.  Essentially, if youíre a low pressure area or anything which is sucking liquid in from a long way away, the stuff which is to the North; because the Earth has a smaller radius out there is moving, going round the Earth once a day, but itís not going very far so itís not moving very fast.  But, the stuff nearer the equator, youíre further away from the axis of the earth.  So, the distance you travel everyday is further so youíre travelling faster.  If you then suck the stuff in towards the central point, the stuff which is going faster, from the South will overtake stuff from the North and it will sort of start to spin around into the center.  Now, this is an effect which does happen, cyclones go anti clockwise in northern hemisphere and clockwise in southern hemisphere.  But, when you start talking about emptying basins and sinks, the problem is this effect is there, but itís absolutely microscopic, itís tiny.

Chris -   People have measured it.

Dave -   People have measured it, yes.  Americans did make a huge bath, several meters across.  They put a little bit of water in it and left it to sit for a fortnight and they pulled the plug out in a very controlled manner.  If you do that, it does always get out anti clockwise in northern hemisphere.  Problem is in a normal sink, itís much more affected by which tap you use to turn it on.  How you move your hands in it within hours before you left it to pull the plug out, and exactly how you pull the plug out.  And so, we did this experiment on the Naked Scientists a while ago and we found itís essentially random in both northern, southern hemispheres.

Kat -   You mentioned about cyclones going different ways.  What happens to the cyclones moves across the equator?  Does it suddenly stops and start going the other way?

Dave -   They generally slow down and I donít think they normally do - Iíve never seen one.

Chris -   It wouldnít be energetically favorable probably for it to do that.

Kath -   So it wouldnít do it, it would grind to a halt.  Crazy.


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Mike Butcher asked the Naked Scientists: Can you confirm (and explain) that water goes around in an anti-clockwise direction down the plug-hole in the Northern Hemisphere and clockwise in the Southern hemisphere? What do you think? Mike Butcher, Sun, 14th Jun 2009

It is true, but only of very controlled experiments. you can't just fill your sink up then unplug it and watch it, its called the coriolis effect and is only a very insignificant force in terms of deciding the direction of the water spiralling, any motion in the fluid before you pulled the plug would mostly determine it. To actually observe the effect you need a very large shallow dish of uniform shape, leave it for a long time so that any of the initial motion in the fluid has pretty much all stopped, and would have to be at constant temperature to stop any convection currents occuring, and then let it drain through a very small hole (you'd have to unplug from the bottom, putting your hand in the fluid and pulling a plug out would obviously give the fluid motion again).

Then the effect can be observed, it would spiral counterclockwise in the northern hemisphere, clockwise in the southern. Madidus_Scientia, Sun, 14th Jun 2009

There was this:
Also a poll that I cannot find at the moment. Chemistry4me, Sun, 14th Jun 2009

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