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Air moving with Earth

  1. Jun 8, 2014 #1
    First post Heey

    Why does the air move at the same velocity as the earth ? Assuming there is no wind.

    I fear my question will be misunderstood so I'll expand a bit. The zero velocity for air (no wind) is the same velocity as the earth. Wind = slower/higher velocity relative to the velocity of the earth.
    How comes the air has the same basic velocity ?

    I read the earth velocity is .47 km/sec. So to stretch it even further If I'm standing still relative to the center of the earth and there was no friction between me and the earth (moving at .47 km/sec but myself at 0mm/sec). I should essentially glide on the surface of the earth. but the air around me should make me eventually go .47km/sec with time. Is that statement true ? and is that what is happening with the air around us, hitting the irrugalirities of the earth so it eventually goes at the same velocity ? And third but not least : how do you write irregularities. ha got it, kept the first one because it was funny.
     
    Last edited: Jun 9, 2014
  2. jcsd
  3. Jun 9, 2014 #2
    That is a good question.

    First of all. The particles and molecules of the air, come from the Earth surface, so they have an "initial velocity" that is non-zero with respect to the center of the Earth (if the Earth is rotating).

    If the Earth did not rotate (no spin), and the temperature was equal at every point, the air (on average) would have zero velocity with respect to the Earth surface (and Earth center, which in this case is the same).

    If there were temperature gradients, for example, poles colder and Equator warmer (as it is in the real case) then there would be North-South flow of air (even if the Earth did not rotate).

    In the real case, we have temperature gradient and also the Earth is rotating (spinning), so we have some North-South component and ALSO the Coriolis effect.
     
  4. Jun 9, 2014 #3
    well you didn't really answer my question.
    I'll expand: Imagine that we have the magical power to stop all the molecules in the air relative to the center of the earth (0m/s). At that point there will be huge wind, air will have a big velocity relative to the ground. Those air molecules will eventually hit trees, mountain, buildings and thus are doomed to be at the same velocity reference as the ground.

    Now my question : Imagine the same scenario but this time the earth is a perfect sphere and there is no friction between the earth and the air molecules. What is the outcome of this ? Will it just stay static or the earth spinning will still exercise a force upon the air due to its rotation?
     
  5. Jun 9, 2014 #4

    boneh3ad

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    Well, saying there is no friction makes no sense here. The important concept is viscosity. Basically, because air has viscosity, it tends to "stick" to surfaces over which it is moving (or surfaces that are otherwise moving relative to the air).

    The same concept can be seen just in your home. Find a dusty table and blow as hard as you can over its surface. No matter how hard you blow, there are still some dust particles, particularly the small ones, that remain. This is because of viscosity and the fact that the air wants to "stick" to the surface so that, right at the surface, the relative velocity is zero. So, for small dust particles, no matter how hard you blow, they still see approximately zero velocity and stay put even though the air farther above the surface has some larger, finite velocity. The region from the surface up until where the air reaches this free stream velocity is called a boundary layer.

    So, moving back to the Earth (or another solid planet), the air is dragged around by the viscosity of the air and the motion of the Earth. If you had a perfectly smooth planet that wasn't spinning and had a stationary atmosphere, then impulsively (suddenly) started it rotating about its axis, the air would begin to be dragged along with it, forming a boundary layer over the surface. Eventually, since the air is just touching up against space, which offers no resistance, the whole atmosphere will be dragged uniformly along with the planet barring the other effects such as temperature or various fluid instabilities.

    If you were to have no viscosity, the air would not move with the sphere.
     
  6. Jun 9, 2014 #5
    My question was more about what would happen in a virtual world where there was no viscosity you were talking about but i got your point. And my question turns out to be "virtual" for a lack of better word. It would be like to ask what would happen to an object in the air if there was no gravitational force. I like wondering but I don't know if those types of question happen to be addressed for the science realm ?

    Anyway thanks for the information.
     
  7. Jun 9, 2014 #6

    boneh3ad

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    Well if there was no viscosity the air would not necessarily follow along with the surface and anything standing on the surface would feel a great wind.
     
  8. Jun 9, 2014 #7
    Since you're inventing this virtual world may be you should tell us what would happen.
     
  9. Jun 9, 2014 #8
    And that great wind would have no effect at all since no body would be able to feel the wind given it's lack of viscosity.
     
  10. Jun 9, 2014 #9

    boneh3ad

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    Well that's not true. Drag exists in inviscid flows. You would feel the effect of the wind from pressure drag, just not viscous drag.
     
  11. Jun 9, 2014 #10

    Drakkith

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    The air is already moving with the Earth's rotation, so even with no viscosity it would still continue to move with the Earth. Remember that the atmosphere, like the rest of the Earth, was formed gradually over a long period of time from an accretion disk and kept its angular momentum from the formation.
     
  12. Jun 10, 2014 #11
    There are some complicated weather effects on the air. The temperature is constantly changing due to, in part, the day and night cycle, and this affects the wind.
     
  13. Jun 10, 2014 #12

    Turning the question around and assuming no air temperature gradients, at what earthly speed of rotation for example, would the boundary air over, say, a still ocean surface at the equator be 100kph?


    IH
     
  14. Jun 10, 2014 #13

    Drakkith

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    There wouldn't be a spot on Earth where this would occur since the air is already moving with the rotation of the Earth. Friction is not keeping it moving.
     
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