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Temperature of air

  1. Jun 9, 2012 #1
    We all know that denser substance sink,while less dense substance float on the denser substance.

    Thus,air with lower density should float on air with higher density.
    That means hot air at higher altitude,cold air at lower altitude.

    In fact,temperature decreases with increasing altitude,why?
     
  2. jcsd
  3. Jun 9, 2012 #2

    D H

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    First off, warm air does rise. Have you ever seen hawks or vultures gliding (no wing flaps) circularly, but spiraling upwards as they circle? Those birds have found a thermal and are using the rising air in that thermal to gain altitude.

    That column of air is however cooling as it rises. The reason is that density decreases with altitude. Imagine a packet of air in that rising column. The packet has some amount of internal thermal energy. It doesn't transfer much of that thermal energy with the surrounding environment because air is a pretty lousy conductor of heat. The rising of that packet of air is close to an adiabatic process. The packet expands as it rises due to the decrease in density with altitude. The packet is performing work on the surrounding air because pΔV > 0. Per the first law of thermodynamics, dU=δQ-δW, the packet must lose internal energy because δQ≈0 and δW>0. Temperature is a measure of internal energy. The temperature drops as the packet rises.

    Another name for this is the adiabatic lapse rate. Google that phrase for more info.

    A deeper reason is that the Earth's atmosphere is heated from below but cooled from above. Except for the ozone layer and clouds, the Earth's atmosphere is close to transparent to the bulk of the incoming solar radiation. The incoming solar radiation mostly heats the ground and oceans. The portion of the atmosphere in contact with the surface is heated by conduction. In short, the atmosphere is heated from below. Except for the Sun, which is quite small compared to the whole open sky, the atmosphere is "looking" upwards at empty space, which has a temperature of 2.73 kelvin thanks to the cosmic microwave background radiation. That's downright cold. The atmosphere loses heat to empty space by radiation. In short, the atmosphere is cooled from above. This heating from below / cooling from above is one of the things that drives the adiabatic lapse rate.

    An even deeper reason is greenhouse gases. Suppose there were no greenhouse gases in the atmosphere. There would be a thin layer near the surface that had an adiabatic lapse rate, but above this the atmosphere would be more or less isothermal. The second law of thermodynamics would dominate over the first. An isothermal atmosphere is the condition that maximizes entropy. The greenhouse effect, coupled with the heating from below / cooling from above, prevents the atmosphere from reaching this equilibrium state. Usually. Sometimes temperature inversions do set up, and when that happens pollution is trapped near the surface. Rising air stops rising at the inversion layer. These temperature inversions can be quite problematic in coastal California, Salt Lake City, and Denver (just to pick a few).

    There is a natural temperature inversion at the tropopause, the boundary between the troposphere and the stratosphere. I briefly mentioned the ozone layer above. The ozone layer absorbs ultraviolet light from the Sun, and thus violates the heated from below / cooled from above paradigm that dominates in the troposphere. Thunderstorms do occasionally punch through the tropopause, but that's an exception rather than the rule. Rising air general stops rising at or before the tropopause.
     
  4. Jun 9, 2012 #3
    Temperature usually decreases with increasing altitude for reasons DH explained above but temperature can also increase with increasing altitude and such an inversion is very stable:

    http://en.wikipedia.org/wiki/Inversion_(meteorology [Broken])
     
    Last edited by a moderator: May 6, 2017
  5. Jun 9, 2012 #4
    Correction, temperature decreases with increasing atmosphere only in the troposphere. The troposphere is a layer of atmosphere that extends from the ground to the point at which the temperature does not change with height. The temperature of the troposphere decreases with height.
    Most sunlight passes through the atmosphere without absorption. Very little sunlight directly heats the atmosphere.
    Most of the sunlight that hits the ground is absorbed and turned into the internal energy of the ground. Therefore, sunlight directly heats the ground. Therefore, the ground is usually hotter than the atmosphere immediately above it.
    The ground has a higher temperature than the troposphere. Thermal convection carries heat energy from the ground into the troposphere because of buoyancy, which you just mentioned.
    There are some caveats to this explanation.
    The troposphere does not extend forever. The point at which the temperature doesn't change with height is called the tropopause. Above this is the stratosphere. The stratosphere is defined as that layer of atmosphere where there is a locally defined temperature that increases with height. The stratosphere is being warmed more by the direct sunlight than the ground.
    The troposphere varies in thickness all over the earth. The thickness of the troposphere at the equator is many miles. This is because the sunlight is hitting the ground directly. However, there is no troposphere at the poles. The sunlight "misses" the ground at the poles. At the poles, sunlight directly heating the atmosphere. So at the poles, the temperature increases from the ground up. At the poles, the stratosphere starts at the ground.
    Even near the equator, there are certain weather conditions where the troposphere effectively disappears. There is a condition called the temperature inversion. The temperature inversion is the condition where the temperature of the ground is lower than the temperature of the atmosphere just above it.
    The rule that you just stated is fully reliable at the equator during the day. At the equator during the day, the temperature decreases with height for a couple of miles upward. However, that rule becomes less reliable at night as one approaches the poles.
     
