View Full Version : Coriolis!

26th May 2002, 15:16
Here's something that has me worried, sleepless nights etc.
In a few weeks I a have MET exams, so I supposedly know all about the CORIOLIS turning wind to the right. Fair enough.
But what mystifies me is this:
What happens to an airplane flying say, over the npole heading south for new york...why will it not end up in LA?

Help me please, for I am confused!!

Captain Stable
26th May 2002, 15:22
The flippant answer is because they actually aim at mid-Atlantic! :D

In effect, the coriolis force doesn't get noticed among all the drift caused by wind, jetstreams etc. Its effects are rather longer-term and the force itself is far smaller, so it works over long term on major air masses.

Somewhere I used to have among my favourites a website that showed all the effects, one by one, on air in the atmosphere, of coriolis, rotation of the earth, heating by the sun, etc. etc. but I can't find it - lost in a HD rebuild, I suspect... :(

26th May 2002, 18:43
First of all, what is coriolis force?

Picture a turntable turning anticlockwise as pictured from the top. Now consider an ant crawling out from the centre of the turntable to the edge of it, in, what looks like the ant to be, a straight line.

Now look at the path the ant travelled form the point of view of an observer looking down on the turntable from an inertial reference frame (i.e. standing still!). The ant actually travelled on a path that is curved to the left. In order to do that, the ant had to be accelerated to the left. It did that by hanging on tightly to the surface of the turntable, which provided a force to the left.

(Actually, if the ant walks in from the edge, or in any other direction that is a straight line with respect to the turntable, he also curves left according to the outside observer.)

Standing as that observer, it's obvious what happened. A force was applied to the ant, and it accelerated in a curved path. But sometimes it's really awkward to stand away from the rotating frame like that. So to work out what happens to the ant more conveniently, we invent a ficticious force called the coriolis force, which always acts to the right when the ant tries to move. As far as the ant is concerned, the force the turntable applies to the left just balances the coriolis force to the right, and he travels in a straight line. That's much easier for the ant than trying to picture the scene from the point of view of the observer! The coriolis force corrects for the fact that the turntable is actually spinning, allowing the ant to use the turntable as its frame of reference.

We do exactly the same on the surface of the earth, when we don't want to solve the mechanics by blasting off into space and looking at the situation from there. We invent a coriolis force, which acts to the right when something moves across the earth (in the N Hemisphere). That accounts for the fact that the earth is not still, but actually spinning. The formulas are slightly more complicated in 3D.

Why doesn't it affect aircraft?

It does, but it's tiny. Even for something travelling at 600 knots, directly towards the axis of the Earth, the coriolis force is about 0.002 G. Compared with all the other forces on the aircraft, that's negligible.

It's only important when you consider things that only have small forces acting on them. A parcel of air in the atmosphere is affected by pressure gradients. Take a sheet of wood half an inch thick, and point the face towards the centre of a low pressure area. Did you feel it pushed towards the centre of the low pressure? No, I thought not. :) Yet it's that pressure gradient that is balancing the coriolis force for the wind aloft. Both forces are very small, of the order of a ten-thousandth of a G. Only with something like air does it become important.

Hope that helps.

Dick Whittingham
26th May 2002, 21:09
Yes, the Coriolis force, aka the Geostrophic force is an imaginary force invented to account for the rotation of the earth. Never mind the details, try this one:

Set up your cold air vents in the centre of your car dashboard so they are pointing between you and the passenger and turn the fan up full. You should feel only a little of the air blast.

Execute a high rate turn right - at low forward speed please, a sort of urban right turn. You will feel the air blast switch and blow full in your face. You are turning right, but the air leaving the vents has appeared to turn left coming out.

This is just what happens to air, pigeons, whales, nuclear submarines and ballistic missiles. The earth turning left underneath them makes them appear to turn right.

Dick W

I. M. Esperto
26th May 2002, 21:13
In long range naval gunnery solutions, coriolis is one of the forces factored in along with such other things as bore erosion, temp., wind, target motion, and gun motion.

26th May 2002, 21:35

don't forget for your Met exams that the Coriolis force can also act to the left. Think about it :)

Good luck,

27th May 2002, 13:28
Coriolis.......and Bernoulli...................

A right pair of scheming twats they were!!!!!!!!!!!!!!!!!!!

27th May 2002, 15:17
Thanks for the replies people. It's very comforting to know that coriolis is barely noticible, I already saw myself calculating coriolis-crab-angles with some super-crp5!! And no mjudk, I'll try not to forget the troubles of the Southern Hemisphere!

28th May 2002, 12:24
I M Esperto,

I think you forgot "Gun canted trunnion elevation error"!
Happy days at Whale Island - twice!

30th May 2002, 17:44
Coriolis effect is the appearance on TV of a soap opera from Lancashire that compels me to press the OFF button.;)

P.S., sorry that is a UK joke. For USA readers, just think "Dynasty" with clogs on!