Debate Magazine

Why Hurricanes Turn Anti-clockwise in the Northern Hemisphere - with Diagrams

Posted on the 06 November 2013 by Markwadsworth @Mark_Wadsworth
There are two stages to understanding this - the "Coriolis Effect" and the "just missing the low pressure" effect.
The Coriolis effect has been explained often enough using the roundabout analogy. Viewed from above the North Pole, the world turns anti-clockwise ("There's a burning sun in the Western world, but it rises in the East").
1. You are standing on the blue spot (near the equator) and some air is moving away from you. Viewed from above the North Pole, this is what it looks like (assuming there is no friction between atmosphere and earth's surface). I'm afraid you have to read these left to right (long story):
Why hurricanes turn anti-clockwise in the northern hemisphere - with diagrams
2. I cut out the eight pictures and stuck them down again to show things from your point of view:Why hurricanes turn anti-clockwise in the northern hemisphere - with diagrams
3. We can do the same exercise for you standing on the blue spot and some air is moving towards you:Why hurricanes turn anti-clockwise in the northern hemisphere - with diagrams
4. I cut out the eight pictures and stuck them down again to show things from your point of view:Why hurricanes turn anti-clockwise in the northern hemisphere - with diagrams
5. I then kept the blue spot constant and traced the arrowheads from 2. and 4. Air moving away from you and air moving towards you is all moving to its own right, i.e. clockwise:Why hurricanes turn anti-clockwise in the northern hemisphere - with diagrams
6. So we see that if a bit of air in the northern hemisphere is moving, it will tend to move to the right i.e. clockwise. But if all the individual atoms were moving in circles, it would all cancel itself out. It needs something to large numbers of them in motion in the same direction, i.e. a region of low pressure, such as is found in the middle of hurricanes (or of most storms, for that matter).
If there were no Coriolis effect, the air would rush straight in, and it would even itself out again quickly. If there were the same pressure all over then we'd be back to each atom going in circles which cancels itself out.
But put the two effects together, and each bit of air drawn towards the low pressure "just misses" the area of low pressure, being pulled slightly to the right. And if they are all doing it, then the air direction around the hurricane is in fact anti-clockwise:
Why hurricanes turn anti-clockwise in the northern hemisphere - with diagrams

Back to Featured Articles on Logo Paperblog

Magazine