Wind, which way?

Discussion in 'Earth Science' started by Kunax, Sep 7, 2004.

  1. Kunax Sciforums:Reality not required Registered Senior Member

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    Wathing the weather(h?) on TV, I notised that the wind always went clock wise around high preasure and anticlock wise around low preasure.

    Is this a constant or just a coinsident?

    p.s. yes i know notheing about weather systems
     
    Last edited: Sep 8, 2004
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  3. river-wind Valued Senior Member

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    it should be that anywhere above the equator, the storm should go couter-clock-wise; south of the equator, clockwise. this is due to the Coriallis effect (sp?), same reason your toilet flushes in the same direction everytime.

    The warm air is sucked up at the eye of the storm, and spreads out, as it cools and sinks back down, it spins, just like going down a drain.

    same deal with tornados, though it is more direct, because a cool (dense) body of air has pushed up ontop of a warmer (less-dense) mass, and is sinking through it. Because airmasses of different tempurature don't easily mix, the cold air find a weak point and dains through the warm air through it.
     
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  5. Kunax Sciforums:Reality not required Registered Senior Member

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  7. hypewaders Save Changes Moderator

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    To illustrate Coriolis "force" to students, I usually employ an office chair that pivots on a vertical axis: Rotate the chair (anticlockwise for N Hemisphere) while lowering your closed hand over it, and spread your fingers. Chair rotation represents Earth's, spreading fingers represent high altitude/pressure descending air mass spreading at the surface. Conservation of momentum explains the Coriolis effect: The spreading airmass just exhibits linear momentum, and its fluid freedom of motion, while the Earth turns beneath.

    To model a low pressure system, reverse your hand motion, closing your fingers and raising your hand. In low pressure systems, the apparent vortex accelerates, because moving inward momentum increases speed with less circumference to travel- like spinning yourself in the chair, and pulling your arms and legs inward. High and low pressure air masses also interact, increasing the wind speeds between them.

    On a slightly related note, I've long been seeking a concise explanation for the force propelling jet streams. I have studied meteorology quite a bit, but do not understand what causes high-altitude rivers of air to zip along at very high speeds (sometimes 300+ Knots). Does anyone out there understand jet streams??
     
    Last edited: Sep 8, 2004
  8. The Singularity The last thing you'll ever see Registered Senior Member

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    The jet streams (there is more than one) are caused by temperature differences in the atmosphere. The most famous jet stream is the Polar Jet Stream ... which controls weather systems across the Northern hemisphere above the 30 degree latitude. It forms where the cold, polar air meets the warm air from the tropics. The large temperature difference between these two consequently causes a large pressure difference.

    Air from high pressure flows in the direction of low pressure. Since the pressure changes so quickly, the air flows between pressure systems very fast and thus the strong winds are formed, which are called jet streams.

    And this happens to circle the entire globe as one continuous jet stream ... especially for the Polar jet.
     
    Last edited: Sep 8, 2004
  9. hypewaders Save Changes Moderator

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    But jet streams don't flow from high to low. There's little correlation that I can see between pressure maps and jetstreams. Or frontal zones. As you mentioned, some JSs encircle the globe- That doesn't sound like high to low to me.

    Further, the areas between high and low pressure system are vast- What squeezes them into tight cores of moving air? Spontaneous wind-weaving? I still don't get it (but thanks for trying to explain, maybe I'm missing something).

    I've read that jetstreams form high up along hot-cold frontal boundaries, because of the shearing effect of a thermoclime. I understand why non-jetstream wind shear forms at an inversion (something like surface-tension on water, and gases molecularly lining up into low-friction planes), but it never organizes into a high-speed narrow river at lower altitudes. Even if a jet stream forms annular thermoclimes (and I don't know if they do) what propels them at such speed? We are talking about millions of tons of atmospheric gases accelerating to Mach 0.5 in tubular rivers... and I still don't get it.
     
  10. The Singularity The last thing you'll ever see Registered Senior Member

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    If you look at a worldwide map, you can see alternating high and low pressure systems circling the globe. It doesn't look obvious because on the map you only see centers of highest and lowest pressures. Everything in between varies between these two extremes and as long as it alternates from high to low at certain degrees, then the trade winds do follow a global path. This is why the jet stream doesn't follow a straight path ... it curves throughout its path into throughs (extreme lows) and ridges (extreme highs) as it circles the globe.

    Jet streams are around 300 km wide ... and to me thats not very narrow. But I don't know what keeps them in "cones" or columns of horizontal air as they circle the globe.

    The reason why there are no lower level jet streams is because the fact that temperature contrast isn't as large as in higher altitudes and the large pressure gradiant is missing. This is a simplistic way of looking at it ... I could explain further but i'm slightly tired right now.

    The thing that propels them at such velocities has to do three things; coriolis force, pressure gradiant, and centripital acceleration. A strong coriolis force with a high pressure gradiant and a strong centripital accel. results in very strong winds ... its almost like a multiplying effect. These three factors contribute with the speed of low level winds and the jet stream. Temperature gradiants does have an impact but it's directly related to the pressure gradiant so it's not worth mentioning again.

    If I find more info on it (and when I'm more alert) I'll post it here.
     
