Why hurricanes spin

A hurricane turns one way north of the equator and the other way south, because the planet turns beneath it. Your sink does not know which hemisphere it is in.

Which way a storm turns

A hurricane is a vast wheel of air spiralling inward toward a centre of low pressure. In the Northern Hemisphere the wheel turns counter-clockwise; in the Southern Hemisphere it turns clockwise. The pattern is consistent, and it was spotted early: in 1821 William Redfield read a counter-clockwise pattern in the damage a hurricane left across Connecticut.

Why the direction matters

Knowing the sense of rotation tells you where the storm hits hardest. In the north, the winds on the right side of the track add to the storm's own forward motion, so forecasters call that the "dirty side"; south of the equator, the same reasoning puts it on the left. The rule also says where hurricanes cannot form. The force that sets the spin vanishes at the equator, so a storm must be at least 300 miles from it before that force can turn the inflow into a circulation.

And it settles a famous kitchen argument. The belief that drains, sinks and toilets swirl one way in the north and the other way in the south is a myth. The same force that organises a storm is far too weak to matter in a basin.

An apparent force

Earth rotates, and everything on its surface rides along. Air that sets off in a straight line over the ground is watched from a surface that has turned by the time it arrives, so its path appears to bend. Meteorologists treat this bending as a force, the Coriolis force, and call it apparent because nothing is pushing the air; the ground is turning underneath. It deflects moving air to the right in the Northern Hemisphere and to the left in the Southern. It is greatest at the poles and zero at the equator.

Now picture the low-pressure centre of a young storm. Air rushes toward it from every side. Each inbound parcel bends to the right of its own path, so air coming from the south slides past the centre to the east, air from the east slides past to the north, and so on around. Every parcel misses the same way, and the sum of the misses is a counter-clockwise swirl. South of the equator the deflection is to the left, and the swirl runs clockwise. Near the equator the deflection is too small to bend the inflow before it arrives, which is why the storm needs its 300 miles.

Two limits keep this honest. First, the Coriolis force turns the air; it does not drive it. The fuel is ocean water of at least 80 °F, warm to a depth of 150 feet, feeding the thunderstorms that power the storm's heat engine. The spin may help that engine organise. Second, the force is tiny per second. Storm inflow covers hundreds of miles over many hours, so the deflection accumulates into a spiral. Water in a sink covers inches in seconds, and its swirl is decided by how it was already moving when the plug came out.

In short

Air converging on a low-pressure centre is bent by the turning Earth: rightward north of the equator, leftward south of it. Every parcel misses the centre the same way, so the storm spins counter-clockwise in the north and clockwise in the south, and cannot form within 300 miles of the equator, where the bending vanishes. The effect needs hours and hundreds of miles to show; a draining sink has neither.