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Thread: Relation between temperature and air pressure

  1. #1
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    Relation between temperature and air pressure

    Hi, I have doubt about the relation between air pressure and temperature. Let's say Earth had an atmosphere 10 times the air pressure with exact the same composition and proportion of gases. This means the temperature would be 10 times higher too? Or the temperature would only be doubled? For example, the temperature of Earth is around 15C with a greenhouse of around 30C. In this case, would the temperature be 30C and the greenhouse 60C? Or the value would be much higher instead? I'm making this questions because of a book i'm writing, about planets around another star and some of them are superearths.

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    Id suggest asking mods (use the report button on your post) for this thread to be moved to the Q&A forum, as more people look at that than this section. Though Im not sure youll get a detailed answer. My guess is that you would need to do some fairly deep analysis to determine the temperature of a thicker atmosphere with the same composition as Earths current atmosphere. I expect it would end up warmer, and temperature wouldnt vary as much by location due to the larger thermal mass and easier heat transport, but composition would probably change pretty rapidly. A lot more oxygen, for instance, would lead to most burnable things going up in flames. A lot more water could be held by such an atmosphere and boiling temperature would shift too.

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  3. #3
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    I think you are mixing different ideas. A fixed mass of gas with no change of heat across its boundaries gets hotter if you change its pressure. Like in an engine cylinder but the walls are insulators. Compression raises the temperature and expansion reduces it. If you pump up bicycle tyres, the pump gets hot. Our atmosphere is warmed mostly by contact with the land and oceans which absorb the sun radiation. So the low atmosphere has a temperature and that value falls as you go up. The upper atmosphere is very thin but there radiation is absorbed, so there are complexities.

    The pressure at the surface is literally the weight of the air above, the weight being due to gravity. If there were more mass in the atmosphere the pressure would be higher. The temperature in that thicker atmosphere would depend on how much radiation is absorbed and how much reaches the ground as before.

    If you pumped up a bicycle tyre in that thicker atmosphere, the pump would get hot, just as before.

    The actual value of temperature in the thicker atmosphere would be caused by the balance of radiation received versus reradiated, just as it is with our atmosphere where the lower temperature of earth radiation, vs the sun, means heat is retained by water vapour and CO2 etc. Plus clouds.
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    Quote Originally Posted by enioguedes View Post
    Hi, I have doubt about the relation between air pressure and temperature.
    Are you referring to Gay Lussac's Law? P1/T1 = P2/T2

    If so, that law only applies for a constant volume. The atmosphere of a planet is not constrained to a constant volume. So if the atmospheric pressure was increased 10-fold, it does not mean the temperature would increase 10-fold.
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    Quote Originally Posted by Swift View Post
    Are you referring to Gay Lussac's Law? P1/T1 = P2/T2

    If so, that law only applies for a constant volume. The atmosphere of a planet is not constrained to a constant volume. So if the atmospheric pressure was increased 10-fold, it does not mean the temperature would increase 10-fold.
    And adiabatic conditions, no heat lost or gained.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  6. #6
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    I will give some examples to make the reason of this post more clear, maybe I can have a better answer, I hope... Ok, I use Space Engine.

    Magnus System: M3V star (it's mass is in the limit for it being or not a red giant one day), surface temperature is 3470K. Peak radiation is around 900 nm (so more near infrared than visible light here).

    Planet Teran: 2.03 Me; 15682 Km; 6g/cm; 1,34 g; 0,1 AU; insolation same as Earth's, translation lasts 24 days; rotation lasts 60 hours; axial tilt is 15. It's atmosphere is 7 atm in pressure, having 85% N2, 12% O2, 1% CO2, 0,6% H2O, 0,3 Ar. Traces of Ne, H2S and CH4. The proportion of gases in atm are: N2 is 6 atm, O2 is 0,84 atm, CO2 is 0,07 atm, H2O is 0,04 atm. It's albedo is 0.4, due to the thicker air and humidity causing more formation of clouds. The greenhouse is 64C, having an average temperature of 35C. Hydrosphere is made of water. It also has a moon with 3 times the mass of luna. Obs: It has around 4 times more oxygen than Earth does but in compensation a lot more N2 to contrast. So... bigger brains and arthropods, I guess?

    I just wanted to know if this is correct to my project. There are other planets similar to Teran in the same system, so based on the answer I could estimate the greenhouse effect based on pressure alone for those.

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    Quote Originally Posted by Swift View Post
    Are you referring to Gay Lussac's Law? P1/T1 = P2/T2

    If so, that law only applies for a constant volume. The atmosphere of a planet is not constrained to a constant volume. So if the atmospheric pressure was increased 10-fold, it does not mean the temperature would increase 10-fold.
    Yup. There's no lid on a planet's gas envelope. It can and does "puff up" and expand further out into space when heated or pressurized.
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