Water spirals in the southern hemisphere

Urban legend has it that the earth’s rotation causes toilets to drain in the opposite direction in the southern hemisphere. Several people have asked me whether this is true. No, the direction the water in a toilet swirls is determined by the angle of the jets that direct water into the bowl.

What about bathtubs? Do they drain opposite directions in each hemisphere? When you pull the plug in a bathtub, there are no jets directing the flow of the water. However, the water will drain clockwise about half the time in either hemisphere. Here’s an explanation from my post Two myths I learned in college:

The Coriolis effect does explain why cyclones rotate one way in the northern hemisphere and the opposite way in the southern hemisphere. The rotation of the earth influences the rotation of large bodies of fluid, like weather systems. However, a bathtub would need to be maybe a thousand miles in diameter before the Coriolis effect would determine how it drains. Bathtubs drain clockwise and counterclockwise in both hemispheres. Random forces such as sound in the air have more influence than the Coriolis effect on such small bodies of water.

5 thoughts on “Water spirals in the southern hemisphere

  1. The links to “Why airplanes fly: a modern myth. Part I, Part II” come up empty. Sure would like to see those presentations!

  2. It’s true that on the tiny scale of a plughole, the Coriolis Effect is extremely weak, producing an acceleration ten million times weaker than gravity. Its influence is thus easily overwhelmed by, say, the tilt or shape of the wash-basin. But if done very carefully, it is possible to detect the effect under laboratory conditions. In the 1960s, teams of researchers in the US and Australia published reports in Nature of delicate experiments in which water was allowed to flow very gently out of very wide, shallow tanks. In both cases, the water spiralled out in the direction predicted by the Coriolis Effect.

  3. Sound in the air? Can that actually affect which direction it goes? I would think the frequency of sound would be too high for that.

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