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Atmosphere temperature gradients at altitudes impossible?

  1. Jun 16, 2007 #1
    atmosphere temperature gradients at altitudes impossible??

    Here's a link to a graph:
    Layers of the Atmosphere

    I don't understand how this is possible.
    The atmosphere is
    • Warm at the surface, (due mainly to infrared heat from earths' surface -ok)
    • Cold at 20km altitude (impossible?? Fully encapsulated by heat sources; Nowhere for heat to go.)
    • Warm at 50km (Ozone layer heated by UV from sun -OK)
    • Cold at 90 km (impossible; fully encapsulated by heat sources)
    • Hot at 500km

    I don't understand how fully enclosed colder regions can exist between heat sources.

    An analogy might be a perfectly insulated pipe with a heat source at either end. No matter what different materials I put in the pipe, it is impossible for the temperature anywhere in the pipe to be lower than that of the the lowest heat source.

    Can anyone explain how the atmosphere temperature gradients are possible?

  2. jcsd
  3. Jun 16, 2007 #2
    You're thinking about heat as being some physical property but you have to recall that heat is actually the average kinetical energy of the individual molecules and hence the individual molecular speeds play an important role. Also gravity acts on individual molecules. So obviously, at altitudes where the atmosphere gets so thin that individual molecules travel a great distance between collision, this speed is getting important. Due to the gravity there is a balance between molecular speeds and altitude. The slower molecules are pulled down more easily, while the faster molecules stay aloft. Some even escape gravity

    Now recall that "slower molecules" is identical to being cooler and the faster molecules is the same as being warmer.

    But if you would be there you'd still get the impression that's freezing cold (at night), since the sparsety of molecules would not give a noticable conduction heating. And out-radiation of EM plays the main role in losing heat.
  4. Jun 16, 2007 #3
    But if the air molecules at an altitude of 20km had less kinetic energy than the bordering, encapsulating air molecules above and below, wouldn't they eventually reach at least the energy of the cooler of the two borders?

    I can't understand how any object can be permanantly cooler, or contain less kinetic energy, than its surroundings. Surely if this occurred, there would be a flow of energy into the cooler object?
    Last edited: Jun 16, 2007
  5. Jun 16, 2007 #4


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    Staff: Mentor

    The pressure/density gradient causes the temperature gradient. In convection in the atmosphere, hot pockets of air rise and the air cools as it rises because it expands.
  6. Jun 16, 2007 #5

    I think I understand that.
    Charles's law! V/T=constant
    So I should be able to calculate that and prove it to myself.


    Thanks to all.

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