Why does air moving north from the equator have to turn?

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Marija Djordjevic  asked the Naked Scientists:

Why does the air, which is moving along the parallel, have to turn on right (north hem)? I see that fact follows from Coriolis force formula, but I can't explain it.

(I fully understand what is happening with the air which has either northward or southward component of the motion).

Can someone explain this to me viewed from the absolute reference frame, i.e. the Frame of the Universe? I asked my professor about this, but he has just repeated that annoying sentence about apparent deflection, but I can't see the reason for deflection......Please help!

What do you think?


Offline lightarrow

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Why does air moving north from the equator have to turn?
« Reply #1 on: 29/04/2009 00:14:03 »
Privet Marija.
Air at equator (for example) is rotating with the Earth. Air at the North pole is rotating much less, so air at the equator is rotating *with respect* to the one at the north so when this air from equator moves to the pole, it has a tangential component of speed with respect to the one already present there.
« Last Edit: 29/04/2009 00:15:58 by lightarrow »


Offline stereologist

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Why does air moving north from the equator have to turn?
« Reply #2 on: 29/04/2009 01:47:45 »
Think of it this way. Imagine trying to keep your figure on a spinning globe. Pretty easy at the poles. Not so easy at the equator. Your arm has to move the full distance of the equator to keep up with the spinning globe.

So area over the equator is also moving with the earth otherwise there would be a powerful wind. Air or water moving away from the equator moves towards a zone where air can move slower to remain stationary relative to the surface. Moving north, the air is moving faster to the east than air which is stationary relative to the surface. So it moves east, not just north.

In the Known Universe science fiction series this was a problem for teleportation. Someone from the poles teleporting to the equator would find themselves about 1600 kph too slow as they materialized. The teleportation system had to add in the kinetic energy to keep people in the destination pod.



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Why does air moving north from the equator have to turn?
« Reply #3 on: 30/04/2009 00:36:28 »
Perhaps your professor gave you the standard text book explination, because it is so hard to try and convey the coriolis to people. Not only that but you go in to the Pressure Gradient Force, the maths of the Coriolis Parameter and end up at the Geostrophic Wind link 1 Geostrophic Wind link 2.

One page I would suggest you take a look at is Bad Coriolis written by Alistair B. Fraser Emeritus Professor of Meteorology Pennsylvania State University , and his FAQ page

A nice and simple explanation of the corolis force / effect / acceloration is given HERE by Meteorologist Jeff Haby:
The Coriolis force is an apparent force. From an earth observer, it is an apparent curving of a wind flow. The earth spins counterclockwise when viewed from the North Pole and clockwise when viewed from the South Pole. Therefore, the Coriolis deflection is the opposite in the Northern Hemisphere as compared to the Southern Hemisphere. If you stand on the North Pole, your body will make a complete counterclockwise rotation in 24 hours. However, if you are on the equator, your body will not rotate but will rather face forward as you move with the earth. The Coriolis force is a maximum at the pole (perfect spin) and a minimum at the equator (no earth generated spin). The earth's linear velocity (distance per unit time) is a maximum at the equator and a minimum at the pole.

An air parcel in the Northern Hemisphere moving from the equator toward the pole will carry its higher angular velocity as it moves north. This will cause the air parcel to deflect to the right of its path of motion. If an air parcel moves north to south in the Northern Hemisphere, it will carry its lower angular velocity with it. Since it is moving into a region of higher angular velocity, the earth will spin underneath the air parcel, causing again, an apparent deflection to the right. The word deflection is used because it is relative to an earth observer. Someone watching the air parcel from space would not see the parcel deflect but would rather see the parcel moving straight and the earth rotating out from under the air parcel.

Even if air moves on a constant latitude (east to west or west to east) it will deflect to the right in the Northern Hemisphere. A wind moving to the east will have its own velocity added to the earth's velocity. This will cause the parcel to deflect to a region of higher velocity, which is right of the path of motion. A wind moving east to west will subtract from the earth's velocity and cause the parcel to deflect to a lower velocity, which will deflect the parcel to the right and away from the equator.

There are also a few experiments / demonstrations you can do to visualise the coriolis force.
This is the first and this is the second. I actually found this nice little video showing the second demonstration.

Here are a few other links that you may find useful:

Elementary Climate Physics By F. W. Taylor

How Do We Understand the Coriolis Force? (PDF File)

Watch this video

Wind Circulation

understanding coriolis

pressure gradient force