[chris (OP)]

A quick glance at the "flight stats" now available via your "on board entertainment system" on the average passenger flight reveals that the external temperature at 35,000 feet is about -70°C. Why should it be so cold at this altitude? After all, in space if you're facing the sun the temperature can be +100°C and space suits are designed to insulate astronauts from heat as well as cold.

So why is it so cold outside my aeroplane, or at the top of Everest (29,000 feet) for that matter?

Chris

[Heliotrope]

*insert half-drunken diatribe regarding leading questions and the go-look-it-up-in-wikipedia yourself thing etc...*

The atmosphere cools down with altitude as one would expect due to the thinning of gasses and the reduction in pressure etc...
However, the outer reaches of the atmosphere do in fact increase in temperature quite dramatically as one exits the thicker parts.
As a point of perception, the temperature from a human perspective will just get colder.
However one will pass through regions of extremely hot but also extremely rarified gas. The temperature of this gas is very high as the atoms are all quite separated and therefore cannot give up their energy to one another and thus cool down so they stay hot.
A human isn't going to feel this.
All you'll get is much, much colder as you get higher.
Then when you're out of the protective envelope of the atmosphere you will enter the realm of the Sun.
Now your temperature is not dependent upon the thickness of the atmosphere. Largely because you're not in an atmosphere anymore.
It is purely dependent upon the amount of incident radiation you can absorb directly from the Sun.
Hence, satellites are hot in the sun but very cold in the shadow of the Earth.
Hence large temperature variations etc...

*I'm not at my best at the moment so any inaccuracies will have to be excused*

[syhprum]

I can understand the surface of the Earth being about 30° warmer than the black body temperature appropriate to our distance from the Sun due to the global warming effects of water vapour, CO2 and methane but why it drops of so rapidly at the modest altitude of 10Km where I believe the the pressure is about 25% of that at ground level and the chemical composition little different I find puzzling 

[ukmicky]

Hmmm
As you say as you get higher and the air gets thinner there is less air but dosent that also
mean their is less a medium to cool you down or for your body heat draw into and as your body is constaintly producing its own heat and as the suns rays are now hitting you with more intensity why dont you get hotter and hotter.

[neilep]

could it also be that at such a high altitude you are not receiving the benefit of cloud insulation also..or part thereof  ?

[syhprum]

I have been pondering on the problem of why stratosphere is about 30° colder than the black body temperature of the earth (-18°C).
I think it is because of the relative transparency of the upper layers that adsorb little energy from solar radiation while it is deprived of infra red radiation from the lower layers by the shielding effect of water vapour, methane and CO2   

[chris (OP)]

I agree. I think that there are a number of factors which keep us cosy at ground level, but keep the peak of Everest (29,000 ft) snow-capped.

At sea-level, each square metre of ground is heated by the Sun at the rate of 1kW. The ground soaks up this energy and then re-radiates it, keeping us warm.

But at altitude the air is very thin, so it has a low heat capacity, it is also very exposed, so there is significant wind-chill, and there is no or little "ground" to soak up solar energy and warm the surrounding air. In fact, because it's high there is very often snow and ice which reflects warm sunlight back out into space, making things even colder...

Chris

[daveshorts]

This is quite a complex subject:

An object will change temperature until the rate it is gaining or loosing heat is zero.
Objects that are not transparent will absorb light/radiation, and also emmit it.
The hotter an object is the more energy it will radiate and the shorter the wavelength.

Globally heat comes into the earth from the sun at various wavelengths, some of this heat is reflected, the rest is absorbed and then reradiated at a lower temperature. If you look at the earth from the outside it looks like it is at about -18°C (thanks syphrum).
[diagram=63_0]

So if the world on average is -18°C why is it a lot warmer than this at the surface?

The major reason for this is the greenhouse effect. Gases like carbon-dioxde are a lot more opaque to long wavelength infra-red light that the earth is radiating than the short wavelength visable light which the sun is giving out. So they reflect some of the IR back at us warming us up - essentially insulating us.
[diagram=64_0]

So the higher you go the less insulation there is between you and cold space, so a piece of ground will loose more heat from radiation at the same temperature, so it will get colder.

Mountain tops are also colder becasue they are often pointy, this means they have more suface to loose heat from for the same amount of sunlight heating them up:
[diagram=66_0]
So at the same height a mountain top will be colder than a flat piece of ground.

Albedo
This is a posh term for how reflective something is. If a piece of ground is white and therefore reflects 90% of the light that hist it back into space it will end up colder than something that is black which absorbs everything.

How can the stratosphere be colder than the earth as a whole?
Syphrum asked how some layers in the stratosphere can be globally colder than the earth as a whole which shouldn't be possible in this model, because the coldest should be the outside of the carbon-dioxide layer which has been measured to be about -18°C

I am not entirely sure but it could well be due to air movements:

If air expands, it gets colder this is because it is pushing on it's container which uses energy and the only energy a gas has is heat, so it must get colder.
So imagine a layer of gas that is pretty much transparent to UV and visible light sitting above a layer with some carbon-dioxide in it which is at about 18°C - it isn't going to gain or loose energy by radiation as it is transparent.

If some of the air in this layer goes upwards for some reason it will expand, and hence cool down
[diagram=67_0]
Because it is transparent it will stay cold, unless it moves back down again when it will warm up.