No Coriolis-Drain Effect?

As someone from the other hemisphere I felt obliged to comment. Or, at least question, the conclusion drawn on that site. Their logic is basically that the rotational force is not strong enough to have any effect. My counter to that is that if you get your head out of the sink long enough and look at the direction weather system like hurricanes and warm and cold fronts travel in the southern hemisphere you will notice that they are opposite to their northern hemisphere counterparts. I believe this is also true of warm and cold ocean currents. The real question is do republicans become democrats when they cross the equator? ;)

Answer the darn question! BTW there is a 'myth busting' web site out there that claims it does NOT rotate the opposite ways (namely toilets)

The thing is, a toilet is configured in a certain way to cause the "whirlpool" effect in a specific fashion. The water does not simply drain out of the toilet, it is forcibly removed through the introduction of additional water through predetermined paths. That's quite a difference from simply removing a plug from a full sink.

I think I am better suited to answer this question as a mechanical engineer than as a Brazilian, so here goes...

The coriolis effects come into play when you are moving in a curve of changing radius - or, an easy way to think about it, when you are on a platform moving in a circle and then you move towards (or away from) the center of the circle. In other words, if you move towards or away from the axis of the earth, your east-west motion can speed up or slow down. In traveling over hundreds or thousands of miles, weather patterns (and ICBMs, etc) move relative to the axis of the earth a good fraction of their distance to that axis - in other words, their distance to the axis of the earth can change by a few percent, which means their east-west motion gets affected. Water falling down your drain gets closer to the center of the earth by INCHES, which is not enough for the coriolis effect to kick in. Or, more strictly, it is not enough for the coriolis effect to overcome viscosity and any pre-existing angular momentum.

It turns out that the pre-existing angular momentum is what causes the little whirlpool you see circling the drain of your sink/bathtub. Just moving stuff around those small reservoirs of water makes the water spin around very slowly about some vertical axis. As the water approaches the drain, this circular motion gets magnified (conservation of angular momentum - like pulling your legs in when you're spinning around in an office chair (or on ice skates) in order to spin faster), and a little whirlpool forms, even if you didn't intentionally spin the water around. (And if you DID intentionally spin the water around, like the toilets John S mentions, then you certainly get a nice whirlpool effect that way).

A physicist friend of mine once told me that someone once did an experiment where round containers of water were allowed to sit for a LONG time before being drained, so that viscosity had time to eliminate angular momentum. The whirlpool effect was greatly reduced but still observable. And the containers spun clockwise about half the time and counterclockwise about half the time - so even THEN, the coriolis effect was not strong enough to overcome the marginal remnants of angular momentum.

Now I'll go check out the link you posted to see what it says... I hope I didn't screw up this explanation...

"Water falling down your drain gets closer to the center of the earth by INCHES"

By that I meant, to the axis of the earth. You don't get closer to the center of the earth by moving around on its surface (maybe just a little bit, but all this still works for a spherical earth), but you do get closer to the axis if you go towards or away from the north pole (which is right on the axis). Hmmm, I wonder if the coriolis effect would become non-negligible in influencing a sink/bathtub drain if you were near the north pole (where north/south motion translates directly into motion towards/away from the axis of the earth, rather than just a component of it at lower latitudes, or none at all near the equator).

But you guys HAVE to know that the only way to settle this for sure is to collect-call some random kid in Australia...