Colder with increasing altitude.

In summary, according to this discussion, the temperature difference between higher altitudes and lower altitudes is due to a number of factors, including decreasing air pressure. Additionally, the increasing distance from the sun affects the temperature of air, as does the presence of clouds.
  • #1
chingel
307
23
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?
 
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  • #2
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
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
chingel said:
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?
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
HallsofIvy said:
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.

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
chingel said:
Why wouldn't the hot air rise and warm the upper layers?

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
chingel said:
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?

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.

Further reading : http://www.usatoday.com/weather/resources/askjack/2004-06-23-cold-upper-atmosphere_x.htm
 
  • #8
sankalpmittal said:
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.

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
Borek said:
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.

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
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
daveb said:
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.

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.

Borek said:
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.

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.:rolleyes:
 
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  • #12
sankalpmittal said:
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.:rolleyes:

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
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
chingel said:
If the air cools as it rises, it cools because it is giving heat to other air around it
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
chingel said:
If the air cools as it rises, it cools because it is giving heat to other air around i

No, it cools down because to expand it has to do work.
 
  • #16
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
chingel said:
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

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
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
chingel said:
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.

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
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
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
jartsa said:
the expansion causes a slight air pressure increase everywhere.

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
haruspex said:
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.

Sounds reasonable.

But the energy to lift the atmosphere travels around the atmosphere as pressure wave and at speed of sound.
 
  • #24
haruspex said:
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.

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
DaveC426913 said:
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

Dave, if

[itex]\frac{V_{1}}{T_{1}}=\frac{V_{2}}{T_{2}}[/itex]

and V2 = 2V1

then

[itex]\frac{V_{1}}{T_{1}}=\frac{2V_{1}}{T_{2}}[/itex]

∴ 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
 
  • #26
chingel said:
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?

Of course, but I was just looking at the immediate consequence of a parcel of air expanding in situ, e.g. by being warmed. The question was, where has the energy gone? The suggestion had been that it had gone into compressing the air around it. Readjustment by convection comes later.
 
  • #27
SHISHKABOB said:
Dave, if

[itex]\frac{V_{1}}{T_{1}}=\frac{V_{2}}{T_{2}}[/itex]

and V2 = 2V1

then

[itex]\frac{V_{1}}{T_{1}}=\frac{2V_{1}}{T_{2}}[/itex]

∴ 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
Well spotted. DaveC's description clearly does not meet the constant pressure requirement of Charles' Law. To meet that, heat would need to be added to maintain the pressure, and at a doubled volume the temperature would also double.
 
  • #28
haruspex said:
Of course, but I was just looking at the immediate consequence of a parcel of air expanding in situ, e.g. by being warmed. The question was, where has the energy gone? The suggestion had been that it had gone into compressing the air around it. Readjustment by convection comes later.

OK, but once the air is already warmed, what stops it from rising and warming the upper layers. If it rises and cools, mustn't the energy go to the other air around it and if it does, doesn't it succed in warming the upper layers (ie give off its energy at a higher altitude as heat to the air around it)? Why aren't the upper layers then warmed by rising hot air?
 
  • #29
chingel said:
OK, but once the air is already warmed, what stops it from rising and warming the upper layers. If it rises and cools, mustn't the energy go to the other air around it and if it does, doesn't it succed in warming the upper layers (ie give off its energy at a higher altitude as heat to the air around it)? Why aren't the upper layers then warmed by rising hot air?

Having expanded due to being heated, it rises (convection) because it is less dense than the air around it. As it rises, it encounters a slightly lower pressure, so expands a little more. This further expansion does more work in the same way as before, cooling the air adiabatically. There will also be some mixing, but in a large parcel of air that's a second order effect.
This rising-expanding-getting dense cycle has diminishing returns, so settles out when the air is at the same temperature as surrounding air. (This is for dry air - moist air is more complicated.)
At the same time, some air must descend to occupy the space vacated. That air gets compressed adiabatically, warming.
Looking at the total picture, the air as a whole is a little warmer than before (we added heat to kick this off), so is a little less dense, so the atmosphere extends a little further into space and has acquired a little extra potential energy. Thus the heat energy lost by the ascending air has not quite all gone into heating the descending air.
 
  • #30
chingel said:
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?

Yes, but it is Mechanical Energy (Work) that is transferred, not thermal energy. What happens inside the cylinder doesn't depend on what the work happens to act on.
 
  • #31
SHISHKABOB said:
Dave, if

[itex]\frac{V_{1}}{T_{1}}=\frac{V_{2}}{T_{2}}[/itex]

and V2 = 2V1

then

[itex]\frac{V_{1}}{T_{1}}=\frac{2V_{1}}{T_{2}}[/itex]

∴ 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
Adiabatic cooling (aka, adiabatic expansion) does not happen at constant pressure.

