Why are the top of mountains cold?

Discussion in 'Earth Science' started by John Connellan, Oct 23, 2009.

  1. John Connellan Valued Senior Member

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    It is my understanding that the reason it gets colder the higher up you go in the troposphere is because you are further from the main source of warmth - the surface level.

    Many people think that the reason why it is cold is due to the pressure difference but I think this explains more why warm air from the surface cannot distribute it's warmth higher up - because as the pressure drops the temperature drops.

    Notice that the pressure has to drop for the temperature to change (temp drops because the air parcel loses energy expanding). If the pressure of gas stayed the same, then the temperature would not depend on it's absoute pressure but instead on the amount of energy it gains and loses due to various processes.

    In fact the adiabatic lapse rate which exlains the loss of energy as a parcel of air moves up tells us that the parcel is often even colder than the surrounding air. Similarly, air descending rapidly down a mountain often gains so much energy that it's temperature can be way higher than the air temperature at the bottom of the mountain was (e.g. Fohn winds).

    Does anybody disagree with this? Is my understanding lacking something?
     
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  3. spidergoat Valued Senior Member

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    Heat is relative to the density of particles bouncing off each other. Obviously, at higher altitudes, there are fewer air particles to interact with one another.
     
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  5. iceaura Valued Senior Member

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    You need to account for the fact that less barrier air does not allow the sun to heat up the mountain more, making the top hot.

    As you get even higher, for example, the air temp in sunlight rises dramatically rather than falling in the lowering pressure.

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  7. quadraphonics Bloodthirsty Barbarian Valued Senior Member

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    How does that not explain why it is cold?
     
  8. weed_eater_guy It ain't broke, don't fix it! Registered Senior Member

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    John, you're half-right and half-wrong I think. The earth's surface and the relatively low cloud layers of the atmosphere absorb alot of heat, but the fact is that it's true, temperature drops as air density and pressure drops. No matter the warmth of the air below, when it convects upward due to whatever weather pattern, it will drop from what it was at lower altitudes.

    Never to miss the opportunity to go editorial on something...

    So way back when "The Day After Tomorrow" came out, there was a part in the movie where they were trying to find why a mammoth would flash-freeze, and their explanation was that air was rushing down from the upper atmosphere so fast that "it didn't have time to warm up". I usually keep my mouth shut in a theater, but some instinct triggered in me where I blurted out "Bullshit!". It doesn't need TIME to warm up, what the hell?! It comes down, it's at a lower altitude, it's compressed, it warms up. It's an immediate thing! But whatever that director needs to justify people running away from the evil -150 degree wind. "We didn't listen!!!" Bullshit.... ugh...
     
  9. christa Frankly, I don't give a dam! Valued Senior Member

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    why was it so hot for the last week, then plummits to supper cold in 3days?
     
  10. John Connellan Valued Senior Member

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    It explains why it cannot warm up at the top of mountains. It doesn't explain why it is intrinsically cold. The reason as i said is lack of heating because air mocules don't heat directly very well from the suns rays.

    What I am trying to say is that I don't think there is a strong relationship between the pressure of air and its temperature
     
  11. John Connellan Valued Senior Member

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    And I agree. But I dont think that when 2 parcels of air are given the same heat energy, that the one with the lower pressure will necessarily be colder (uness its pressure is changing)

    Yes, that would probably be a very hot Fohn wind indeed

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  12. Nasor Valued Senior Member

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    Are you sure about that? The temperature of a gas only increases as pressure increases if volume stays constant. If the air's pressure is rising because more air is being crammed into a proportionally smaller area (the area near the surface), then you get isothermal compression and the temperature wouldn't necessarily change.

    Also, in the (admittedly fantastical) scenario of a bunch of upper-atmosphere air "flash freezing" and so falling down to the surface, the falling air would [/i]already[/i] be at the density of the lower air, so it wouldn't heat up as it sank. If the mass of cold air wasn't already above the density of the air it was falling through, it wouldn't be falling..
     
