Physics

Do Toilets in the Southern Hemisphere Really Swirl the Opposite Direction?

Common and persistent misconceptions tend to be rooted in truth. The idea that toilets flush in opposite directions in opposite hemispheres is a great example. Any frequent traveler with enough equator-crossing experience can tell you that this is not the case. But why don't flushed toilet swirl in opposite directions in each hemisphere? It turns out there is a real physical phenomenon fueling this misconception. It's called the Coriolis effect.

Turn, Turn, Turn

The Coriolis Effect makes wind, water, and virtually every other free-moving thing curve with the rotation of the Earth on its axis. The two basic concepts at play here are the spherical geometry of the Earth and Newton's First Law.

The Earth is about 24,000 miles in circumference at the Equator, and it makes a complete rotation once per day. This means that someone standing still in Ecuador is actually moving about 1,000 miles per hour faster than someone standing still on the North Pole.

Now compare both those people to someone standing still at the 45th parallel north – on the border between Wyoming and Montana, for instance. There, the ground is moving at roughly 750 miles per hour relative to the Equator and the North Pole.

Newton's First Law states that objects in motion tend to stay in motion. This means that if you stood in Billings, Montana and threw a baseball with enough force to reach Quito, Ecuador, you would miss your target because Quito rotated along the Earth's axis faster than Billings, Montana did.

Now, imagine the difference in velocity in the ground under the northernmost edge of a hurricane and its southernmost edge. Hurricanes can be several hundred miles in diameter. It's a big difference.

Hurricanes spin counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere because of their enormous size. Their hundred-mile diameters lead to huge differences in ground velocity between the northernmost and southernmost edges of the storm system.

In order for Coriolis forces to be noticeable, the systems they impact have to be very large. Its effect on small fluid systems — like toilets — is so small as to be virtually undetectable.

Science in Your Toilet

It turns out that the Coriolis Effect does impact a flushing toilet, but to such a tiny degree that observing it requires negating every single other force that could possibly act on the water.

In 1962, an MIT fluid mechanics expert named Ascher Shapiro performed an experiment to observe the Coriolis Effect in a large, shallow dish of water. He had to carefully control the conditions of the experiment and even compensate for the minuscule force of air drag in order to notice a tiny counterclockwise movement as the water drained.

A team in Sydney Australia later performed the same experiment and demonstrated the opposite results.

What this means in practice is that the direction a toilet flushes has more to do with the shape of the bowl and incoming water's inertia than it does with the rotation of the Earth. Another legend debunked!

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Delve into one of the Coriolis effect's biggest roles — hurricanes — in "Weather: An Illustrated History: From Cloud Atlases to Climate Change" by Andrew Revkin and Lisa Mechaley. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Austin Jesse Mitchell July 14, 2016

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