Does water swirl the other way in the Southern Hemisphere?
Does water swirl the other way down south?
They say that in Australia water swirls down the plughole the opposite way to Europe. Supposedly the Earth’s rotation is to blame. Is there any truth in it?
The equator races fastest
The Earth turns once a day, so a place on the equator goes round the Earth’s whole circumference in a day: it races east at about 1,670 km/h. Central Europe is closer to the pole and its circle is shorter, only about 1,100 km/h. At the pole the ground just spins on the spot.
Whatever flies towards the pole veers
If you fire a cannon north from the equator, the ball carries its fast eastward motion with it. But towards the pole the ground beneath it moves more and more slowly, so the ball gets ahead of it and veers to the right as you look in the direction of flight. In the north, movement in any other direction veers right too; in the Southern Hemisphere everything veers left.
Hurricanes really do spin the other way
In the middle of a hurricane the pressure is low, so air is drawn in from hundreds of kilometres around. Currents from the south and from the north veer right on the way, so the whole vortex spins anticlockwise. In the Southern Hemisphere it’s the other way round, and hurricanes spin clockwise.
A sink is too small for it
But the deflection is so weak that it only shows when the motion has enough time and nothing else moves the water more. Water in a sink drains in a few seconds. It’s set spinning far more by the shape of the sink and the motion left over from filling it.
It can be done, with great patience
In 1962 Ascher Shapiro at MIT near Boston filled a round tank almost two metres wide and left the water to settle for a day. The water turns ever so slightly along with the Earth, and when he then drained it slowly for twenty minutes, the rotation at the plughole grew stronger, like a figure skater pulling in their arms. The vortex spun anticlockwise every time, and three years later the same experiment in Sydney gave the opposite vortex.
The trick on the equator
In Kenya or Ecuador tourists are shown a basin in which water a few steps north of the equator swirls one way and a few steps south the other. It’s a trick: the guide pours the water in or turns the basin so that it spins the way he wants. What’s more, right on the equator the deflection is zero.
See for yourself
Fill the sink, let the water settle for a while and then gently swirl it one way with your finger. When you pull the plug, the vortex will turn the way you set it spinning. Try the other way too: your finger matters more than the hemisphere.
So does it swirl the other way?
The Earth’s rotation deflects everything that moves across it, but only weakly. Hurricanes are huge and spin for days, so they turn a different way in each hemisphere. A sink’s vortex is decided by the shape of the sink and how the water was run in, and the Earth’s deflection only shows up in a big tank left to settle for a day.