# Colder with increasing altitude.

1. Apr 20, 2012

### chingel

I have read some threads on this topic, but I am still confused. Why does temperature drop as you go up a mountain? I have read that since pressure depends on the weight of the air on top of you and as you increase your altitude the amount of air over you decreases, therefore pressure decreases, gas expands and expansion makes its temperature drop. The rising hotter air gets continually cooled due to expansion (or would it start rising at all?). Gas loses internal energy when it has to expand against a force, i.e. do work. But isn't exactly the same amount of work the gas does received as internal energy of the lump of gas next to it that the work is being done upon?

2. Apr 20, 2012

### Pkruse

Air can only be warmed in two ways. The ground can conduct heat to it, or it can absorb light energy passing they it. Clean air and thin air absorb less heat than thick dirty air near the ground, and of course air at altitude can't get as much heat as air near the ground. Everything works to make air at altitude cooler.

3. Apr 20, 2012

### Pkruse

Also, clouds are a radiation heat shield to keep more heat on the ground. So above the clouds the air is also losing heat to the cold of space much more quickly.

4. Apr 20, 2012

### HallsofIvy

In addition to what pkruse said, gas expands as it contracts (and so gets colder) becauser the gravitational force on it decreases. There is no "work being done" on the "lump" of gas next to it.

5. Apr 20, 2012

### chingel

Could you elaborate on that? How does it get colder due to the decreasing gravitational force while expanding as it contracts? I don't understand that statement. Is it just that the molecules convert kinetic energy to potential energy while rising?

I thought the heat energy absorbed and radiated is negligible compared to the heat from the ground. Is the radiation of heat from the air above the clouds big enough to explain a significant portion of the temperature difference? Why wouldn't the hot air rise and warm the upper layers?

6. Apr 20, 2012

### Staff: Mentor

This part I can easily answer. It would expand and cool down on rising, so it will be not able to warm up the upper layers.

7. Apr 20, 2012

### sankalpmittal

I knew many people give the reason that air pressure decreases , and so it becomes less dense at higher altitudes and so its less heated. So its colder at higher altitudes.

When you go to a hill station , you feel a lot colder than plains. But do you feel problem in breathing ? No , because air density difference is negligible. Also you go to a hill station which is not at much higher altitude.

The reason is that sun radiates heat energy in shorter wavelength. It does not heat air through which it passes. Hence it heats the plains. Land re-radiates heat at longer wavelength which heats air and each consecutive lump of air gets heated till most of heat is dissipated as it reaches at lumps of air at hill station. Moreover one can include many other points of Pkruse. Hill station gets heated directly but is heated less much as you can see the case here. Same was the point of HallsofIvy though precise.

8. Apr 20, 2012

### Staff: Mentor

To some extent it depends on what you mean by a "hill station", but in general you are wrong. When you go up it is not only becoming colder, but also the pressure and the gas density go down. At around 2000 m pressure is already at 80% of the sea level and if you are not well trained you will already feel the difference in breathing during physical activity. Because of the low partial pressure of oxygen you may also get altitude sickness, especially over 2500 meters.

9. Apr 20, 2012

### chingel

But wouldn't the energy lost as it expands and loses temperature have go into the air around it, nevertheless increasing its temperature?

Gas loses heat when it does work, gains heat when work is done on it. Some gas expands and does work, other parts get work done on them. At least in my understanding, I didn't understand why the beforehand poster said no work is done on the lump of air.

10. Apr 20, 2012

### daveb

These seem rather poor and incomplete explanations since by the logic of most of these answers the air should always get colder as altitude increases. But at the stratosphere, the temperature increases as altitude increases due to the presence of ozone which absorbs UV light. Then in the mesosphere temperature again decreases with increasing altitude again, and then increases again in the thermosphere (due to low density of air). Wikipedia has a pretty good explanation. Though pkruse has the right idea when talking only about the troposphere.

11. Apr 21, 2012

### sankalpmittal

This might confuse the OP further though. He meant to ask "why colder with increasing altitude ?" His question can be phrased as - "Why its colder at mountains ?" or "Why its colder at higher elevations like hill stations ?"

The stratosphere lies between 10,000 m to 50,000 m. Jet planes fly so high. So his question remains limited to troposphere only.

In general , though you are cent percent correct.

Why ? I have gone to this hill station : http://www.mapsofindia.com/nainital/nainital-tourism.html

It was approximately 1939 m high. It was rather cold there compared to plains. But I did not feel any problem in breathing there. Also , I am just in high school and not in army so I am not trained at all.

Last edited: Apr 21, 2012
12. Apr 21, 2012

### Staff: Mentor

You are doing a classical mistake of taking your own partial experience and assuming that's how it works in general. It is called anecdotal evidence and carries no weight in general.

