Does water always swirl the same way when it drains?

This question was asked by Sue from York, UK. 

It’s a commonly accepted idea that in the northern hemisphere (the top half of the globe) draining water swirls in a different direction than it does in the southern hemisphere (the bottom half of the globe). But is there any scientific basis for this?

“Sink 3” by Mattman4698 is licensed under CC BY-NC 2.0 

The idea has come about because of something called the Coriolis force, or Coriolis effect. This is where the rotation of the Earth affects the movement of storms and wind, causing them to turn anticlockwise in the northern hemisphere and clockwise in the southern. It makes sense to think that this could apply to water draining from sinks, baths and toilets but in fact it doesn’t because the amount of water is far too small.

If you fill a sink with a few inches of water and then allow it to drain, the water could swirl either way. This is true whichever hemisphere you’re in. The direction of the water may be affected by the position of the tap (left, right or central), the shape of the sink, how smooth it is and lots of other factors, but your location isn’t one of them.