Yes, Mulder was actually wrong once...

So, I remember this episode of the X-Files when Mulder and Scully were tracking down some sort of devil teacher at a school. Mulder goes to get a drink from the drinking fountain and notices that the water is draining counter-clockwise. He promptly informs Scully and cites it as an indication of a nearby demonic presence, because water always drains clockwise in the northern hemisphere due to the Coriolis force. I'm sorry, Mulder, but you're just wrong. The Coriolis force has NO BEARING on the direction that water drains either in a drinking fountain or in a toilet. (Lisa Simpson is another important cultural figure who has misinterpreted this remarkable phenomenon.)

As I'm sure you clever folks have figured out, I'm giving a little lesson on Coriolis today, one of my favorite subjects in oceanography. (This is all within the scope of trying to tell you all a little more about what's going on in my life. At the moment, Rachel's life = school school school school. However, I didn't really think you'd want to hear about the nuances of my arguments with MatLab, and how I'd like to smash my computer into a wall on a regular basis. So instead, you get to learn about my favorite parts of this whole oceanography business.) Without Coriolis, the oceans wouldn't circulate the way they do and the earth's climate would be completely different than what we have. There would be no ocean gyres, and large scale cyclones (think giant storm cells like the ones that plaster the midwest with snow) would not happen the way they so.

So, what the story? Well, the thing is, the Earth is spinning. I know, you're all tempted to go /palmface and say "well, duh," but that's really the key to the whole story. We are standing on a rotating reference frame, and that completely changes the way things appear to move from our perspective. When you are in a rotating reference frame and attempt to move something in a straight line, it will appear to curve to one side or the other (depending on which direction your reference frame is spinning. In the case of the earth, which is spinning counter-clockwise, things curve to the right. If you're in the northern hemisphere. Hence Mulder's delusion about the drain.) The word appear is very important, because the kicker of Coriolis is that if you abruptly stop the spinning reference frame, you'll discover that the object in question is actually moving in a perfectly straight line. It is just the movement of the reference frame that makes it appear to curve or spin. (So, if the Earth stopped spinning, we'd find that the large ocean gyres are in fact giant sloshing pools that move back and forth with no spin at all.)

So, some important points about Coriolis:

1) It's strength is dependent on latitude. There is NO Coriolis at the equator, but the farther you go toward the poles, the stronger the effect. (To give you a visual, if you are standing still on the equator, the movement of the earth is carrying you in a straight line - you're not actually spinning at all. If you're standing still directly on one of the poles, the movement of the planet causes you to spin like a top. Therefore, no Coriolis at the equator, lots of Coriolis at the poles.)

2) It is an important force over large scales of time and space. Coriolis is an acceleration, so the longer the time spent under its influence, the more noticeable the effect.

3) The effect is more noticeable on something moving slowly over a distance than on something moving quickly over the same distance, because it's going to take the slow moving object longer to get from point A to point B, and time is what will really determine the magnitude of curvage experienced.

4) While Coriolis is a powerful force on our planet, it is not particularly strong. Meaning that is is easily counteracted by other forces, like pressure gradients and friction. This is particularly true on small scales.

This leads to why the Coriolis force really has nothing to do with the toilet/water fountain:

1) The water is moving too quickly

2) A toilet bowl is not exactly a large spatial scale

3) In both cases, the water is already being pushed a specific direction. There's no way Coriolis can counteract that.

Now, I know that this is a lot of information, but here's the take home message for today: in the northern hemisphere, the Coriolis force causes a moving object to curve to the right. In the southern hemisphere, things curve to the left. If you're questioning why this is at all relevant to your life, and thinking that perhaps listening to Rachel ramble like a mad scientist is in fact a waste of your time, then check back tomorrow. Because without Coriolis, the planet we know and love would be a very very different place.