[Physics FAQ] - [Copyright]
Slight update 2022 by DK.
Original by Matthew Feinstein.
Question: Does my bathtub drain differently depending on whether I live in the northern or southern hemisphere?
Answer: No. There is a real effect, but it is far too small to be relevant when you pull the plug in your bathtub.
Because Earth rotates, a fluid that flows along Earth's surface feels a "Coriolis" acceleration perpendicular to its velocity. In the northern hemisphere, Coriolis acceleration makes low-pressure storm systems spin counterclockwise; but in the southern hemisphere, such storm systems spin clockwise because the direction of the Coriolis acceleration is reversed. This large-scale meteorological effect leads to the speculation that the small-scale bathtub vortex that you see when you pull the plug from the drain spins one way in the northern hemisphere and the other way in the southern hemisphere.
But this effect is very weak for bathtub-scale fluid motions. The order of magnitude of the Coriolis acceleration can be estimated from size of the "Rossby number" (see reference 1 below). The effect of the Coriolis acceleration on your bathtub vortex is very small. To detect its effect in your bathtub, you would have to get out and wait until the motion in the water is far less than one rotation per day. This would require removing thermal currents, vibration, and any other sources of noise. Under such conditions, never occurring in a typical home, you would see an effect. To see what trouble it takes to actually see that effect, see reference 2 below. It discusses experiments that have been done in both the northern and southern hemispheres, which verify that under very carefully controlled conditions, bathtubs do drain in opposite directions due to the Coriolis acceleration from the Earth's rotation.
Coriolis accelerations are significant when the Rossby number is small. So, suppose we assume a Rossby number of 0.1 and a bathtub-vortex length scale of 0.1 metres. The Rossby number is defined as
Rossby number = v/(2 L ω) ,where v is the speed of a fluid element, L is the scale of the fluid motion (assumed to be about 0.1 m above), and ω is Earth's rotation speed of one rotation per day, or about 10−4 radians/second. Solving the above equation for the fluid speed v, we find that the Coriolis acceleration in a bathtub is significant for fluid speeds of less than 2 × 10−6 metres/second. This is a very small speed. How small is it?
Well, we can take the analysis a step further and calculate another, more famous dimensionless parameter, the Reynolds number:
Reynolds number = L v density/viscosity .
Assuming that physicists bathe in hot water, we have L = 0.1 metres as above, v = 2 × 10−6 metres/second, density = 103 kg/m3, and viscosity equals about 8 × 10−4 pascal seconds. So the Reynolds number is about 0.25, which is very low.
Now, life at low Reynolds numbers is different from life at high Reynolds numbers. In particular, at low Reynolds numbers, fluid physics is dominated by friction and diffusion, rather than by inertia. That is, at low Reynolds numbers, the time it would take for a small piece of fluid to move a significant distance due to an acceleration, is greater than the time it takes for that piece to break up due to diffusion.
Ideas about which way a bathtub will drain have also been cited for giving the direction water circulates when you flush a toilet. This is surely nonsense. In this case, the water rotates in the direction determined by the pipe that carries the water from the cistern to the bowl.
1.  J. Pedlosky, "Geophysical Fluid Dynamics", Section 1.2.
2.  L.M. Trefethen et al., Nature 207, 1084–5 (1965).