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Debunked: Does water drain in a different direction in each hemisphere?

You remember this from The Simpsons, don’t you?

Image: water drain via Shutterstock

IN THIS SERIES, TheJournal.ie takes a look at an urban myth, old wives’ tale, or something that your mammy told you years ago to see if there’s any truth in it.

We’ve all learned an awful lot from The Simpsons.

You might remember an episode where the family travel to Australia. In the American Embassy, they come across a contraption that reverses the direction of water swirling down a toilet, since in the Southern Hemisphere will flow counter-clockwise, and in the Northern Hemisphere clockwise.


This belief is pretty commonplace, and while we shouldn’t always take what we see on The Simpsons as gospel truth, the science behind it does sound as though it could add up.

The Coriolis Effect, sometimes called the Coriolis Force, is said to be what causes the different directions of flow.

Not the easiest of topics to explain to detail, it is defined as “the deflection of an object moving above the earth, rightward in the northern hemisphere and leftward in the southern hemisphere”.

Speaking to TheJournal.ie, Dr Rowan Fealy of NUI Maynooth said the best way to visualise this is by imagining standing on the North Pole and throwing an object south.

Because the Earth rotates beneath the object, the object will not land where you expect it, but instead slightly to the right.

If you were standing on the South Pole, the same would happen, although the object would end up on the left.

This rotation of the Earth can cause swirls in our weather systems and in our oceans.

So does it affect the water draining from your bath? Anyone who has ever traveled to the opposite hemisphere and paid attention to the phenomenon will tell you, quite simply, no, this myth is “utter nonsense” as Dr Fealy put it.

The Coriolis Effect is very small. It only impacts slow moving very large objects, typically in the order of hundreds of kilometres, like very large ocean currents and parcels of moving air.

“The effect on your bathtub would be extremely limited to non-existent.”

He said that the shape of the drain or the container from which the fluid is flowing can affect the direction of flow. Some will flow clockwise, others might flow counter-clockwise.

That said, some reports mention a 1908 experiment in which a tank of water was left for 24 hours to let the fluid become completely still. The plug was carefully removed, and while the water first drained straight out of the tank without any rotation, it eventually began to swirl.

Is there a myth you’d like debunked? Email nicky@thejournal.ie

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About the author:

Nicky Ryan

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