Asked • 03/27/19

Why does Coriolis determine the rotation direction of a cyclone but not my drain?

It is a common misconception that Coriolis is responsible for the direction the water swirls down a toilet, tub or sink drain. E.g. does a toilet flush the other way in the southern hemisphere? (If it does, it is due to the construction of the basin, not Coriolis). This misconception has been debunked many times and leads to a new issue I ran into recently. I was talking to someone about low pressure systems (cyclones, e.g. mid-latitude storms and tropical cyclones) spinning in the opposite sense in the the north and south hemispheres and they questioned this fact because "Coriolis does not determine how something spins, Mythbusters showed that with a toilet". This demonstrates a lack of understanding in how Coriolis works and leads to this question: Why does Coriolis determine the sense of rotation of a cyclone but not my drain? How can it work on one phenomenon but not the other?

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