  13. Orleander OH JOY!!!! Valued Senior Member

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    why are the tops of the mountains so cold? because that's where the snow is?? :shrug:
     
  14. John Connellan Valued Senior Member

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    Very true Orleander. Many people dismiss the effect on surface air temperatures that overlying snow has. In fact, because the tops of very high mountains receive a huge amount more radiation than at the surface (50-80%), if you took the snow off the mountain top, there would be quite a rise in local temperature

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  15. James R Just this guy, you know? Staff Member

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    It's more the other way round - the snow is there because it's cold.
     
  16. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I have not read all the other replies, but you have it basicaly correct, except the part I made bold. That is normally false, because of one minor point you seem to have overlooked:

    When a mass of air rises and expands it adibatically cools, as you said; however, if it had moderate or high humidity, then some of the water vapor in that mass will condense, releasing 540 calories / gm of liquid water formed.

    This internal heating will keep that air mass WARMER that the surrounding air which is now only at the adibatic lapse temperature. Thus, the rising air "floats" higher sort of like a bubble, but with net upward force on it, it is CONSTANTLY accelerating. (Can toss an unfortunate airplane upward too.) This accelerating rate of rise makes for more adibatic cooling, more condensation of H2O, etc. I.e. a postive feed back system and quite often in summer time with moist air, this process can continue and drop the temperature of that rising mass of air below 0C* so ice forms and falls downward, but the upsteaming air may carry the ice back up some and the end result is called "hail" when the ice ball is too large to be repeated lifted up more often by the upward streaming air of a "thunder head cloud."

    Dry air would behave ALMOST exactly as you suggested.

    ---------------------
    * The air layer that mass of 0C of rising air is pushing up thur is probably at -35C or even colder. So 0C is WARMER than the surrounding air. Planes often fly in air that is colder than -35C, if memory serves me correctly. Some strong thunder heads have lost all their water by the time they are at 35,000 feet, but they got a lot of upward vertical speed earlier and can still toss the plane up, but I don't think this is what is called "clear air turbulence."
     
    Last edited by a moderator: Oct 25, 2009
  17. John99 Banned Banned

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    heat gets held onto surface objects longer.

    http://en.wikipedia.org/wiki/Thermomagnetic_convection
     
  18. Montec Registered Senior Member

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    Hello John Connellan

    The ability for air to increase and hold heat is related to the time of flight between colliding molecules and the rate per unit volume of said collisions. The longer the time of flight between collisions the greater the chance for CO2 and other "green house gases" to emit infrared radiation that was gathered by collision with a "hot" N2 or O2 molecule.

    Also when two "non-green house" molecules collide there exist a possibility for infrared radiation to be absorbed. "Green house gases" absorb infrared radiation and then transfer said heat energy to N2 and O2 molecules which makes the air warmer.

    Taking all the above into consideration will help explain why temperature varies with pressure altitude.

    As a side note adding more CO2 to the atmosphere will increase the rate at which the air heats up and cools down.

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  19. John Connellan Valued Senior Member

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  20. quadraphonics Bloodthirsty Barbarian Valued Senior Member

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    Not seeing a difference here. In my world, "no heating" = "intrinsically cold."

    It's changes in pressure that are at issue here, not just pressure as such, but supposing that's what you meant to say, you're clearly wrong. This is just ideal gas law stuff.
     
  21. John Connellan Valued Senior Member

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    There is a difference. As I said, many people think it's the lower pressure that directly makes the air cold at higher elevations. This is not true.

    Obviously. Reread the 3rd paragraph of my first post.
     
  22. quadraphonics Bloodthirsty Barbarian Valued Senior Member

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    It is the lower pressure, as opposed to simply "low pressure."
     
  23. John Connellan Valued Senior Member

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    Not even the lower pressure directly makes air cold. All it does is indirectly make the environment cold by not allowing convection of heat from the surface. The only direct cooling occurs when pressure is actively lowered (by adiabatic processes) and this does not occur much at the higher levels of the troposphere (ie. there is not much vertical air movement).

    However I always get the impression from talking to people that low ressured air always has to be lower compared to high pressure air. Obviously these same people do not think too much about the conservation of energy laws etc.

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    That's all I was trying to explain really.
     

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