Bathtub-drain vortex direction and the Coriolis myth

Settled in 1965 by Ascher H. Shapiro (MIT, 1962); Lloyd M. Trefethen, R. W. Bilger, P. T. Fink, R. E. Luxton, R. I. Tanner (Sydney, 1965).

The claim that bathtubs drain oppositely in each hemisphere is centuries-old kitchen folklore attached to the same Coriolis force that steers cyclones. In 1962 Ascher Shapiro built a 6-foot symmetric tank at MIT, let it settle for a full day to kill residual motion, and showed a slow, reproducible counterclockwise vortex driven by Earth's rotation. Three years later a Sydney team led by Lloyd Trefethen repeated the experiment in the Southern Hemisphere and found the predicted clockwise reversal, closing the loop. The catch is that the effect only survives in large, still, carefully isolated basins; an ordinary bathtub or sink is swamped by leftover currents and asymmetric plumbing, so its everyday swirl direction is essentially random. The puzzle is thus resolved: the physics is real, but the popular household version of the claim is not.

It shows how a force too weak to matter in daily life can still be teased out and confirmed with a big enough tank and enough patience.

Explanations that circulated

• Any home toilet or sink will drain in a hemisphere-determined direction — false; at that scale residual currents, basin asymmetry and how the water was poured dominate over the tiny Coriolis force.
• The Coriolis-driven effect is pure myth and never real — also wrong; in a large, symmetric tank left undisturbed for ~24 hours, the effect is real and reproducibly hemisphere-dependent.

References

• Bath-Tub Vortex, A. H. Shapiro, Nature, 1962
• The Bath-Tub Vortex in the Southern Hemisphere, L. M. Trefethen, R. W. Bilger, P. T. Fink, R. E. Luxton, R. I. Tanner, Nature, 1965