Naked Science Forum
Life Sciences => The Environment => Topic started by: freecw on 02/06/2011 20:41:11
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I just finished listening to the podcast about metallergy and they said that the temperature decreases as altitude increases. I was just wondering what caused this? With solar radiation i thought it would be the opposite, as the distance with from the sun increases, the temperature should decrease.
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With solar radiation it would be that way -- you are quite right. BUT the lower atmosphere does not absorb the light that the upper atmosphere lets through. Instead, the sunlight heats up the ground and the oceans, which, in turn, heat the atmosphere from below.
The temperature of the atmosphere follows quite a complicated pattern -- it only gets colder up to about 15 km altitude -- then it starts to warm as you get higher. The reason is that that is where the ozone is, and ozone does absorb the UV light from the sun -- much of it, anyway -- and in doing so it warms the surrounding air. At 50 km altitude, the atmosphere is only about 10 deg C cooler than at ground level. From 50 to 85 km altitude, the atmosphere gets cooler again, because there is not much ozone above 50 km.
Above 90 km the atmosphere gets an increasingly high temperature as you go higher, because it is busy absorbing the very high energy and dangerous radiation from the sun. At hundreds of km altitude, the air temp is thousands of degrees! But although the "temperature" is high, the actual heat content is low, because the air up there is so extremely thin. Atoms or molecules in the uppermost atmosphere will hit you very hard (high temperature) but not very often (low heat content).
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Thanks for the great explanation.
As far as the lower atmosphere, I believe it is a bit more complicated than radiated heat.
If it was only radiated heat, then one would expect the high plains areas such as Colorado to be equally hot as the low lying areas, say down in the Mississippi Valley. Likewise, the high elevation ice cap in Greenland wouldn't necessarily cause more cooling than the lower elevation land nearby in Canada.
Perhaps part of it is also wind convection and air density. If a wind blows from low elevation Oregon/California up to Colorado (ignoring mountains), the air density near the surface necessarily decreases, at least at the surface. And, due to the ideal gas law, it would necessarily cool the air. As it drops back down to the Mississippi valley, the density near the surface would increase again and it would get warmer.
Although, perhaps this difference is more noticeable in the winter when there is less solar radiation.
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Yes, I am sorry that I did not make it quite clear.
The first point is that radiation from the ground is not the important thing, but contact conduction and convection. Only water and carbon dioxide can absorb the infrared radiation emitted from the ground, and they are (usually) less than 1% of the air between them; oxygen, nitrogen, and argon cannot. But air in contact with the warmer ground or ocean will itself warm up through this direct contact.
The second point is that there is some quite complicated stuff about convection circulation. In the first place convection will carry the warmer air that has been in contact with solar heated ground or ocean upward. But as this warmed air rises and is replaced by cooler air from above, the rising air will expand and cool, while the sinking air will compress and warm up because of the fact that atmospheric pressure gets rapidly lower as you go to higher altitude. This warming and cooling is always associated with compression and expansion of gases like those in air. (You may have noticed how a gas cylinder cools as gas is released from it, or a bicycle tyre warms up as you pump it)
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Very nice Damocles, a pleasure reading you. What importance do you give to cloud formations, scattering etc?
( And write that SF :) I'll guarantee I'll read it )
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What importance do you give to cloud formations, scattering etc?
I am a chemist, not a meteorologist! However, a bit of simple logic will help out here. On the one hand clouds and other particulate matter clearly make a big difference to local temperatures. On the other hand they do not seem to have as much effect on temperature vs altitude profiles, though they can produce effects like low level inversion layers, etc.
But as far as a global consideration of temperature decrease with height in the lowest few km is concerned, they cannot have much overall relevance, simply because at some times they are present and at others they are not!
Above 15 km there is almost no water vapour, and so no clouds of water ice. In the polar winter and spring, there are sometimes Polar Stratospheric Clouds of water ice or nitric acid ice, and there are sometimes plumes of particles resulting from major volcanic eruptions at any latitude. The detail of the temperature vs height profile can change quite a bit, but the general structure, with temperature minima near 15 and 85 km, and a maximum near 50 km, does not.
Actually, the tropopause (minimum "near" 15 km) characteristically has a hat shape -- rising from about 8 km at the poles to about 12 at each tropic, and then doing a sudden jump to about 17 or 18 km in equatorial regions.