Synchronized videos get to the bottom of hemispheric toilet swirl mystery

Coriolis Effect in a southern hemisphere kiddie pool.
Coriolis Effect in a southern hemisphere kiddie pool.
Image: Veritasium

Which way does the water in your toilet bowl swirl when you flush?

If you live in the northern hemisphere, it swirls counter-clockwise, while if you live in the southern hemisphere, it swirls clockwise. Right?

Not quite.

Two new synchronized videos posted by a couple of science-minded YouTube stars show that the swirl of your toilet water has more to do with the force of your toilet's flush than the hemisphere in which you live. But they don't stop there.

Derek Muller and Destin Sandlin, the minds behind the Veritasium and Smarter Every Day YouTube channels, respectively, do show that water (and even hurricanes or cycloness) preferentially spins counter-clockwise in the north and clockwise in the south, you just might not be able to see it with your toilet water.

Sandlin and Muller set up the same experiment in the northern and southern hemispheres to test the Coriolis Effect — the governing principal behind the water swirl idea. The two YouTubers set up kiddie pools in Huntsville, Alabama and Sydney, Australia on platforms to keep them somewhat isolated from forces that could potentially jostle the water about, influencing their results. They let the water sit for 24 hours before draining it.

Once the water started draining, they found that water draining from the kiddie pool in the southern hemisphere swirled clockwise, while water in the north swirled counter-clockwise.

This happens because of the Earth's rotation. The pool may be stationary on Earth, but it competes one rotation each day, as the Earth turns. The side of the pool close to the equator travels farther each day because of that rotation, Sandlin and Muller explain. Because of this, the water on that side of the pool has more momentum when it starts to drain, dictating the water's swirl. The side of the pool closest to the equator is different in each hemisphere, making water swirl counter-clockwise in the north and clockwise in the south.

The Coriolis Effect is particularly noticeable on a large scale when it comes to the spin of cyclones in the southern hemisphere and hurricanes in the north.