Does water drain in different directions in different hemispheres?

You’ve almost certainly heard about this before. The idea was used as the basis for an episode of The Simpsons, about toilets flushing the opposite way in the southern hemisphere.

I’ve also written about this many times, including in my first book Bad Astronomy, and also here on the blog. In the latter case, it was about debunking fraudsters tricking tourists at the Equator about the effect.

Here’s the thing: This effect is real. It’s called the Coriolis Effect, and is due to the Earth’s rotation. I have an explanation of how it works on that debunking page linked above. The Coriolis Effect is why cyclones rotate, and why they also rotate in opposite directions in the northern and southern hemispheres.

So it’s real, but does it work on smaller basins, like a sink or toilet? The effect is very small, so it’s most noticeable over large areas… and it turns out to be far too small to practically affect your toilet and sink on an everyday basis. Worse, toilets don’t just drain; the water is usually injected into the bowl at an angle, which completely overwhelms the tiny Coriolis effect. Sink drains and basins can have imperfections in them that can also influence the spin of the water.

But what if you controlled for all those effects? Could you see the Coriolis effect over something as small as, say, a kiddie pool?

Yes! And my friends Destin from Smarter Every Day and Derek Muller from Veritasium went to great lengths to show it. Very great lengths: literally opposite sides of the Earth.

They set up identical pools, one in the US and one in Australia, and drained the water from them. They created two videos, meant to be seen side by side. For best viewing, go to Smarter Every Day, but here are the two videos below, stacked vertically. You’ll have to synch them, and they give instructions on how to do that.

Isn’t that wonderful? And very well done. Although it’s not obvious in the video, they repeated their experiments several times, and got the same result every time.

I could nitpick, and say that to be completely thorough, they should swap the pools, taking the northern hemisphere one to Australia and vice-versa, to make absolutely sure there was nothing in the pools themselves causing this, but to be honest I think they’ve done sufficient work here to show what’s what.

This is fascinating to me. I had read that this effect isn’t strong enough to be seen on scales this small, but in fact Derek pointed me to a paper from 1969 that shows it can be done! Very cool indeed. Also, be sure to watch their explanation of how this works in the video after the experiments are completed; it’s one of the best and easily understood I’ve ever seen.

I’ll note this doesn’t really change what I’ve been saying all along: While the Coriolis Effect does drive the rotation of big weather systems, it doesn’t have anything to do with how sinks or toilets drain. Look at all the trouble Derek and Destin went to to show it at all! Clearly, the effect is so tiny on small scales that it’s wiped out by small eddies and imperfections in the basins unless you go to Herculean effort to eliminate them.

My congratulations to Derek and Destin for doing such a fun experiment! I’m flushed with pride for them.