You don't need a weather man to know which way the wind blows

May 9, 2007 11:50 PM

Does the atmosphere rotate? In other words, is there a net shift in air around the globe, east to west?

I got totally dumbfounded in a discussion about this, and Google didn't seem to help. Does the atmosphere have a net- rather than localized- rotation around the globe, or are all the east winds basically equivalent to the west winds? Do the Easterlies and Westerlies (http://ww2010.atmos.uiuc.edu/guides/mtr/hurr/gifs/mvmt1.gif) cancel out?
Example: In the midwest US, it would seem that the whole atmosphere has a general shift from West to East. But in the Caribbean it seems to go, on average, East to West. Does this all even out in the end, or is there a net rotation in relation to the terra firma?

posted by conch soup to science & nature (13 comments total)

Yes, the atmosphere rotates, on average, at the same speed as the earth below. The real speed of the atmosphere is less at the poles, therefore, than at the equator. All those localised "easterlies" and "westerlies" are the result of localised effects of the coriolis force / Hadley / Polar / Ferrel cells.
posted by Jimbob at 12:04 AM on May 10, 2007

The diameter of the earth is 8000 miles, more or less. This is equivalent to a circumference of about 25000 miles.

It turns once a day.

If the atmosphere didn't rotate, then the wind velocity at any point on the equator would be 25000Mi/24hrs, or 1024 MPH, a pretty stiff wind.

Another fluid (the oceans) also rotates in conjunction with the earth. Air is just a thinner ocean, ubounded by shores.
posted by FauxScot at 12:29 AM on May 10, 2007

I guess I should clarify- by "net shift around the globe" I mean is there a net shift in relation to the already rotating earth.

I'm asking whether the atmosphere rotates slightly faster or slightly slower than the earth (ground and water) does.
posted by conch soup at 12:34 AM on May 10, 2007

Try searching for global circulation.
posted by zamboni at 12:34 AM on May 10, 2007

As someone who is in this science field, I don't really know the answer for sure. The prevailing westerlies in terms of mass*speed are pretty damn significant, but the trade winds (which have an east-to-west component) cover a -lot- of square mileage.

The term you're referring to is the zonal component and I think you may be able to find some clues with a search like this, though at first glance a lot of the results look extremely arcane.
posted by rolypolyman at 12:55 AM on May 10, 2007

Conservation of momentum in the atmosphere is a complex process, but basically the earth/ocean/atmosphere system must conserve angular momentum. Angular momentum is transferred from the earth to the atmosphere by the tropical easterlies, where air is rotating faster than the earth and transferred from the atmosphere back to the earth by the westerlies in the mid-latitudes, where the wind is rotating slower than the earth.

These are not the only torques acting on the atmosphere, but all combined they even out in a momentum budget. If they did not, then for example, more and more momentum would gradually be imparted to the atmosphere and winds would get stronger and stronger while the earth slowed down. The fact that this does not occur indicates that a balance is achieved. However there are temporal anomalies, like El Nino, that can cause a imbalance that can be measured in a temporary change in the earth's rotational speed.
posted by JackFlash at 2:00 AM on May 10, 2007

Well, obviously I said that backwards. The easterlies rotate slower than the earth and pick up momentum from the earth. The westerlies rotate faster than the earth and return momentum to the earth. These are primarily frictional forces. One of the more prominent is called mountain torque and is the effect of westerly winds impinging on the Rocky Mountains.
posted by JackFlash at 2:09 AM on May 10, 2007

I would suggest that you would have to look carefully at atmospheric movement caused by the Coriolis Effect and at thermal models before looking at any winds which might be attributable to drag of the atmosphere against the vacuum of space (assuming this drag exists - I am not a Physicist).
posted by rongorongo at 5:52 AM on May 10, 2007

My guess would be that the atmosphere would very slowly revolve opposite to the Earth's rotation.

Why? Conservation of angular momentum. All the air is rotating around the center of gravity, and it's doing so (pretty much by definition) at a higher altitude than the land. It's farther from the rotation center, and therefore must rotate a little more slowly. Taken a whole, it would slowly move 'backwards'. (example: bucket whirled at the end of a rope. If you lengthen the rope, the bucket slows down; if you shorten it, it speeds up.)

The Earth is 8000 miles in diameter, and the atmosphere is generally defined as ending at 50 miles above the surface, so the net motion from this effect is going to be very small.
posted by Malor at 6:40 AM on May 10, 2007

Those were my original thoughts exactly Malor, due to the angular momentum part. But then I started thinking about friction, coriolis effect, the upper atmosphere and the coffee table discussion got way over my head.

Hey- some of these answers have been super helpful, even though I'm sure this is a very complicated and hard to measure topic. AskMefi still seems to be the best place to come for those near-impossible science quandaries.
posted by conch soup at 7:10 AM on May 10, 2007

My guess would be that the atmosphere would very slowly revolve opposite to the Earth's rotation.

Why? Conservation of angular momentum.

That would be the case if you recently outgassed the atmosphere from the center of the earth as in your lengthening bucket example. Instead most of the atmosphere condensed from a widely dispersed disk. Conservation of momentum causes the disk to speed up as it contracts. But in any case, that time is long past. The atmosphere has been coupled to the surface of the earth by frictional effects for billions of years so, on the whole, they rotate synchronously.
posted by JackFlash at 10:45 AM on May 10, 2007

The 'super-rotation' of the atmosphere with respect to the surface of the planet is one of the big mysteries about Venus.

The planet itself takes 243 Earth days to rotate once on its axis (i. e. one Venusian day is 243 of our days long), but the winds "are so powerful and fast that they circumnavigate the planet in only four Earth days."
posted by jamjam at 6:05 PM on May 10, 2007

Jamjam, Venus is a very mysterious case that astronomers and physicists don't really understand. The high velocity winds are limited to a thin layer at the cloud tops about 60km (200,000 ft) above the surface where the air is very thin. At the surface and above the cloud layer there is no wind movement at all.
posted by JackFlash at 6:59 PM on May 10, 2007