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Why there is always cold at high altitudes. e.g. at peak of mountains. Also as we go high from see level, temperature starts decreasing, so why is it.

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Variation of temperature with altitude is more complicated than your question states. –  RedGrittyBrick Oct 16 '12 at 20:39

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I'd like to add to the answers already given. Indeed, the atmosphere is transparent to shortwave radiation from the Sun, but absorbs a lot of the longwave radiation from the Earth; that's why we have the greenhouse effect, that's why the Earths has a liveable climate and that's part of the reason why we have the lapse rate we observe. But why is it colder at the Tibetan plateau, which is a large, flat area at roughly 4 km elevation? Aren't we equally close to the local surface there as when we are at sea level?

Let's assume the Tibetan plateau receives the same intensity of solar radiation as lower areas at the same latitude. In reality, it probably receives more due to the dry climate. Then it should heat up more, shouldn't it? But it doesn't. The system Earth-Atmosphere can be considered to be in a local Radiative-Convective Equilibrium (see the diagram from Kevin Trenberth below). This means that the energy flows "in" and "out" cancel out by energy transport due to radiation and convection. In other words: what goes in, must go out (this is not really true locally, because there are large-scale flow patterns known as wind). Now, the Earth surface emits radiation according to its temperature with $P = \epsilon \sigma T^4$. Some of this radiation is aborbed by greenhouse gases (or clouds) in the atmosphere: water vapour, carbon dioxide, methane, and others. Then the atmosphere heats up, and again radiates according to $P = \epsilon \sigma T^4$; part of this radiation goes into space, and part goes back to the surface. The greenhouse gases keep the surface of the Earth warm like a blanket.

Now at the Tibetan plateau, the atmosphere is much less dense, because the elevation is so high. Therefore, radiation emitted by the surface is not absorbed much, but mostly exits straight into space. This means that the surface cools down. To return to the blanket analogy: Tibet has a much thinner blanket than lower elevations do.

Now I have made a number of severe simplifications, because in reality it depensd on day/night, on clouds, on atmospheric flow such as wind, on humidity, and on other factors. But whereas the explanation given by others explains why it gets colder higher up in the free atmosphere, I think it doesn't really explain why it is colder at the Tibetan plateau.

enter image description here

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A sub-question: why is the blanket Thinner on the Tibetan plateau? Gravity? Wolfram Alpha says (bit.ly/VuX0qn) that the gravity on sea level is 9.83 vs. 9.818 on this plateau, not much difference there. –  duality_ Oct 27 '12 at 13:26
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@duality_ I think that deserves to be its own question, but in short: due to its fluid nature, the atmosphere is roughly homogeneously distributed over the geoid. Therefore, the higher above the geoid an observer is, the less atmosphere (less blanket) there is above the observer. The local gravitational acceleration is of secondary importance only. –  gerrit Oct 27 '12 at 13:30
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So it kind of spilled from the plateau down to the lowlands, like a liquid would? I think I understand now. –  duality_ Oct 27 '12 at 13:32
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@duality_ I think that analogy is correct. –  gerrit Oct 27 '12 at 13:33

Gravity pulls air down closer towards lower altitudes. For this reason the atmospheric pressure is higher at lower altitudes. This higher pressure further results in a higher temperature as first described by Gay-Lussac's Law (Pressure is Proportional to temperature).

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It's a consequence of Ideal gas law. For 'n' moles of gas, $PV=nRT$. As we go higher and higher, the atmospheric pressure decreases, causing the temperature to drop (since they're directly proportional...)

I got this one from Howstuffworks, which shows the variation of pressure with altitude in pounds per square inch. (PSI)

Variation

For more info, see Lapse Rate in Wiki. It also provides a good definition.

