The Coriolis Effect

by Brian Hancock on 4 Feb 2003 Bruce Schwab from the Around Alone competitor Ocean Planet explains the very tricky Coriolis Effect, a phenomenon that contributes to winds rotating around pressure systems rather than blowing directly from a region of high pressure to a region of low pressure.

It’s time to put the parties and shore news aside and take a science lesson, so with great thanks to Bruce Schwab who has saved me from having to writing a story today, here is Bruce’s explanation of the Coriolis effect:

'Ever wonder why things like weather systems rotate one way in the Southern Hemisphere and the other in the Northern Hemisphere? Or why do they rotate at all? Me too.'

Let's start with a rough description of how weather and wind works. Wind flows from areas of high pressure to areas of low pressure trying to fill up the low. How soon a low fills up depends on what is happening to the air in the upper atmosphere above it, but that's another subject. What concerns us is that if you look at a map or chart from above it will show which way the wind is blowing and it appears to be going in a circle around the low. It is also going in a circle around the high and sometimes it looks like two wheels or gears spinning together. Why doesn't the air go straight at the low? What is the Coriolis effect? The answer, which seems unbelievable, is that the air IS going pretty much straight, or starts out that way, but there is something else at work....let me explain.

As you know, the earth is rotating once every 24 hrs or 1000 miles per hour at the equator (the earth is roughly 24,000 miles in circumference there). Imagine that you are standing at the north pole and a friend of yours, let's call him Bob, is standing on the equator. Let's also say the two of you are giants and are tall enough to see each other over the curvature of the earth. If you are facing Bob and can see him it seems like the both of you are not moving, but he is actually going 1000 miles an hour faster than you! You are not moving, but are slowly rotating with the earth beneath your feet so that you keep facing him.

Now let's say that you and Bob have this thing going with paper airplanes and like to throw them at each other. The both of you are really good and can throw them far enough to hit each other and at 1000 miles per hour (just bear with me here....), you smile, take aim with your super paper airplane and fire one off directly at Bob at 1000 mph. Since Bob is on the equator, which is half way to the south pole (or 1/4 of the way around the world) he is 6000 miles away so it should take six hours for your airplane to get to him on the equator.

Now here's the weird thing: you watch in amazement as your airplane zooms off to the right and misses Bob by thousand of miles!! The reason is that Bob is not where he was when you threw the airplane, but has moved 6000 miles in the 6 hours due to the rotation of the earth.

Your airplane is going straight, BUT THE EARTH IS TURNING UNDERNEATH IT making your airplane look like it is going in a curved line. The earth is rotating to the east so anything that moves towards the equator looks or acts like it turns west, since the earth is going faster there than wherever the moving object started.

Conversely, anything moving towards the north or south pole has more rotational speed than where it is going, so it looks like it turns to the east.

Are you with me?

This is what happens to the air as it moves from high to low pressure. In the Northern Hemisphere the air flowing south from a high goes to the west, and air flowing north from the high goes to the east starting the circular flow that you see on the map. This is what we call the Coriolis effect.

Draw two high pressure areas out on a globe, one on the north side of the equator, and one on the south side, and you'll see why the two highs turn in opposite directions to each other.

Keep in mind that the wind doesn't really start out curving, but the earth is actually rotating underneath it.

In reality the wind eventually does get drug along by the friction of the earth but we only need to get so far into this....this is about as much as I have figured out so far anyway.

I hope this makes sense! If you get it you can now explain the Coriolis effect to amaze your friends with your knowledge of this earth, our Ocean Planet.

By the way, Gustave-Gaspard Coriolis was an 18th century math major in France.'