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# Cannot get my head around the Coriolis force!

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# Cannot get my head around the Coriolis force!

15th Sep 2008, 12:02

Join Date: Mar 2005
Location: UK
Posts: 518
Cannot get my head around the Coriolis force!

A friend of mine asked me to explain to him what a weather front is and we ended up discussing the Coriolis force. It made me realise that my understanding of it must be wrong.

1. Why is moving air across the earth's surface deflected by it's rotation? Surely the atmosphere is not stationary, i.e it rotates with the earth. Yes, there are fluctuations in the atmosphere due to uneaven heating and pressure gradients, but the surface of the earth doesn't rotate "underneath" the atmosphere. So why can't air proceed directly from high to low pressure?

2. If moving air is deflected to the RIGHT in the northern hemisphere, then why do low pressure systems rotate anti-clockwise?

3. what is it that makes weather systems track west to east (in the northern hemisphere)? Considering that the earth rotates in an anti-clockwise direction (when viewed from the north pole) I would expect the opposite.
15th Sep 2008, 12:36

Join Date: Aug 2006
Location: Suitcase
Posts: 233

On a merry-go-round in the night
Coriolis was shaken with fright
Despite how he walked
'Twas like he was stalked
By some fiend always pushing him right

or :
Fc = -2mΩ v
Where v
is the velocity of the particle in a rotating system, and Ω is the angular velocity vector and the × symbol represents the cross product operator. Times all that by the mass of the object and voila! Coriolis Force.

Got all that?

A video here might help if you are still in the dark.
15th Sep 2008, 13:08

Join Date: Mar 2005
Location: UK
Posts: 518
Doesn't answer any of my questions.

A video here might help if you are still in the dark.
The air surrounding the merry-go-round is stationary where as the earth's atmosphere rotates with the earth. If the air surrounding the merry-go-round was rotating around the same point, would the ball still veer?
15th Sep 2008, 13:59

Join Date: Jan 2003
Location: The Heart
Posts: 812
The air isn't attached to the Earth by any force other than gravity and friction. Think of a cup of tea. The fluid can rotate freely.

Incidentally, the Earth's core is also fluid and the crust proportionally thinner than an eggshell. The crust is free to rotate; very possible that this is the reason for the three great moves of magnetic pole. Not that the magnetic pole has moved but the crust has.
15th Sep 2008, 17:57

Join Date: Sep 2006
Location: Scotland mainly, rather than at home.
Posts: 387
The video is good. I think it will answer your question if you look at it again. The ball in the merry-go-round analagy actually travels in a straight line, but the perception is of a curve ball due to the rotation of the merry-go-round. Think of an air mass of moving in the same very simplified way, and don't over complicate it for the purposes of passing the exam. When you are flying this won't really come into your thinking, then, 10 years later, on PPRuNe there's be a question mentioning Coriolis, and you'll think "Hey I remember that, what was it again?".
15th Sep 2008, 19:38

Join Date: Aug 2002
Location: UK
Posts: 559
The air surrounding the merry-go-round is stationary where as the earth's atmosphere rotates with the earth. If the air surrounding the merry-go-round was rotating around the same point, would the ball still veer?
The ball would still veer because it has inertia, but it would not veer to quite the same extent because the rotating air would try to accelerate it in the opposite direction.

However, you are slightly missing the point of the video. The ball being thrown, IS the air moving on the Earth. You are right that the atmosphere is part of the Earth and is rotating with it, but WIND and movement of tropical storms, is movement of air RELATIVE to the Earth's surface - just like the ball in the video.
15th Sep 2008, 20:39

Join Date: Sep 2004
Location: Maasmechelen, Belgium
Age: 47
Posts: 182
Question 2: If you look here, it may give you some answer (see 4.0).

Last edited by Moira; 15th Sep 2008 at 20:53.
15th Sep 2008, 22:50

Join Date: Jul 2000
Location: Everywhere
Posts: 787
I think I can have a crack at one of your questions, TotalBeginner.

