Coriolis effect determines which way drains swirl
The Coriolis effect does influence large-scale atmospheric and oceanic rotation (such as hurricanes), but it is far too weak to determine which way water drains in a typical sink or toilet. Residual water motion from filling and any asymmetry in the basin or drain overwhelm the Coriolis force at sink scales.
What we know
The Coriolis effect is a real phenomenon. It causes freely moving air and water masses to deflect right in the Northern Hemisphere and left in the Southern Hemisphere due to Earth's rotation. This effect is significant in large-scale systems like ocean gyres (thousands of kilometers across) and hurricanes (hundreds of kilometers across), where the Coriolis force has time to accumulate over long distances and durations.
However, a typical sink or toilet is about half a meter across. The Coriolis force acting on such a small volume is many orders of magnitude smaller than any residual motion in the water from filling, washing, or flushing. Penn State atmospheric scientist Alistair Fraser has written that the direction of draining in a sink or toilet is determined entirely by any net rotation already present in the water and by the geometry of the drain, not by Earth's rotation.
Scientific American confirms that the ultimate direction of flow in a drain is essentially random and can go either way in either hemisphere on different occasions. Under carefully controlled laboratory conditions, with extremely still water and a symmetric basin, the Coriolis effect can be demonstrated in sink draining. These experiments require hours of waiting for residual motion to dissipate and special equipment. Under everyday conditions, the effect is unmeasurable.
The myth may have been reinforced by street vendors near the equator who charge tourists to see water drain in opposite directions on either side of an arbitrary 'equator line', using a trick of directing water flow with their hand during the pour.
Common claims
- Water drains clockwise in the Northern HemisphereFalse - direction is random in sinks
- Toilets flush differently in AustraliaFalse - toilet design determines flush direction
- Coriolis effect is real and important for large systemsTrue - valid for weather systems and ocean currents