In an episode of the Simpsons, Bart Simpson watched how water in his bathroom sink went down the plughole anticlockwise. His sister Lisa, always the know-all, said this was caused by the Coriolis effect, created by the spin of the Earth, and that in the southern hemisphere water drains clockwise.
To get to the truth, Bart rang people at random in the southern hemisphere and eventually got through to a boy in Australia, who found that sinks and toilets there did indeed drain clockwise. And the Australian boy’s father threatened to sue Bart for the cost of making the six-hour phone call by reverse charge.
But this was one of the rare occasions when Lisa Simpson was actually wrong. Water going down a plughole in the north or south hemisphere is just too small to come under the influence of the Coriolis force. But it does have a big impact on much larger scales, making moving objects appear to deflect.
So, if you fired a big gun at the equator towards to the North Pole it would appear to swing to the right in the northern hemisphere, and towards the left in the southern hemisphere if fired at the South Pole.
The Coriolis effect influences global wind patterns and gives the UK its prevailing south-westerlies, as winds blowing from the subtropical highs towards the low pressure in the north are deflected to the right. This deflection is also a major factor in why winds blow anticlockwise around low pressure systems and clockwise around high pressure in the northern hemisphere, and vice versa in the southern hemisphere.
