Asked by: Roger Evans, Argyll
As the Earth spins, the speed of rotation experienced at the equator is much faster than at the North Pole. This difference causes the ‘Coriolis effect’ – a force which appears to deflect winds towards the right in the northern hemisphere.
In a low pressure weather system, air flows inward, but this deflection twists the air flow towards the right, creating an anticlockwise swirl of winds. In a high pressure system, air flows outward, and the deflection results in a clockwise rotation. The Coriolis effect deflects winds towards the left in the southern hemisphere, so weather systems here spin in the opposite direction.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun facts.