The Coriolis Conspiracy

[Whirlybird]

Well, it all started when I was trying to practice for my next week's FI test by giving AerBabe briefings on the whole Met syllabus.:eek: Coriolis Force has just never made sense to me. Yes, OK, wind blowing from high to low pressure gets turned to the right (in the Northern Hemisphere) by the apparent movement of the earth. That's fine and dandy. But how do you get from there to wind blowing parallel to isobars? And what happens to winds which were originally easterly or westerly; the examples given are always from north to south, or vice versa. Well, AerBabe and I decided Coriolis doesn't exist; it's all an evil conspiracy to confuse us. I'm about to join the conspiracy; you see, I can explain it well enough to probably totally confuse my future students into believing I know what I'm talking about. But I don't. :confused: Can anyone explain?

[Tinstaafl]

Coriolis force doesn't really exist. It's a compensation due to our frame of reference viz. We're relating the airmass movement to a constantly revolving platform - the Earth's surface.

The effect can be shown by making a circle & rotating it (representing the Earth viewed from above a pole). Centre of the circle is of course the pole, circumference is the equator.

Rotate the circle West to East while moving a pen in a straight line from pole to circumference. Do the same again from circumference to pole. Do it again at some diagonal. The pen represents an air mass.

Each time, the line will curve as seen on the circle even though the movement of the pen (the 'airmass') was straight.

The impetus that sets the air mass in motion is a pressure differential. Any time there is N-S component in the velocity then this effect will be seen.

Even an E-W movement will eventually have a N-S component eg Highs/Lows to the N or S.

[Aussie Andy]

Tinstaafl - thanks! I've never really understood it, and your explanation is easy to understand - brilliant!

[Keef]

An elegant answer! I like it better than mine.

Mine was similar - it relates to airmass and earth. At the pole, the earth isn't moving at all. At the equator, it's fair belting round. Therefore as the air moves North or South, the ground under it slows down or speeds up "relatively", with the airmass thus acquiring an apparent east/west velocity relative to the earth.

And then the water goes down the plug'ole and the experiment is over.

[t'aint natural]

I first understood it properly when Mike Smith produced a grammophone record, which he must keep in his office for the purpose. He turned the record while the airmass, his finger, went in a straight line across it. And lo, the light went on in my head. Hours of studying text had failed to drive the point into my thick skull.

[Final 3 Greens]

All I can sya is that it's really freaky that the little vortex around the plughole goes in a different way Dunnunda to here!

Must be one hell of a force to act on such a microsystem ;)

[bluskis]

Now can we please have the simple explanation of Corriolis as applied to gyroscope precession.

[bookworm]

Can't add to Tinstaafl's excellent explanation of the coriolis force itself. Whirlybird also asked about how you get from there to wind blowing parallel to isobars. Here's a mental model that might help.

Imagine you have a car with slightly slippy tyres, and a fault with the steering that means that it always pulls slightly right (its "coriolis force"). The faster you drive it, the more it pulls to the right.

Start driving it down a slope, like a hillside. It turns to the right. It accelerates, so it turns more to the right, and so on. Eventually it reaches a point where it has turned 90 degrees and is moving directly across the slope -- the force accelerating it down the slope (to the left) is exactly the same size as, but opposite to, the force that's pulling it to the right (up the slope). It's in "equilibrium".

If it were to turn further right, it would be going up the slope, it would slow down, the pull to the right would decrease, and so it would start going downhill to the left again. If it were to turn left, it would be going down the slope, it would speed up, the pull to the right would increase, and so it would start going uphill to the right again.

If it were to slow down, the pull uphill to the right would decrease, it would start sliding down the slope and would speed up again. If it were to speed up, the pull uphill to the right would increase, it would start turning up the slope and would slow down again.

So there's only one thing it can do -- drive along at a particular speed (that's its "geostrophic speed") at right angles to the slope (i.e. along the contours).

If you're not into cars, try thinking of a a skier with heavy backpack on her right hip. :)

That's what it feels like to be a air packet of the geostrophic wind.

Hope that helps.

[Whirlybird]

Tinstaafl,
I spent much of my PPL, and some more time recently, drawing lines on oranges in much the way you describe, and sorry, after everyone else's nice comments, but it doesn't convince me, good though it sounds. What about a wind that starts off due easterly? And why don't we end up with all winds going west? And what happens to a wind that starts off as a westerly?

bookworm,
I think your explanation might make sense if I was convinced of Coriolis in the first place; I'm not quite sure. But...for the analogy to make sense, surely the slope would have to move, not the car. Otherwise, what happens when your car goes uphill? Or am I just confused - which is certainly possible.

I've read about this in several books now, and no-one's yet logically explained the jump from a rotating earth that is supposed to bend winds, and winds blowing parallel along isobars no matter what direction the original wind or the isobars go in.

