Question Does water go down the drain counterclockwise in the northern hemisphere and clockwise in the southern hemisphere?

Answer

It all depends upon how the water was introduced and the geometric structure of the drain.

Handwashing over a sink. Amanda Mills, CDC photographer, 2011. Centers for Disease Control and Prevention, Public Health Image Library.

One can find both counterclockwise and clockwise flowing drains in both hemispheres. Some people would like you to believe that the Coriolis force affects the flow of water down the drain in sinks, bathtubs, or toilet bowls. Don’t believe them! The Coriolis force is simply too weak to affect such small bodies of water.

In his work “Sur les equations du movements relative des systems des corps” (1835) the French engineer Gaspard Gustav de Coriolis (1792-1843) first described this force. The Coriolis force is caused by the earth’s rotation. It is responsible for air being pulled to the right (counterclockwise) in the Northern Hemisphere and to the left (clockwise) in the Southern Hemisphere.

The Coriolis Effect is the observed curved path of moving objects relative to the surface of the Earth. Hurricanes are good visual examples. Hurricane air flow (winds) moves counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere. This is due to the rotation of the Earth. The Coriolis force assists in setting the circulation of a hurricane into motion by producing a rightward (clockwise) deflection that sets up a cyclonic (counterclockwise) circulation around the hurricane low pressure.

What happens at the equator? The Coriolis force is too weak to operate on the moving air at the equator. This means that weather phenomena such as hurricanes are not observed at the equator, although they have been observed at 5 degrees above the equator. In fact, the Coriolis force pulls hurricanes away from the equator.