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.