Some people suggest that the reason toilets, sinks and other types of drains swirl in the direction they do as the water drains out is due to the Coriolis Effect, also known less pedantically as the "Coriolis Force".

Well, sort of, in a way, but...

Into each "drain event" there are a whole lot of different factors that determine which way the water in a drain is going to spin, among them:

  • The shape of the drain, and the basin the water is in
  • Whether it is level or tilted
  • Which direction the water is moving in when you open the drain

So. in an idealized toilet experiment (using the ideal toilet installed outside Plato's cave?), with perfectly still water1 to begin with, a perfectly round and level drain, and a means of opening the drain that didn't horizontally disturb the water, The Coriolis Effect just might cause the water to swirl clockwise in the southern hemisphere, and counterclockwise in the northern. But any of the other factors might overwhelm the Effect and cause the water to swirl the other way.

The Coriolis Effect affects weather systems because they happen over much larger geographical areas than a single basin of water. So, ok, if someone were to pull the plug on the North Atlantic Ocean, the water might swirl counterclockwise.

1Since pouring water into a basin leaves all sorts of random currents hither and yon, the toilet would have to be left to sit for a week. Eww2.

2Oh wait, it has to be pure water, impurities will also cause problems.