I used to believe the Coriolis Force influenced drainage direction in northern and southern hemispheres (probably because of that Simpsons episode), until this thread made me read this from wikipedia (seems legit). I really like the part that says if the Coriolis force did affect drainage direction, it would drain counterclockwise in the northern hemisphere.
"Draining in bathtubs and toilets
A misconception in popular culture is that water in bathtubs or toilets always drains in one direction in the Northern Hemisphere, and in the other direction in the Southern Hemisphere as a consequence of the Coriolis effect. This idea has been perpetuated by several television programs, including an episode of The Simpsons and one of The X-Files.[1] In addition, several science broadcasts and publications (including at least one college-level physics textbook) have made this incorrect statement.[2]
Some sources that incorrectly attribute draining direction to the Coriolis force also get the direction wrong, stating that water drains clockwise in the northern hemisphere. If the Coriolis force were the dominant factor, drain vortices would spin counterclockwise in the northern hemisphere.
In reality the Coriolis effect is a few orders of magnitude smaller than various random influences on drain direction, such as the geometry of the container and the direction in which water was initially added to it. Most toilets flush in only one direction, because the toilet water flows into the bowl at an angle.[3] If water shot into the basin from the opposite direction, the water would spin in the opposite direction.[4]
When the water is being drawn towards the drain, the radius of its rotation around the drain decreases, so its rate of rotation increases from the low background level to a noticeable spin in order to conserve its angular momentum (the same effect as ice skaters bringing their arms in to cause them to spin faster). As shown by Ascher Shapiro in a 1961 educational video (Vorticity, Part 1), this effect can indeed reveal the influence of the Coriolis force on drain direction, but only under carefully controlled laboratory conditions. In a large, circular, symmetrical container (ideally over 1m in diameter and conical), still water (whose motion is so little that over the course of a day, displacements are small compared to the size of the container) escaping through a very small hole, will drain in a cyclonic fashion: counterclockwise in the Northern hemisphere and clockwise in the Southern hemisphere—the same direction as the Earth rotates with respect to the corresponding pole."
