Why does water swirl clockwise?

photo by Ignacio Sanz

Here was the situation, you were being abducted and kept in a secret place. Where in the world were you? You asked to use a toilet and as you flushed the water down the toilet bowl, you observed the direction the water swirling down, you then figured out that you were in the southern hemisphere. See? Now your problem is halved.


This is no laughing matter, in 1995, Mr Simpsons "Bart" from International Drainage Commission conducted a vast-scale experiment and survey in both North America and in Australia [3] and confirmed of this effect. ;-)

This is an effect known as "Coriolis Effect" - named after Gaspard-Gustave Coriolis, a french scientist who was first to express this effect in mathematical formula in 1835 [2].

Coriolis effect is caused by the rotation of the Earth and the inertia of the object experiencing the effect. This effect causes moving objects on the surface of the Earth tend to veer to the right in northern hemisphere and to the left in southern hemisphere. The study of Coriolis effect is being applied into many areas ranging from predicting the movement of water-flow in the ocean, predicting weather patterns, ballistic missiles, satellites, and working out which hemisphere you were in when being abducted.

Oh, a word of warning, if you toilet or bathtub doesn't behave accordingly, blame it on the manufacturers. ;-)


References
[1] Myth 3: Does Water Swirl Counter Clockwise in the Southern Hemisphere, blog.sciencegeekgirl.com
[2] Coriolis Effect, wikipedia.org
[3] Bart vs. Australia, wikipedia.org