  6. Jun 10, 2012 #5
    Correction, temperature decreases with increasing altitude in the troposphere and in the mesosphere.
     
  7. Aug 5, 2012 #6
    what is the height of the stratoshere from sea level
     
  8. Sep 28, 2012 #7
    Acknowledging the usual habit of this forum to explain everything and how it works, would you please find some literature that documents the physics of lapse rate?
     
  9. Sep 28, 2012 #8

    davenn

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    Hi gryllida
    welcome to PF

    google is a wonderful thing, type your question in there .... the physics of lapse rates
    and there's a mass of links to good information on the subject :smile:

    cheers
    Dave
     
  10. Sep 28, 2012 #9
    Thanks for the welcome. I tried web search and landed on a huge number of blog posts and somesuch but what I'm looking for is a *book* that I can actually refer to in my research, and that dedicates a few pages to a description of physics behind the lapse rate process, or at least some substantial amount of attention to the discussion of the physics behind it rather than just equations.
     
  11. Sep 28, 2012 #10
    I see a lot of words and I am sure they are all extremely complete answers.

    Bottom line. There is less air pressure higher up--fewer molecules per unit volume. Fewer molecules are hitting your thermometer per unit time even if they are individually hitting with the same force they did at a lower altitude.
     
  12. Sep 28, 2012 #11

    cjl

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    But if each molecule hit with the same "force" (I'll assume that means the same kinetic energy), the temperature would be the same. Temperature is defined as the mean kinetic energy per particle. For the temperature to drop, each molecule must have less energy - simply having fewer molecules does not guarantee a lower temperature.
     
  13. Sep 28, 2012 #12
    but the ones that do hit you give you the energy you measure (feel) the same one won't give you the same energy next time unless it steals it from another one, which is not as likely to happen. Also between the time you measure one of those molecules and another you have radiated some of your temperature away yourself.
     
  14. Sep 28, 2012 #13

    D H

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    That has nothing to do with temperature. You are talking about energy per unit volume. Temperature is closer energy per mole. Think of it this way: The intergalactic medium can reach temperatures of 100 million kelvin or more. Suppose you were in an uninsulated spacesuit in this medium. Would you vaporize? No. You would freeze. That intergalactic medium is extremely tenuous.
     
  15. Sep 28, 2012 #14
    Please note that the temperature decreases with height only in the troposphere. The temperature of the other layers of atmosphere does not decrease with height. For instance, the temperature of the stratosphere increases with height.
    There are two reasons that the temperature of the troposphere decreases with height. The first reason is that a parcel of air that is already in the upper troposphere loses internal energy by radiating heat energy to outer space. The second reason is that a parcel of air that rises in the troposphere loses internal energy by doing work on the ambient air. As the air parcel rises, it expands due to the drop in pressure. As it expands, it does work on the ambient air.
    The temperature of a parcel of air decreases as the internal energy of a parcel of air decreases. From a thermodynamic standpoint, it is interesting that the internal energy of the parcel decreases two ways: by releasing heat and by doing work. In the troposphere, both forms of energy transfer are going in the same direction.
    Wikipedia gives a good description of both mechanisms that cool the air in the troposphere. Here is both a link and a quote from this source. I found it merely by Googling “lapse rate”. However, please note that the “lapse rate” is the second reason given for the decrease in temperature in the troposphere. The first reason given is the heat radiated into outer space by a parcel of air near the top of the troposphere.

    http://en.wikipedia.org/wiki/Troposphere
    “The temperature of the troposphere generally decreases as altitude increases. The rate at which the temperature decreases, , is called the environmental lapse rate (ELR). The ELR is nothing more than the difference in temperature between the surface and the tropopause divided by the height. The reason for this temperature difference is the absorption of the sun's energy occurs at the ground which heats the lower levels of the atmosphere, and the radiation of heat occurs at the top of the atmosphere cooling the earth, this process maintaining the overall heat balance of the earth.
    As parcels of air in the atmosphere rise and fall, they also undergo changes in temperature for reasons described below. The rate of change of the temperature in the parcel may be less than or more than the ELR. When a parcel of air rises, it expands, because the pressure is lower at higher altitudes. As the air parcel expands, it pushes on the air around it, doing work; but generally it does not gain heat in exchange from its environment, because its thermal conductivity is low (such a process is called adiabatic). Since the parcel does work and gains no heat, it loses energy, and so its temperature decreases. (The reverse, of course, will be true for a sinking parcel of air.)”
     
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