As an average, the International Civil Aviation Organization (ICAO) defines an international standard atmosphere (ISA) with a temperature lapse rate of 6.49 K(°C)/1,000 m (3.56 °F or 1.98 K(°C)/1,000 Ft) from sea level to 11 kilometres (36,000 ft). From 11 to 20 kilometres (36,000 to 66,000 ft), the constant temperature is −56.5 °C (−69.7 °F), which is the lowest assumed temperature in the ISA.

A natural question should've come by now, "Why does pressure decrease then..?"

Because the air molecules here at lower altitudes would experience the weight ($mg$) of the molecules above them thereby a net compressive force is produced. (seems a bit funny 'cause, how a molecule weighing some micro kg's would provide such a force?). Consider this way. They aren't weightless. They do have weight which is smaller. But, considering as a whole, the net weight is BIG. As altitude increases, the downward force also decreases due to the lower population of molecules.

Response to comment: Indeed, the number density ($n/V$) also influences temperature inversely (as suggested by @Prathyush) but its effect is quite a bit low, I think... Hence, neglecting such issues wouldn't be a problem as I'd say :-)

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What about the number density of molecules that decreases with height? –  Prathyush Oct 16 '12 at 15:07
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"Directly related" isn't very good for the description of pressure and temperature in the atmosphere. First, I would imagine that modeling the ideal gas as expanding adiabatically as it rises gives a slightly better estimate than isovolumetric. Also, there are some more complicated effects such as water vapor concentrations that matter quite a bit, as I remember. –  Mark Eichenlaub Oct 16 '12 at 15:20
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No I am taking about n/V. Which is inversely related to temperature. Perhaps It is not a significant effect. ps: A molecule weights much less then nano Kgs, remember 1 Gram weight = 10^23 molecules. We would be crushed to death if each molecule weighted nano Kilos. –  Prathyush Oct 16 '12 at 15:27
    
@MarkEichenlaub: Hello Mark. First of all, Thanks for the info... I mentioned "Directly related" inorder to indicate $P\alpha V$ (now revised). But, I think this itself is a better suite to the question rather than including complex tasks. If you've seen any issues in my answer, comments would be helpful.. :) –  Waffle's Crazy Peanut Oct 16 '12 at 15:27
    
It would be useful to see if its significant. Even a one percentage change will correspond to a couple of degrees change in Temperature. –  Prathyush Oct 16 '12 at 15:49

Crazy Buddy is quite correct that it's because gas expands and cools as it rises, but there is more to it than that.

For something to be be heated it must either absorb EM radiation, or it must be heated by some hot object it's in contact with. Air doesn't absorb light so it can't be directly heated by sunlight. Instead the sunlight passes through the air and heats the ground, and the ground heats the air.

The expansion comes in because the hot air that is heated by the ground rises. However as it rises it's volume increases and therefore it's temperature decreases. So the decrease in temperature with height is indeed due to expansion, but this is only the case because air is heated from below by the ground.

If air absorbed light directly it would heat up independantly of the ground and we would not see the same temperature variation with height. In fact exactly this effect happens in the stratosphere. In the upper reaches of the stratosphere ozone molecules absorb ultraviolet light and heat up, and in the stratosphere temperature increases with height instead of decreasing.

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+1 for mentioning the primary source of heat and Ozone in the Stratosphere. Certainly if there were no Sun we'd still have a temperature gradient due to changes in pressure, but that's not the biggest factor on Earth. –  ghoppe Oct 16 '12 at 18:30
    
In places of high altitude too we have ground to heat up the air then why its still cold. It is ( as @CrazyBuddy pointed out) just because of low pressure ? –  user10001 Oct 16 '12 at 19:17
    
@John Rennie : thank you very much for your answer. –  android developer Oct 17 '12 at 6:54
    
@John Rennie: Why then the temperature of different places can not become the same because they are in contact and can exchange heat? –  richard Nov 19 '13 at 14:56
    
@richard: do you mean different heights, or different areas e.g. different countries? –  John Rennie Nov 19 '13 at 15:21

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