Which Way Does Water Drain in the Northern Hemisphere?
The science (and a surprising twist) to a longstanding myth.
Our daughter is visiting Australia, so I asked her to flush some toilets and fill some sinks then pull the plugs, for an admittedly less-than-perfectly-controlled scientific experiment to see which way the water would rotate (it’s fun to be a science writer’s daughter!). Because, as we all know, water rotates counterclockwise in a bathtub drain in the Northern Hemisphere — just like a hurricane—and clockwise south of the equator, right?
Well… as she said after several tests: “Jury still out.”
What I didn’t tell her, and which she’ll find out when she reads this, is that the notion that water will drain a certain way depending on where you are on the planet is a total myth. But there’s a teensy weensy grain of truth behind the myth, and some very interesting planet-wide physics, too, not to mention some less interesting but easy-to-grasp plumbing physics.
Let’s start with the big picture:
Year-round, the sun strikes Earth’s equator more directly than the poles, so the equator is consistently warmer. Molecules in warm air are more energetic, so they get pushed farther apart, which means warm air has lower pressure. This warm air rises, and then it has…