  11. The Singularity The last thing you'll ever see Registered Senior Member

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    Actually, I just found this:

    This is from an aviation website but it does explain some of it better than i did.

    http://www.avsim.com/avwx/avsim_wxus_jetstream.html

    Note: Apparently I was wrong about the polar jet being a continuous "river" of air. It's not actually a continuous flow but a series a jets that are in the order of 1000 km in length with weakened areas along the path.

    Another source I found is this:

    http://www.phys.ufl.edu/courses/met1010/chapter11-1.pdf

    Starting on page 7 of that PDF file describes it with some pictures. It's not a conplete description but it does show it to some degree.


    To tell you the truth, there are no real websites that explains it completely ... at least none that I know of. If the answer is somewhere out there, then it would be in my textbook.
     
    Last edited: Sep 8, 2004
  12. hypewaders Save Changes Moderator

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    Thanks, Singularity. That's good info.
     
  13. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    hypewaders' model of Coriolis "force" was not very clear to me. I provide an extract from my book, Dark Visitor, which is longer but most readers easily understand:

    "... At the equator, both the land and the air are moving at about 1000 miles per hour, if seen from the moon (or any other point traveling through space with the earth). This is because at the equator it is approximately 24,000 miles around the earth and the air must go around in 24 hours. Citizens of Ecuador don't notice this speed because they are also going around at 1000 miles per hour. This "free speed" is why all space-rocket launch-centers are as near to the equator as the launching nation can place them. Relative to the land, the speed of the moving air (we call it wind) is much less, but these high equatorial speeds have an enormous effect upon the climate.

    At Lisbon's latitude the distance around the earth is only about 3/4 the circumference of the equator, so there the air is only moving at 750 miles per hour to go around with Lisbon in 24 hours. If you could instantly move a big chunk of this Lisbon air to the equator, the earth would be running under it so fast that the wind would come out of the East at 250 miles per hour.

    Obviously a mass of Lisbon air can not be transported instantly but recall from the discussion of a non-spinning earth's climate that the surface winds should tend to come from high latitudes and move towards the equator. Thus, as Lisbon's air moves south, it tends to head west also as it is not going around the earth fast enough to keep up with the land (or ocean) spinning under it. By the time this air from Lisbon gets near the equator, it is mainly moving towards the West as seen from the deck of a ship in the tropical ocean. On its trip southward, this air mass gained most of equatorial speed from friction with the surface – making steady winds towards the west, not 250 mile / hour gales!

    Local weather phenomena such as warm clouds rising and cold air descending tend to keep the lower parts of the tropical atmosphere mixed and all moving towards the west, at least as the air mass moves west from Africa, but recall from the non-spinning earth model, this warmer air must also tend to go towards the poles to replace the air which is going toward the equator. Thus all year long east of the Caribbean region there is a general climatic circulation of air along the path of typical hurricanes - westward from Africa but turning northward.

    Columbus may not have understood why (as I hope you do now) but he knew you could find good reliable winds to the west if you first went south from Spain/Portugal along the West Coast of Africa towards the equator. Even today these dependable winds from the east in the tropics are call the "trade winds" because they were so important for trade between the new and old worlds when ships were moved only by the wind.

    As surface air that was once near Lisbon or Oslo travels west from Africa these trade winds acquire both heat and moisture from the ocean. This moisture (water vapor) represents a tremendous store of energy, which as the weather begins to cool in the fall, can condense (rain) and release this energy in the form of a hurricane. In the winter, even more energy (heat) can be gently released as this moisture condenses into snow. This helps to keep Oslo etc. from becoming too cold. Even when you understand it, it is hard to believe, but true, that lots of snow helps keeps Oslo warmer than Boston!

    In summary, one can say that the excess heat of the tropics and the spin of the earth combine to give a clockwise circulation (when view from space) to the climatic wind patterns in the Northern Hemisphere. Or more generally, for both hemispheres, it is true that the prevailing winds over the oceans in the tropics move towards the west and then turn towards the poles. Thus in the Southern Hemisphere the climatic circulation is counter clockwise.

    The clockwise circulation in the Northern Hemisphere is also caused by the fact that by the time this tropical air mass is at Boston's latitude, it is now traveling too fast towards the east for the surface under it, making winds to the east from the west. The same is true in the Southern Hemisphere but it is a counter clockwise circulation that takes air from Argentina eastward towards Southern Africa. The "pole seeking" equatorial air is moving too fast towards the east as it travels toward the poles. (Just as the "equator seeking" mass of polar air from Lisbon or Oslo was moving too slow as it went south.) Thus, at America's latitude, the prevailing winds come out of the west and blow towards the east because they still "remember" part of their equatorial speed. ... The "force" that causes north or south moving masses of air or water to be deflected sideways (the clockwise turning in the Northern Hemisphere just discussed) is called the Coriolis force, after the French engineer Gaspard Coriolis who explained to Napoleon why his long-range cannon fire was always falling a little to the right of the target if the target was north or south of the gun, but was "on target" if the gun was aimed either east or west. (Napoleon began his career as an artillery officer and was always interested in science1.)..."

    Footnote 1: Perhaps he intuitively understood the germ theory decades before either Pasteur or Semmelweis. He supported the development of canned food. It can be argued plausibly that canned food was more responsible for his military victories than his generalship. (He could mount long campaigns in distant lands and yet feed his army, but his supply of canned food was no match for the Russian winter.) As Caesar said: "An army travels on its stomach."

     

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