Thanks, guys, but please try again.

[bluskis]

A google search under coriolis effect gives a link to 'a fairly simple explanation' by Dave van Domelen. This link gives a reasonable explanation of the apparent coriolis effect involved in weather systems. As I read it it is a case of relative motion, no actual coriolis force involved in the theory, but forces are provided by neighbouring air masses and gravity in practice.

Note: I think he may have made an error in his diagram showing the N-S arow in the southern hemisphere.


Another google search under 'coriolis force gyroscopes' gives a link to 'dictionary meanings' which contains a description of a 'real' coriolis force acting in a mechanical system.


Sorry, I couldn't post working direct links to these sources.

[AerBabe]

Well, I have another problem now. To get my car out from where it's parked right now I need to drive it down a steep road and I'm sure it's pulling to the right. Bookworm - I hold you fully responsible for me being stuck here until someone can airlift me out. :D

[englishal]

All winds start of N-S in the Northern hemisphere....

As a result of the tropical cell [or whatever its called] which lifts warm air from the equator and dumps it up North, resulting in high pressure north of this zone and low pressure to the south of this zone and thus the quasi stationary polar front. When you get a disturbance in the QSP front, say a ridge of low pressure converges up into the northerly high pressure area, then coriolis acts on this causing rotation, and hence a low pressure area...Then the winds are determined by this low pressure area. In the summer the QSP front tends to be further north and in the winter further south, which is why in Europe we get more low pressure areas during the winter months. You also get a jetstream running along these quasi fronts....

I know what I'm trying to say....I just can't explain it very well :D

Bye
EA

[andrewc]

Final 3 Greens

Coriolis forces aren't strong enough to show up in any normal
situation involving bath tubs...so which way the water goes
down the plug hole is independent of your hemisphere,

Sorry if I missed some irony in your post,

-- Andrew

[Final 3 Greens]

andrewc

Sorry if I missed some irony in your post

It was in there somewhere with the coppery bathtub piping ;)

[bookworm]

Whirly

OK, we need to separate two separate issues here:

1) Why does the coriolis force exist (and what does it do) when we try to consider dynamics in a reference frame that spins with the earth?

2) Given that it does exist, how do we get from there to winds blowing parallel to isobars?

I didn't comment much on 1. Unless you want it in vector algebra, which many find isn't very satisfying as an explanation, all I can suggest is that you keep playing with oranges and turntables.

What's important is that you discover out of that process, and I don't think you're convinced yet, is that coriolis force acts to the right -- not the east, not the west, not the north or south, but the right (in the northern hemishere). Because it only acts on moving bodies, there's always a "right". Try it with a paper disk on an anticlockwise turntable. Run your pencil at constant speed in a straight line outwards, inwards, in the direction of rotation, against the direction of rotation. Look at the pencil traces. They always bent to the right as you look from where you put the pencil down to where you lifted it.

Now let's look at 2, and my car-on-slope model. The slope is like the pressure gradient on the surface of the earth. Its contours are like the isobars.

If you were to look at the scene from a distant star, yes, the slope (or the pressure pattern) would rotate with the earth. The whole scene would look very complicated, and we'd never work out which way the wind went. So we pretend that the earth is standing still, but in order to do so we have to invent this fictitious force (actually forces -- there's centrifugal force in the same category too). We fix the earth/slope/pressure-pattern but have to remember that if anything moves in this artificially fixed frame of reference, it feels a force to the right-- not the east, not the west, not the north or south, but the right.

The only stable state of motion for the car to be in is with the top of the hill on its right. The gravity of the hill pulls it left, the coriolis force pulls it right. If it had the top of the hill on its left, both forces would be acting downwards -- they wouldn't balance, they'd add. It's just like the wind: the anticyclone (high pressure) is always on its right.

If the car starts to track slightly right (going uphill), it slows down, so the coriolis force decreases. Now it has the same gravity pulling it down the slope to the left, but a smaller force pulling it up the slope to the right. So it gets pulled to the left, back onto its original track.

Any better?

[Whirlybird]

EUREKA!!!!!!!!!! :) :) :) :) :)

bookworm ,

You are absolutely, totally, wonderfully bloody brilliant!!!!! The turning to the right I'd sort of understood. The difference between real and apparent forces (which was driving me nuts) you made clear by your distant star analogy. And once I'd got that clear, the car=coriolis, slope=pressure gradient, actually made sense. Many many thanks.

It may be that I'm stupid or have a block here, but judging by the groans and blank looks I've received when mentioning Coriolis (from CPLs and instructors too) I somehow doubt that I'm unique. I'm going to print out your explanation so I can use it in the future. Thanks again!