Science Puzzle
Which Way Does the Storm Spin?
Hurricanes north of the equator always spin anticlockwise. Cyclones south of it always spin clockwise. No storm has ever been recorded getting this backwards.
What forces every storm on the planet to obey this rule?
The Answer
The Earth is spinning underneath the air, and it spins at different speeds depending where you stand. At the equator the ground races east at over 1600 kilometres per hour, because it has the furthest to travel in a day. Near the poles it barely creeps.
Now imagine air rushing south toward a low pressure centre in the northern hemisphere. That air carries the eastward speed of the higher latitude it came from, which is slower than the ground it is arriving over. So the ground outruns it, and from our point of view the air appears to curve to its right. Air arriving from the south, moving over slower ground, races ahead and also curves right. Every incoming stream veers right, and the whole system winds up turning anticlockwise.
South of the equator the geometry mirrors, every stream veers left, and storms turn clockwise. This apparent deflection is called the Coriolis effect. It is far too weak to affect your bathwater, despite the myth, but over hundreds of kilometres and several days it sets the spin of every hurricane on Earth.
The principle: The Coriolis effect. Because Earth rotates and its surface speed varies with latitude, moving air appears deflected right in the northern hemisphere and left in the southern.