Just because you had no problem breathing doesn't mean your endurance was not compromised.

To know how it is you should take a group of people, put them to some kind of endurance/fitness test at sea level and at 2000 m, then judge the situation based on the test results. And such a research has been done many times in the past, performance goes down with the height/lower air pressure.

13. Apr 21, 2012

### chingel

Yeah I mean why does the temperature decrease in the stratosphere. So I understand that the major heat source is the earth and further up you are further away from the heat source. But still, why doesn't the air rise and warm the somewhat upper layers? Uneven heating of the ground can cause strong winds on the ground, why not between the hotter and colder layers of the stratosphere? If the air cools as it rises, it cools because it is giving heat to other air around it and then why couldn't we have such a circulation, air moving up, giving the heat away and going back down?

14. Apr 21, 2012

### DaveC426913

No, it does not cool by conduction, which is what you're describing. It cools because, the same amount of thermal energy is distributed throughout a larger volume. This is called adiabatic cooling.

If you put a gallon of air in an insulated container that had a piston, then operated the piston to increase the volume to 2 gallons, the air's temperature would drop. This is Charles' Gas Law.

V1 / T1 = V2 / T2

So, if V1 doubles to V2 then T1 will be halved to T2

15. Apr 21, 2012

### Staff: Mentor

No, it cools down because to expand it has to do work.

16. Apr 21, 2012

### chingel

But if on the other side of the piston you also had air whose volume would decrease by the same amount, wouldn't the energy lost one one side of the piston be transferred to the other side? And isn't it the same way in the case of expanding air, air does work on other air?

17. Apr 21, 2012

### DaveC426913

The energy is not lost, the 2 gallon volume of air contains the same amount of thermal energy, it is simply distributed throughout a larger volume.

There are lots of way you can complicate the system, true, but the simpler you make it, the easier it is to see the fundamental principles without obfuscation.

18. Apr 21, 2012

### chingel

How is the energy not lost if the temperature drops? Isn't temperature average kinetic energy per molecule, and the number of molecules surely stays the same.

19. Apr 23, 2012

### sankalpmittal

Energy is not lost. Its conserved. Though you can say that when temperature of a body drops , its total internal energy in transit , which it possesses , is reduced. But total energy in an isolated system is again conserved.

20. Apr 24, 2012

### haruspex

If a parcel of air expands a little, for whatever reason, it expands the total volume of the atmosphere that much. This means the average height of the molecules in it has increased. So work is done adding to the potential energy. This is quite different from a closed box, and the reason it does not warm another parcel of air.

I think a lot of the confusion over this topic arises because people think of this process as causing the air to be colder higher up. If you could stop the convection, by inserting baffles all the way up, the temperature gradient would be far steeper. The tendency of air to cool when lofted to a lower pressure altitude inhibits such movements, and they only occur when the temperature gradient is steep enough to overcome it. So the expansion doesn't cause it to be colder higher up, it just limits convection's ability to prevent its being so.
At the tropopause, the gradient is no longer steep enough and convection largely ceases.

21. Apr 25, 2012

### jartsa

A cube shaped block of air whose volume is one cubic kilometers expands with force 100 Giga Newtons. That is the force of air pressure on one side of the cube.

If the volume increases 20 %, the energy of expansion will be 200 meters * 100 Giga Newtons = 20 Tera Joules.

That's 20 Kilo Joules per one cubic meter, which has mass of about 1 kg, which cools about 20 degrees when 20 Kilo Joules of energy leaves it.

Now we want to know where the energy goes. Well it goes everywhere at speed of sound. Because the expansion causes a slight air pressure increase everywhere.

22. Apr 25, 2012

### haruspex

No. The atmosphere is not a closed container.
Average air pressure at Earth's surface must be total weight of atmosphere divided by area. Increasing the volume of some of the air does not increase its total mass. If anything, it will decrease total weight slightly because the average height of the air molecules increases.
The energy goes into the atmosphere's gravitational potential energy.

23. Apr 26, 2012

### jartsa

Sounds reasonable.

But the energy to lift the atmosphere travels around the atmosphere as pressure wave and at speed of sound.

24. Apr 29, 2012

### chingel

As some amount of air rises and expands, other air must take its place. Wouldn't the overall potential energy of the atmosphere stay the same because the average height of the air molecules stays the same, they just swap places?

25. Apr 29, 2012

### SHISHKABOB

Dave, if

$\frac{V_{1}}{T_{1}}=\frac{V_{2}}{T_{2}}$

and V2 = 2V1

then

$\frac{V_{1}}{T_{1}}=\frac{2V_{1}}{T_{2}}$

∴ T2 = 2T1

from which we end up with Charles's Gas Law, which states that, at constant pressure, the volume of an ideal gas is directly proportional to its temperature.

http://en.wikipedia.org/wiki/Charles's_law