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Coriolis force.....what force?

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Helen Czerski Helen Czerski | 09:00 UK time, Thursday, 1 September 2011

Distance travelled ~ 626'155'200 km

Last week we all watched Hurricane Irene march up the east coast of the US, a gigantic atmospheric whirlpool that caused a huge amount of damage. Out in the mid-Atlantic, tropical storm Katia has just graduated to being Hurricane Katia on her way west, and there are likely to be more as the season goes on. These storms start as isolated disorganized thunderstorms, but as they grow, they also start to rotate.

satellite image hurricane irene

(This GOES-13 satellite image is of Hurricane Irene just 28 minutes before the storm made landfall in New York City. The image shows Irene's huge cloud cover blanketing New England, New York and over Toronto, Canada. Shadows in Irene's clouds indicate the bands of thunderstorms that surrounded the storm. Credit NASA/NOAA GOES project.

And while watching the stunning satellite images of these mammoths of the atmosphere lumbering across the ocean, I thought about why that is.

Spinning tops and sycamore seeds, wind turbines and the wheels on the bus... lots of things in our world go round and round. But all of those examples are solid objects with an obvious central axis to rotate around. A hurricane is a pattern of flowing air and there's no visible container to hold that air together. The air on one side of it is not attached to the air on the other side of it. Even an average hurricane can be 1400 km across, covering 13 degrees of latitude. What could possibly be happening to make that much of our atmosphere spin in such a coordinated way?

The short answer is that a hurricane spins because our planet spins, and the link is a funny thing called the Coriolis force. It's funny because it's not actually a force at all, but a consequence of the fact that we're going round and round in circles while we're looking at the weather. Ours is an odd point of view, but since we're stuck with it, science came up with the idea of this extra force to compensate. It's a neat psychological trick - instead of accepting that we're the ones doing odd things, we claim that we're normal and the weather is behaving oddly by adding in the Coriolis force. So what is this mysterious force?

Imagine that you're on a roundabout. You want to play catch with a friend, but they don't like roundabouts so they're standing on the ground a few metres away. The roundabout is going round anticlockwise and you are standing on the edge facing outwards. Next time you pass your friend you throw the ball to them, and as soon as it's in the air, it travels in a straight line. You keep rotating around, so it looks to you as though the ball is mysteriously curving to the right. It isn't, it's just that you're rotating to the left. This magic force that seems to have taken over the ball from your point of view is called the Coriolis force. Try it, next time you're at a children's playground. You'll find that whatever direction you throw the ball in, if you're on a roundabout going anticlockwise, the ball always looks as though it's curving mysteriously to the right. And since in the northern hemisphere, Earth is rotating anticlockwise beneath us, anything that moves always ends up slightly to the right of where you think it should be.

Let's get back to that hurricane. Its centre is a huge area of low pressure, so all the air is trying to rush from the outside where the pressure is high towards the middle. But as it moves inwards, all that air is curving to the right because of the Coriolis force. The last part of the puzzle is something you can see in action on our roads all the time. Next time you're at a roundabout, stop and look.


Every car comes in towards the roundabout and has to turn left. But that means that the flow around the whole roundabout has to be to the right. And this is why hurricanes always spin anticlockwise in the northern hemisphere. All that inward rushing air is turned to the right, so the whole system goes round to the left.

These forces are relatively small, and to see their effects you need a very large weather system. So the spin of a hurricane is just a reminder that we travel 17 thousand miles every single day in the UK, as we spin around the Earth's axis. I feel dizzy just thinking about it...


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