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Why is the air less dense higher up you go?

  1. Jun 18, 2009 #1
    Hello everyone :smile:

    I have a quick question why is the air less dense higher up you go. I can understand air expands and the volume is bigger hence less dence but that only happens if it is warm, higher you go it is cooler so how does air expand. Thank you!! :smile:
     
  2. jcsd
  3. Jun 18, 2009 #2

    Borek

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    Staff: Mentor

    Think gravitation.
     
  4. Jun 18, 2009 #3
    Thanksfor the reply. I can understand how this would create higher pressure lower down but how is this related to density. Warm air is more dense than cold air so how is cold air at the top be less dense.
     
  5. Jun 18, 2009 #4
    Temperature is more or less equivalent to the total kinetical energy of the individual molecules. Density is the total mass (amount) of the molecules in a certain volume.
     
  6. Jun 18, 2009 #5
    Thanks a lot for your reply :redface: it makes sense but now I'm bit confused about density and I'll ask my question in form of a picture

    [​IMG]
     
  7. Jun 18, 2009 #6
    Pressure is average force per area. If you have fewer molecules hitting an area over a given time, and/or the molecules are traveling more slowly, then there is less pressure.
     
  8. Jun 18, 2009 #7

    Borek

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    Quite the opposite - assuming constant pressure, warm air is less dense.

    But you can't directly compare densities, not taking pressures into account. The higher you go, the lower the pressure - so even if the air is colder, its density is lower.
     
  9. Jun 18, 2009 #8
    Yes that is what I meant I just wrote it wrong way around. Why is density lower with lower pressure. For example in my picture. The gas molecules at the red area, if I want to look at the density of these molecules do I take the blue box or the yellow box as my volume.

    Thanks for the help so far :smile:
     
  10. Jun 18, 2009 #9

    Borek

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    Start with ideal gas equation:

    [tex]n=\frac {PV} {RT}[/tex]

    Density is mass per volume, mass is number of moles times molar mass. Combine it all.
     
  11. Jun 19, 2009 #10
    What we feel as air pressure is the weight of the air above us, thus the higher you go the less air there is above you. To remain in equilibrium the force must be equalised in all directions, thus why it's even in the horizontal directions. Remember that air is compressible - it occupies less volume (gets denser) the more it is squeezed, thus it's denser where the pressure is higher near the ground and as you get higher the amount of weight from a given volume of air decreases, so the pressure squeeze eases off increasingly rapidly with altitude.
     
  12. Jun 20, 2009 #11

    Astronuc

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    Air is less dense with altitude because there is less mass above a given elevation. It is the weight (force due to gravity) of the mass of air above a given elevation that produces the pressure.

    At sea level, the air has a pressure of 1 atmosphere (1 atm, or ~14.7 psia, or 0.10132 MPa). With increasing elevation, the pressure decreases to the vacuum in space.

    Heating occurs near the earth's surface via conduction from the earth's surface and direct heating, which is proportional to the density.

    http://www.grc.nasa.gov/WWW/K-12/airplane/atmos.html
    Heating is also a function of composition, which also varies with altitude.
    http://www.physicalgeography.net/fundamentals/7b.html
    http://en.wikipedia.org/wiki/Ozone_layer
     
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