Does water always drain clockwise?

Or anticlockwise, in the northern hemisphere. And precisely on the equator it’s supposed to run straight down the plughole with no swirling at all.

It’s a wonder this myth persists when each of us daily has several opportunities to test it out (if you don’t, that may be why people have been avoiding you). In a daring piece of original research I just tried it by dropping a scrap of paper into an emptying sink. The paper sailed merrily to the bottom in tight anticlockwise loops.

So that’s that, then. But not quite. While the phenomenon may be bogus, it’s supposed cause, the Coriolis force, is quite genuine. It’s one of those oddities that emerge when you live on a rapidly spinning ball instead of a motionless sheet.

Different parts of the earth’s surface revolve at different speeds — points on the equator whiz around at over 1600 kilometres an hour while here in the middling latitudes things move more sedately.

As a result if you are flying north (in the southern hemisphere) you find that, because you’re moving more slowly than the ground beneath you, you move a little west as well. Coming south just the opposite happens: you’re moving faster than the ground so you move a little east, too. Though a bit harder to explain, it turns out that any movement in the southern hemisphere is deflected to the left.

Mostly this goes unnoticed. The force is so small that we can’t feel it. But we can detect its effects on the atmosphere where it makes low pressure systems swirl clockwise and high pressure systems anticlockwise (reversed north of the equator). Air rushing inwards to fill a low pressure region is deflected left and pretty soon the winds are spiralling clockwise into the heart of the system. Everything is reversed for a high pressure system.

If someone pulled a huge, cartoonish plug from the bottom of the South Pacific, the water would drain clockwise. For smaller bodies of water, like your shower or kitchen sink, the Coriolis force is overshadowed by other, more prosaic effects.

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