Does water drain in opposite directions in each hemisphere?

No. The popular claim says water swirls counterclockwise in the northern hemisphere and clockwise in the southern. The Library of Congress says both directions occur in both hemispheres, and that the Coriolis force is too weak to affect such small bodies of water. The direction depends on how the water was introduced and on the geometry of the drain. The claim rests on a real principle, but it does not apply at the scale of a bathroom. [1] [2]

Because Earth rotates, anything moving across its surface appears to curve, since the ground below turns at different speeds at different latitudes. Points near the equator move faster than points near the poles. Over large distances and long times this adds up. Low-pressure storm systems spin counterclockwise in the northern hemisphere and clockwise in the southern, and hurricanes follow the same pattern. The drain idea extends this large-scale effect down to a bathtub. [2] [3] [4]

Fluid dynamicists compare the flow with Earth's rotation using the Rossby number. A small value means the Coriolis force matters, and a large value means it is negligible. A hurricane has a Rossby number of order 1. For a bathroom sink, the authors writing in The Conversation say the number is more than 10,000 times larger, so the Coriolis force is negligible. Susanne Horn of Coventry University told Live Science that a household drain does not feel rotation. [4] [2]

Other factors dominate. The water may already be rotating from how it entered the basin, how someone moved in the bath, or how the plug was pulled. Basins and drains are rarely perfectly symmetrical, so the same sink can drain either way on different days. Toilets are designed to guide the water, so the direction is set by the pipe feeding the bowl. [2] [3]

Under extreme control the effect can be seen. A 1962 experiment described in Nature, by an MIT researcher, used a large, carefully prepared tank in which leftover motion was allowed to die away. The water developed the predicted counterclockwise rotation. Three years later, University of Sydney researchers took similar precautions and reported the predicted clockwise rotation in the southern hemisphere. A tub would need to be nearly still, with motion far below one rotation per day and without thermal currents or vibration, which does not happen in a typical home. [2] [3]

Some tourist sites show water draining in opposite directions on either side of the equator. The authors in The Conversation conclude that the Coriolis force is not responsible, because the system is too small. They also note that different videos show water spinning in different directions within the same hemisphere, which would not happen if Earth's rotation were the cause. They do not deny what the tourists see, only the explanation offered for it. [4]