1. Imagine two people walking around a rock. One is further from the rock than the other and therefore has further to travel to get all the way around. If they both do 360 degrees in the same time, the one furthest from the rock is moving faster.

Now, imagine air moving around the pole, except this time the faster moving air is around the equator. Whether it travels north or south, it will have to slow down - because the air there has less distance to travel around the earth's pole, its rotational axis.

Which was will it go when it slows down? Well, it wants to continue in the same state of motion that it had at the equator. As the earth rotates anticlockwise, when viewed from the North Pole, then this means it will try to carry on going anticlockwise (when viewed from the North Pole), whether in the northern hemisphere or southern hemisphere. It just so happens that if you are moving away from the equator into the northern hemisphere, then this ends up being a force to the right.

2. Air always tries to go from high to low pressure - sorry if I'm being too obvious, But I'm just thinking through my typing, in case I make a gross error Ok, stick with me. So you start with air going from high to low pressure, regardless where you are.

So, assuming that there is any gap between the equator and the centre of the low pressure area at all, the air moving towards it from the equator will try to turn right, because is is dragged off due to inertia as it slows down (a force to the right).

However, it still wants to go to the lower pressure. Therefore it is dragged back towards the low pressure (a force to the left). But as it moves further away from the equator it is is pulled right, which is opposed by this force to the left. So in practice, because the initial movement is to the right away from the equator, circulation is set up in an anticlockwise direction.

3. Again, inertia force. Dragged off to the right in the northern hemisphere. However, my understanding is that, near the equator, wx systems actually move more east-west; the air near the surface is subject to the effect of the relatively fast-moving ground (due to large circumference about the pole) and the upper level air is left behind. The inertia effects of air moving away from the equator produce the opposite effect away from the equator, i.e. upper level air gets dragged ahead of the surface.

Hope that helps, and hope it's accurate too!
22nd Sep 2008, 13:44

Join Date: Jun 2007
Location: Herts
Posts: 145
Here's my way of thinking....

take a parcel of air in contact with the ground at the equator. Because of surface friction it will be travelling at the same speed as the ground at that point. Let's say that the speed of the parcel is 100kts. the direction of travel is the same as the direction of rotation of the earth.

Now deflect this parcel Northwards to a higher latitude. The speed of the earth's surface at this latitude is (for example) 50 kts. (This part of the earth has the same rate of spin as at the equator, but the earth at this latitude has a lower diameter).

So the earth is moving at 50 kts, but the air is moving at 100 kts. So it will move faster than the surface, and travel eastwards. If you look at the track that this parcel of air takes, it will appear curved to the right relative to the earths surface.

A similar parcel of air near the North Pole will have a very low speed, but if you deflect it southwards towards the equator it will be moving slower than the earths surface at that point. The effect is that the parcel will appear to deflect westwards (ir to the right of the direction of travel) relative to the earths surface.
18th Jan 2009, 18:50

Join Date: Mar 2006
Location: Northumberland
Age: 33
Posts: 369
Hello,

I hope i'm not hijacking but i've had a few issues with this which i'd like to add:

Firstly, i watched the video posted by Wynsock. When the air is travelling west to east, with the earth's rotation, does it increase speed and that is the reason it heads 'right' towards the equator? If so, when travelling east to west against the rotation, does it slow and therefore head 'right' towards the 'slower' air nearer the pole?

My main query is that I have read that the faster you travel (the example was a north to south in the northern equator), the more effect the coriolis force has. Why is this? (It actually says 'the faster the airflow, the greater the coriolis effect, no airflow = no corilois effect)

Is it in a similar vein to my theory above? I'm thinking along the lines of having slower air at the poles and very fast air at the equator as in rsuggitt's post, and i imagine that the faster air will always head towards the equator and the slower air will always head towards the poles. I'm not sure how correct that is?

Am i over-complicating this?

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