What you are describing is what happens at the surface: it is heated and expands at constant pressure. That's what causes it to rise.
 
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  • #32
haruspex said:
Well spotted. DaveC's description clearly does not meet the constant pressure requirement of Charles' Law. To meet that, heat would need to be added to maintain the pressure, and at a doubled volume the temperature would also double.
Which is why that's not what happens. Again, Dave said adiabatic cooling.

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter6/adiab_cool.html
 
  • #33
russ_watters said:
Adiabatic cooling does not happen at constant pressure.

well, right, I was just pointing out the error in terminology

I was *only* pointing out that Charle's Gas Law, which Dave mentioned, requires constant pressure. It was offtopic, I guess, technically, because I wasn't trying to add anything to the discussion of why it gets colder with increasing altitude.
 
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  • #34
Yes, you're right, sorry. Dave used the right term for what was happening but applied the wrong equation. That's what I get for only reading half a post. :redface:
 
  • #35
chingel said:
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?

This thread seems to have lost its way a bit (so what's new?)
I quote the original question and the simplest answer must explain the steady state, static condition, not involving convection / mixing / adiabatic cooling / heating by the ground etc..
Consider a simple column of a gas air, in equilibrium in an insulated cylinder. In an equilibrium situation the air at the bottom of the column will not rise if it is MORE DENSE than the air above it. This can occur even when it is 'warmer', as long as the pressure is greater. So a temperature / pressure profile can occur which will not allow convection but where the temperature at the bottom is higher than the temperature at the top.
The atmospheric pressure is approximately halved for every 5km increase in height (for constant temperature). Applying
P1V1/T1 = P2V2/T2
to this simple model of a column seems to indicate that a mass of air at sea level at a temperature 300K will occupy a smaller volume than the same mass of air at 5km height (half the pressure), if the temperature at 5km is greater than 150K. (Someone else please check this)
This result is extreme and not realistic but it makes the point, I think. There are many other factors at work but it does put to bed the notion that the Hot Air must rise up through the Cold Air.
 
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<h2>1. Why does it get colder with increasing altitude?</h2><p>As you go higher in the atmosphere, the air becomes less dense. This means that there are fewer molecules to trap and retain heat. As a result, the temperature decreases with increasing altitude.</p><h2>2. How much colder does it get with increasing altitude?</h2><p>The rate at which the temperature decreases with increasing altitude is known as the lapse rate. On average, the temperature decreases by about 3.5 degrees Fahrenheit per 1000 feet of altitude.</p><h2>3. Does the temperature always decrease with increasing altitude?</h2><p>No, there are some exceptions to this general trend. For example, in the stratosphere, the temperature actually increases with altitude due to the presence of ozone, which absorbs UV radiation and warms the air.</p><h2>4. How does the temperature change with altitude in different regions of the world?</h2><p>The temperature change with altitude can vary depending on factors such as latitude, topography, and weather patterns. In general, colder regions tend to have a steeper lapse rate, meaning the temperature decreases more rapidly with altitude.</p><h2>5. Can the temperature at high altitudes be accurately predicted?</h2><p>While there are mathematical models that can estimate the temperature at different altitudes, it is difficult to accurately predict the exact temperature at a specific altitude due to the many variables that can influence it, such as local weather conditions and atmospheric composition.</p>

1. Why does it get colder with increasing altitude?

As you go higher in the atmosphere, the air becomes less dense. This means that there are fewer molecules to trap and retain heat. As a result, the temperature decreases with increasing altitude.

2. How much colder does it get with increasing altitude?

The rate at which the temperature decreases with increasing altitude is known as the lapse rate. On average, the temperature decreases by about 3.5 degrees Fahrenheit per 1000 feet of altitude.

3. Does the temperature always decrease with increasing altitude?

No, there are some exceptions to this general trend. For example, in the stratosphere, the temperature actually increases with altitude due to the presence of ozone, which absorbs UV radiation and warms the air.

4. How does the temperature change with altitude in different regions of the world?

The temperature change with altitude can vary depending on factors such as latitude, topography, and weather patterns. In general, colder regions tend to have a steeper lapse rate, meaning the temperature decreases more rapidly with altitude.

5. Can the temperature at high altitudes be accurately predicted?

While there are mathematical models that can estimate the temperature at different altitudes, it is difficult to accurately predict the exact temperature at a specific altitude due to the many variables that can influence it, such as local weather conditions and atmospheric composition.

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