# Why does the temperature decreases with increase in altitude

1)what is entropy ?
2)why does the temperature decreases with increase in altitude

Related Other Physics Topics News on Phys.org
Clausius2
Gold Member
1) Once my physics teacher told me the entropy is like God, some people believes in God but other do not believe in it. The entropy has a variety of definitions (depending on probabilistic or macroscopic-thermodynamic meaning). The definition I'm going to give you is just that given by the 2nd principle:

Entropy (of a closed system): a thermodynamic variable which suffers an increasing in an adiabatic process.

You can imagine a lot of definitions, all of that based on the thermodynamics equations.

2) Usually, the density decreases with temperature: $$\rho=\frac{P}{R_g T}$$. So that, lighter air masses are in higher atmospheric layers. But this is not always so. By contrast, the temperature usually decreases with height in the Troposphere, but remains constant during some height above the Tropopause (11Km--> T=-50ºC). In the Stratosphere the temperature is increased untill the Stratopause (50Km--->T=+10ºC). Above of here, the temperature suffers increasings and decreasings, for instance reaching 1000ºC in the Termosphere (400Km).

Why the temperature does not decrease always with the height? Well, there are some physical criterions which enhance the stability of a hotter mass of air above another cooler. These stability criterions are derived from the Hidrostatic's equations. But you can imagine it like a balloon filled with hotter air crashing into the roof of your room that avoids its elevation.

Last edited:
why does that temperature decrease with height commonly

lakshmi,
Your first question, Entropy is called as the degree of randomness, randomness means the fredom. matter always opt to be more random hence it is used to define the state of matter.
2. fall in temprature with height is due to the Charles law, it states that with change in pressure or volume or both temprature also changes (this is converse of actual law). Hence with increse in height both pressure and density of the air decreases which results in low temprature.

Clausius2
Gold Member
lakshmi said:
why does that temperature decrease with height commonly
I've never seen such impolite person. I've just explained that to you just before, and you answer with such bad manners.

LURCH
I don't think he was being rude, he simply re-phrased his question. He asked why temp gets colder at higher altitude, and you answered that it doesn't allways do so, so he asked why does temp usually gets colder at higher altitude, thus making the question more precise.

Lakshmi, if it helps at all, you can think of entropy as the movement of energy from places where there's more to places where there's less. It's the reason a hot cup of coffee cools down in a normal-temperature room, and warms up the room a little bit while doing so. The cup gets cooler, and the room gets warmer, untill the two are the same temperature, and then the process stops. Of course, entropy itself never stops, just the flow of heat energy from the cup to the room. The room is still radiating heat out to the rest of the universe.

Keeping in mind that matter is just energy in its "condensed" form, you acn see that this principle applies to absolutely every physical phenomenon. Stars lose mass by converting matter to energy in a fusion reaction, this energy is radiated otu into space, slightly warming the temperature of the space between the stars, while taking energy away from the stars themselves. It's all a moving of energy from where there's a lot to where there's not.

Clausius2
Gold Member
LURCH said:
I don't think he was being rude, he simply re-phrased his question. He asked why temp gets colder at higher altitude, and you answered that it doesn't allways do so, so he asked why does temp usually gets colder at higher altitude, thus making the question more precise.

Lakshmi, if it helps at all, you can think of entropy as the movement of energy from places where there's more to places where there's less. It's the reason a hot cup of coffee cools down in a normal-temperature room, and warms up the room a little bit while doing so. The cup gets cooler, and the room gets warmer, untill the two are the same temperature, and then the process stops. Of course, entropy itself never stops, just the flow of heat energy from the cup to the room. The room is still radiating heat out to the rest of the universe.

Keeping in mind that matter is just energy in its "condensed" form, you acn see that this principle applies to absolutely every physical phenomenon. Stars lose mass by converting matter to energy in a fusion reaction, this energy is radiated otu into space, slightly warming the temperature of the space between the stars, while taking energy away from the stars themselves. It's all a moving of energy from where there's a lot to where there's not.
I'm desiring that Lakshmi write to you something as an acknowdegement more than re-phrasing again his question. Let's see....

i am soory clausius but i didnt get satisfactory answer from you :shy:

Clausius2
Gold Member
Cheers! He spoke.

And what do you think this forum is? It is of a well-given-birth person to acknowledge any response, in spite is statisfactory for you or not. This is not a supermarket where you buy responses.

Ended. You don't need to be worried. Doesn't matters.

pervect
Staff Emeritus
lakshmi said:
2)why does the temperature decreases with increase in altitude
http://farside.ph.utexas.edu/teaching/sm1/lectures/node55.html

does a pretty good job of explaining the "adiabatic atmosphere". This sort of atmosphere gives a decreasing temperature with height.

Note that adiabatic means "no heat transfer", so the adiabatic gas law is the pressure volume relationship that is obtained when a gas is thermally isolated from it's surroundings.

Imagine a packet of air which is being swirled around in the atmosphere. We would expect it to always remain at the same pressure as its surroundings, otherwise it would be mechanically unstable. It is also plausible that the packet moves around too quickly to effectively exchange heat with its surroundings, since air is very a poor heat conductor, and heat flow is consequently quite a slow process. So, to a first approximation, the air in the packet is adiabatic. In a steady-state atmosphere, we expect that as the packet moves upwards, expands due to the reduced pressure, and cools adiabatically, its temperature always remains the same as that of its immediate surroundings. This means that we can use the adiabatic gas law to characterize the cooling of the atmosphere with increasing altitude. In this particular case, the most useful manifestation of the adiabatic law is

$$p^{1-\gamma} T^\gamma = constant$$
I'm afraid I don't quite understand which of these assumptions fails at large altitudes - if you read the earlier section of the URL that I didn't quote, it's probably either the "air is transparent" or the "air is thoroughly mixed" assumption.