In A Nutshell
Although the Coriolis effect is strong enough to influence the direction of travel for things covering great distances, like airplanes and missiles, it has no control over the way water flushes down a toilet or drain. Some toilets do indeed flow clockwise while others flush counterclockwise, but this is only in response to other forces that have more immediate power than the Coriolis effect. Thus, despite popular belief, the direction a toilet flushes or a sink drains does not change based on hemisphere.
The Whole Bushel
The Coriolis effect is often thought of as a mystical power that controls the movements of everything on Earth. In reality, it’s a basic scientific happening that gets far more credit than it deserves. Some even say it causes toilets to flush in opposite directions in the Northern and Southern Hemispheres—a belief that it downright false.
So what is the Coriolis effect? Essentially, it is the apparent deflection of moving objects as they travel away from their point of origin. We say “apparent” deflection because the objects seem to move in an arc but are actually traveling in a straight line in space. The deflection between Point A and Point B is caused by the eastwardly spin of the planet.
The classic example used to understand this phenomenon is to imagine the Earth flattened out like a spinning merry-go-round. If someone was to sit in the center of the merry-go-round and attempt to throw a ball directly to a friend sitting on the outside edge of the ride, the ball would always seem to turn away from the friend just before he could catch it. In this example, the ball is really moving in a straight line and only appears to sway off course because the friend on the outside edge is moving faster than the person in the middle (he moves faster since he has a greater distance to travel to make one rotation in the same amount of time as the person in the center). Similarly, on Earth, the equator spins much faster than the North and South Poles (the Earth’s axis of rotation).
Now, if that same pair of friends could play a game of catch across the whole globe, with one person standing at the equator and the other standing at the North Pole, then the ball would deflect just as it did on the merry-go-round. That’s because the distance is far enough for the Coriolis effect to have an impact.
The Coriolis effect can’t, however, influence movements on smaller scales, such as the pitch of a baseball or the way water flushes in the toilet. That’s because other forms of energy in the immediate environment overpower the relatively weak Coriolis force. So, in the case of the toilet, the shape of the bowl and the angle the water is pushed into the bowl control the direction of the flush—not the spinning of the earth.
That being said, the Coriolis effect does have a major influence on wide-range and far-traveling things. For instance, pilots and boat captains must account for the Coriolis effect when charting their travel paths, since they are moving great distances over the globe. This means that planes generally do not fly in a straight line, even if the destinations are directly across from each other. Also, weather patterns, hurricanes, winds, and ocean currents are all impacted by the Coriolis effect. In fact, it actually causes gyres (ocean surface currents) to flow in opposite directions in the Northern and Southern Hemispheres (clockwise in the north and counterclockwise in the south), which is perhaps what sparked the idea of backward-spinning toilets in the first place.