Coriolis - east/west movement

[Graham Borland]

I'm swotting for my PPL Met exam, and I'm struggling to get my head around this Coriolis thing. The Thom book glosses over it, explaining how it works for north/south wind, but completely failing to explain how it works for east/west wind.

I've found this wonderful site (http://www.dvandom.com/coriolis/) which has lots of photos and diagrams, and a search on PPRuNe has thrown up a few old threads, but it hasn't clicked for me yet.

North/south is easy: the air is moving faster or slower than the ground underneath it, as its latitude changes. But for due east/west wind, there is no change in latitude, so that simply doesn't apply. The site mentioned above tries to explain it with rubber bands, basketballs, and diagrams with arrows pointing in various directions, but I think I'd need to see a physical 3D model to understand it properly.

Can anyone help?

[Mariner9]

You dont have to understand the coriolis mechanism for PPL, just how it applies to wind direction.

All you need to know is a moving air parcel (moving in any direction) is deflected to the right in the northern hemisphere, and to the left left in the southern hemisphere. From that Buys Ballots law (which you definately need to know) arises.

[bookworm]

It's easiest to understand the 2D case first, thinking about the paths on, say, a flat, rotating disc. Make a disk of paper with a drawing pin in the centre.

Hold a pen still on the disc. Rotate the disc anticlockwise around the pin and observe the pen track on the disc. Obviously, it's curved. OK, that was just practice... ;)

Now, while rotating the disc anticlockwise, move the pen steadily radially outwards ("south", away from the pin, which is the north pole) in a straight line with respect to the table. Note how the pen track curves to the right. Now do the same, but move the pen radially inwards (north). Again, the track curves to the right. That's not hard to visualise.

This time, move the pen tangentially across the disc as you rotate the disc. Regardless of whether you move it with (east) or against (west) the rotation of the disc, you'll still find that the track you made curves to the right with respect to the direction of motion of the pen.

Thus if an object has a constant velocity in one frame of reference, it appears to curve to the right in a frame rotating anticlockwise with respect to it. That's what we observe, in our anticlockwise rotating frame, as corilois force on the surface of the earth. Note that it's not "to the east" or "to the west" or "to the north" or "to the south". It's to the right with respect to the motion of the object.

[flybymike]

Bookworm, next time I have a clear mind, a spare half day, a paper disc, a drawing pin and a life ...I will try to figure out exactly what it is you are talking about....:)

[Sallyann1234]

Don't be disparaging - it's ten minutes well spent.

[GOLF_BRAVO_ZULU]

Nobody so far has mentioned Ferrel's Law that is a simple rule saying that particles/objects moving over the Earth in the Northern Hemisphere will tend to be deflected to the right due to Earth rotation. In the Southern Hemisphere they well tend to move to the left. http://www.enotes.com/earth-science/ferrel-slaw Just knowing that life is ordered that way does help to visualise actual events. With direct regard to meteorology, the Buys Ballot Law serves very well too http://www.metoffice.gov.uk/corporate/library/factsheets/factsheet11.pdf
.
O Level Geography was so useful in the '60s.

[xrayalpha]

Graham,

Come to Strathven and learn to fly our C42 microlight. Then you won't need to know anything about the Corialis thingy!

And once you get your NPPL microlights, your non-Corialis microlight met exam can count for an NPPL light aircraft rating so you don't have to sit a JAR met exam!

Oh, don't you love the rules!

Sorry about the thread drift from met to air law!!

Very best,

Colin

[Black Jake]

Try this as an explanation. Air moving over the planet will follow the shortest route = a great circle. With the exception of the equator, all great circles have a North South component. Therefore any air moving East to West (except at the equator) will also have a North South component and be subject to Coreolis, the further away from the equator, the more the North South component and therefore the stronger the Coreolis "force".

[Graham Borland]

Thanks - one point I was missing was the Great Circle thing. Air which starts to head due west (in the northern hemisphere) will gradually acquire a southbound heading as it follows its Great Circle track, so will start to be affected by Coriolis. Still, since the southbound component of its movement is much smaller (initially) than if it was moving due south, so the Coriolis effect should be much less noticeable. Is that the case?

[bookworm]

next time I have a clear mind, a spare half day, a paper disc, a drawing pin and a life

Don't forget "a clean shirt". I've got ink all over mine now... ;)

[bookworm]

Still, since the southbound component of its movement is much smaller (initially) than if it was moving due south, so the Coriolis effect should be much less noticeable. Is that the case?

No, I'm afraid not. The coriolis force is the same whichever direction the movement is in (as long as it's across the surface of the earth rather than up or down).

[italianjon]

I found the easiest way to get my head round it was to think of a ball on a piece of string...

Just start spinning it, and as you give it more string, i.e. increase the radius the angular velocity slows, and as you shortern it speeds up.

So take mass of air which is stationary on the surface it is actually spinning around the north/south pole axis at the same speed as the rotation of the earth. The distance to the pole axis from the surface is the radius.

As the mass moves towards the north pole the distance to the axis, i.e. the radius reduces and so the mass speeds up, hence moves towards the east, and the reverse is true when the mass moves south, it moves to the west as it slows. (In the northern hemisphere - The reverse effect happens in the southern hemisphere but the theory is the same, the shorter the radius the faster the angular velocity)

There will be no coriolis effect for a true east-west movement as there is no change in radius, other than the forces exterted by neighbouring masses which may be affected by coriolois.

[bookworm]

There will be no coriolis effect for a true east-west movement as there is no change in radius,...

Thus unfortunately your model is misleading, since there is a coriolis force for "true east-west movement".

[Graham Borland]

I think it has finally clicked for me.

An easterly wind in the northern hemisphere will start out moving due west, and will gradually turn to the south along its great circle track. However, the ground has spun round to the east in the meantime, with the result that the track followed by the wind is further west than the equivalent great circle on the ground. Is is therefore perceived as having turned to the right from the perspective of a ground-based observer looking along this great circle track. (Even though it may still be on the left (south) of the east-west parallel from where the journey commenced.)

Is that more accurate?

Edit: No, I think this is just a restatement of my earlier misunderstanding. :(

[vihai]

Here is the explanation for the presence of a north-south component of the Coriolis force when traveling along a parallel:

http://en.wikipedia.org/wiki/Eötvös_effect