You may have heard that water flowing down the plug hole in a sink or bath always swirls anti-clockwise in the
Northern Hemisphere, and clockwise in the Southern Hemisphere, and that this is due to the Coriolis effect.
The Coriolis effect (or Coriolis force) was first postulated by the French scientist Gaspard-Gustave Coriolis in relation to the behaviour of water wheels in 1835, but so far as the Earth is concerned, it is the deflection of the atmosphere and the oceans, and large objects on the surface of the Earth, due to the earth’s rotation around its axis. Cyclones, and jet streams in the upper atmosphere, are two of the more obvious phenomena caused by the Coriolis effect, but the effect also causes certain types of waves to form in the oceans. However because the earth spins relatively slowly, the apparent force that its rotation generates only becomes significant over large distances or long periods of time.
The Coriolis effect became important in external ballistics for calculating the trajectories of very long-range artillery shells. The most famous historical example was the Paris gun, used by the Germans during the First World War to bombard Paris in 1918 from a range of about 120 km (75 miles). The distance was so far that the Coriolis effect was substantial enough to affect trajectory calculations. Incidentally, the shells of the gun reached a height of 40 kilometers (25 miles, 131,000 ft) and were the first man-made objects to reach the stratosphere.
However, the direction in which water flows down a plug hole is not influenced by the Coriolis effect, which is tens of thousands of times weaker than other factors such as the existing disturbance in the water, the angular momentum that causes the initial vortex, and the shape of the bowl.
Despite this, popular entertainment has maintained interest in the Coriolis effect. Bart vs. Australia, the sixteenth episode of the sixth season of The Simpsons, starts with Bart Simpson noticing that the water in his bathroom sink always drains anti-clockwise (counterclockwise in the USA). Bart does not believe Lisa, his sister, who explains that this is due to Coriolis effect, and that in the southern hemisphere the water drains the other way round. To confirm this, Bart makes phone calls to various countries in the southern hemisphere, ending up with a call to Australia. Here a little boy, who lives in the outback, confirms, having also checked with his neighbours, that the toilets and sinks are all draining clockwise. The plot continues with Bart being sued by the boy’s father for the cost of his six-hour ‘collect’ call, with Australia indicting Bart for fraud, the USA wanting to send him to prison to placate the Australian government, Bart having to make a public apology in Australia, and so on.
However in 1962, Ascher Shapiro, a researcher at the Massachusetts Institute of Technology in Boston, USA, was able to demonstrate the Coriolis effect on draining water, and this was later repeated by scientists in Sydney, Australia. But this was only achieved by using a perfectly circular bath 1.8m in diameter and 15cm deep, and by allowing the water to stand for 24 hours so that any currents from filling would die down. A small outlet, on the outside, meant that the water took about half an hour to drain away. Under these conditions, the Boston researcher reported a tendency for water to swirl anti-clockwise (viewed from above), whilst the scientists in Sydney described seeing water swirling clockwise.
So to observe the Coriolis effect at home, you would need a large but shallow circular bath, and one that’s not affected by any vibration or disturbance, as well as plenty of time.