Wednesday, 6 November 2013

Why hurricanes turn anti-clockwise in the northern hemisphere - with diagrams

There are two stages to understanding this - the "Coriolis Effect" and the "just missing the low pressure" effect (see explanation here)

The Coriolis effect has been explained often enough using the roundabout analogy.

1. Imagine you are viewing the northern hemisphere 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") so our observer (the blue dot on the equator) moves round anti-clockwise.

Some air (red) is moving away from him towards the North Pole, some air (green) is moving towards him, and we assume that there is no friction between the earth and the atmosphere, so the whole atmosphere is slipping westwards (from the point of view of the observer).

2. We can eliminate the anti-clockwise rotation and look at from the point of view of the observer, who thinks he is static, by rotating the images from step 1, keeping the blue dot at the bottom of the picture.
3. We can then plot the apparent course of the red and green air. They both appear to be rotating to the right, i.e. clockwise.

4. 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:

5. 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:


Dinero said...

Wow you have done a lot of work .

The last diagram is very helpful. It illustrates why the wind would go from the south and the north to to half way between equator and pole. I'm not sure about the first five diagrams.
paulc156 gave a good explanation in the comments on the earlier post about hurricanes 1 Nov.

A K Haart said...

My version works okay for me.

DBC Reed said...

@M The just missing low pressure areas is the most (only)coherent explanation of why hurricanes are anti clockwise I've read: I buy it and have already internalised it as an explanation. Still worried by the Moon being upside down in Australia mind you. Does it appear to flip as soon as you cross the Equator? We should be told.

Mark Wadsworth said...

AKH, but you are a scientist, I need nice simple diagrams, which I ended up having to do for myself.

DBC, thanks.

The Moon does not flip over, you are looking at it standing on your head. If you are at the equator directly underneath the Moon, you can choose whether to look at it right way up or wrong way up.

James Higham said...

Fascinating - shall link.