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

14 June 2009

Question

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?

Answer

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 you're 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 it's not going very far so it's not moving very fast. But, the stuff nearer the equator, you're further away from the axis of the earth. So, the distance you travel everyday is further so you're 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 it's absolutely microscopic, it's 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, it's 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 it's 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 don't think they normally do - I've never seen one.Chris - It wouldn't be energetically favorable probably for it to do that.Kath - So it wouldn't do it, it would grind to a halt. Crazy.

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