Cold Air

  1. If cold air is more dense, why is higher altitude air colder and less dense?
  2. jcsd
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  3. russ_watters

    Staff: Mentor

    Two reasons: space is cold and air cools as it expands(it expands as it rises)
  4. Does its heat sink while the air rises?
  5. Does the air move up through the heat while the heat stays relatively stationary?
    If warm air is thinner than cold air and it expands as it cools, does it somehow re condense?
    I'm trying to get my mind around cold dense air versus cold thin air.
    So if you took a thin plastic milk carton, (one of the good ones, with the screw on cap) up to a mountain top on a cold day, screwed on the cap and drove down into the valley into a hot day, would the milk carton crush in on itself? or would it expand and maybe pop?
    Last edited: Aug 20, 2011
  6. DaveC426913

    DaveC426913 15,940
    Gold Member

    Well, the expansion due to heat will far outstrip the compression due to rise in air pressure, so it will most definitely expand.
  7. So, in this case cold air is not more dense than warm air, and therefore, cold air is not necessarily more dense than warm air, but only in some situations.
  8. Why do you think cold air is by definition 'more dense'?

    Consider a pneumatic tyre just after you have pumped it up.

    Is the air inside more or less dense than the surroundings?

    And is it hotter or colder than the surroundings?
  9. I think cold air is more dense than warm air because if I put an empty water bottle in the freezer over night it will crush in on itself. But I don't know if this principal is universal for cold or hot air.
    I think your question is a good one. I think I can make a way to get a thermometer inside a can and then pressurize it to see if it changes.

    I squirted a blast of air out of a can for cleaning keyboard dust. I noticed that the can got cold. I assume that the can being pressurized must have been endothermic, therefore the release of pressure must be exothermic. Thus, the temperature decrease.
  10. Obviously pressure is very important.

    At constant pressure air is colder when it is less dense. If you increase the pressure the density of the air will increase (as you are effectively squashing the air into a smaller volume). Consider the case where you increase the pressure on the air, and yet the density remains the same -- in this case something must be fighting against the squeezing tendency due to the increasing pressure, that something is the increase in kinetic energy of the gas particles, that something is an increase in temperature -- the rise in pressure is met with a rise in temperature if density is to be conserved.

    So to the OP. What happens to the pressure at higher altitudes? What are the implications for this on the temperature and density of air?
  11. Aren't pressure and density synonymous? At least when it comes to air?
  12. Evo

    Staff: Mentor

    The cold is from the gases (propellants) in the can.
  13. No. There is an interplay between pressure, density, and temperature.
  14. russ_watters

    Staff: Mentor

    Heat is not a thing, separate from the air. The air has a certain amount of heat, which it carries with it as it rises.
    If there is moisture in it, the moisture can condense. That's how a thunderstorm works.
    The atmosphere has a pressure gradient to it, so it gets less dense as you go up. But at any particular altitude, warmer air is less dense than colder air.
    It would collapse.
    Not quite, no. At any altitude, any non-rigid volumes of air will have the same pressure, but may have different density.
    Last edited: Aug 21, 2011
  15. DaveC426913

    DaveC426913 15,940
    Gold Member

    Huh. I thought it would expand but, running some numbers through the formulae, you're right - if we use a tall enough mountain.

    Going from Everest to Sea level would triple the pressure, causing the volume of the gas to be reduced to one third.
    Going from -20C to 20C is actually going from 253K to 293K, which would only cause an expansion by one seventh.
    Net effect is a collapse.
    Last edited: Aug 21, 2011
  16. I think enough examples have now been provided to demonstrate that what happens in a gas is more complicated that one simple statement will allow. Further what happens also depends upon outside circumstances.

    The question of does a gas cool or warm when it expands depends upon how the expansion is generated and whether the gas is confined or free.

    So if we heat a gas in hot air balloon, the gas expands into the confines of the balloon, increasing the pressure and temperature whilst reducing the density.

    By constrast if we expand a gas from a cylinder of compressed air into the open air, the gas cools as it expands and its pressure and temperature and density reduces.

    Mathematically the controlling physics are known as the Gas Laws and the Laws of Thermodynamics.

    go well
  17. russ_watters

    Staff: Mentor

    Actually, I read too fast and didn't see the cold mountain part, but thanks for bailing me out! :biggrin:

    Regardless, yes, atmospheric pressure drops very quickly as you go up in altitude.
  18. I confirm this as I have inadvertently done this experiment in reverse. Buy a packet of crisps at sea level. Drive up a thousand metres or so and look at your crisp packet. In my experience the crisp packet has expanded.
  19. In atmospheric physics, there are two kinds of density.

    One is mass density, that is, kilograms per cubic meter. The mass density of a volume of air depends upon its composition, its temperature, and its pressure. Colder humid air is less dense than slightly warmer dry air at the same pressure, because the mass of a vapor molecule is less than the mass of a dry air molecule. However, air at high elevations is less dense than air at lower elevations because the number density is less.

    The second density is number density, that is the number of molecules per cubic meter. The formula follows Avogadro's Law and is n= P/kT. Here n is the number of molecules per cubic meter, P is the pressure in Pascals, k is Boltzman's Constant, and T is the temperature in Kelvins. Obviously, the number density of a parcel of air depends upon both the temperature and the pressure. The number density of high altitude air is less than that of low elevation air at the same temperature because the number density reflects the Maxwell molecular speed distribution for any given temperature. This is a common exercise in discussions of kinetic gas theory and statistical mechanics. That is, showing that the number density reflects the barometric formula.
  20. DaveC426913

    DaveC426913 15,940
    Gold Member

    It will though it is not as simple as that. You tested pressure change only, did you test temp change? Were your crisps* exposed to below freezing temps a thousand metres up?

    *whatever crisps are...
  21. I think this is a case for the nspcc.

    (National Society for the Prevention of Cruelty to Crisps)
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