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Many websites say that water drain in different directions in Northern, Southern hemispheres. Some people say that Coriolis effect causes it. Some people say that it's not true.

  • Yes, this is a duplicate, though the accepted answer there is incorrect. Ben Crowell's answer is correct. – ProfRob Nov 26 '14 at 12:55
  • Sorry I didn't see that post –  Nov 26 '14 at 13:08
  • If you search for "Ascher Shapiro" you will find details of a 1960's experiment at MIT that verifies the coriolis effect, in a tank six feet wide and 6 inches deep – DJohnM Nov 26 '14 at 16:15
  • More: The note is Nature 196, 1080-1081 (15 December 1962) | doi:10.1038/1961080b0. Costs $US32.00 – DJohnM Nov 26 '14 at 16:21

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The coriolis acceleration is given by $a_c = -2 {\mathbf \omega} \times {\bf v}$, where $\omega$ is the angular velocity of the Earth and $v$ is the velocity of the water. The cross-product means that the largest this acceleration can be is $2\omega v$.

If we assume the swirling water moves at maybe 0.1 m/s and $\omega = 2\pi/86400$ rad/s; then $a_c = 7 \times 10^{-6}$ m/s$^2$. Thus, compared with gravity, this is extremely small and cannot be important on small scales.

Another way of looking at this is to calculate the Rossby number $v/2\omega L$, where $L$ is a length scale. This accounts for the fact that even a small acceleration may be noticeable over a long time or when something travels a long distance. Systems with a small Rossby number are strongly influenced by the coriolis effect. For a typical bath, with $L \sim 1$ m, the ratio is several hundred; but if you drained a lake of size $>10$ km where the water moved at 1m/s, then the coriolis force would probably become apparent.

For the real reasons a vortex is formed by water draining see Why water in the sink follow a curved path?

Community
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ProfRob
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It is not true. I tried both directions and both worked. It depends, therefore, on the initial conditions.

Vladimir Kalitvianski
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I agree with Vladmimir, but want to add two more points

1) if you run water out and twist it one way with your hand it will run out that way, but then if you twist it the other way it flows out the other way - an experiment to try at home.

2) In the atmosphere the Coriolis effect does change the way air flows around a depression (region of low pressure) in the North / South hemispheres. This is, of course, relevant to atmospheric physics and weather prediction.

tom
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I agree with Vladimir too. I tried both directions and both worked. I think that the shape of water basin may course. If it's true, there is a question. If water drain in different directions in Northern, Southern hemispheres which direction water drain in equator?