The temperature of the troposphere decreases with altitude whereas it is opposite in the stratosphere and again fluctuates. Why is this so?
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2$\begingroup$ If you do some homework you will find perfectly good explanations like this one. $\endgroup$– Jan DoggenAug 25, 2015 at 16:04
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2$\begingroup$ @JanDoggen - That's not a very good explanation. For one thing, it omits the extreme importance of greenhouse gases. For another, it contradicts itself: "Temperatures in the thermosphere can exceed 1000°C" versus "The higher layers contain few gas molecules and are very cold." $\endgroup$– David HammenAug 26, 2015 at 2:30
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1$\begingroup$ @DavidHammen I noticed that too when I read it earlier, but I figured they were talking about local effects (I understand that it would dissipate basically instantly since there are no nearby molecules). Is it possible that some molecules could locally, both in time and space, achieve that temperature? $\endgroup$– Isopycnal OscillationAug 26, 2015 at 5:23
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The strong heat flux through the Earth's atmosphere, the presence of greenhouse gases, convection, and mixing conspire to push the troposphere away from thermodynamic equilibrium (isothermal atmosphere) and toward an adiabatic atmosphere. The natural drop in density with altitude means an adiabatic atmosphere is subject to a lapse rate, a nearly linear drop in temperature with increasing altitude.
Temperature rises with altitude in the stratosphere because this is where the ozone layer exists. The troposphere is primarily heated from below. In contrast, the stratosphere is primarily heated from within via absorption of ultraviolet sunlight by oxygen and ozone. A similar situation exists in the thermosphere, which is where the extreme ultraviolet and higher frequencies of sunlight are absorbed.
The thermosphere is where atmospheric temperatures attain their highest values. (BTW, you wouldn't feel those extreme temperatures if you put on a spacesuit and took a spacewalk in the thermosphere. There's hardly anything there that high up.) In between the thermosphere and stratosphere is where atmospheric temperatures attain their lowest values. This is the mesosphere. Most of the extreme wavelength radiation has already been absorbed in higher layers. The mesosphere is too thin to support an oxygen-ozone cycle. All that's left is radiational cooling supported by a strong greenhouse effect, making the top of the mesosphere (the mesopause) extremely cold.
answered Aug 25, 2015 at 15:29
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$\begingroup$ how will global warming affect the stratosphere or is that a separate question ? :-) $\endgroup$– user1066Aug 26, 2015 at 8:56
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1$\begingroup$ @gansub - That is a separate question. $\endgroup$ Aug 26, 2015 at 10:21
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$\begingroup$ From @Emily: The low temperatures in the mesopause cannot be explained only by radiative balance (difference to measured temperature is about 90K in summer hemisphere and 60 K in winter hemisphere). Summer mesopause is cooler than expected by radiative balance because of ascending air parcels (adiabatic cooling). In the winter hemisphere air parcels descend and temperatures are higher in stratosphere/mesosphere/mesopause than expected because of adiabatic heating. The key-word here is the so called Brewer-Dobson-Circulation. $\endgroup$ Nov 19, 2016 at 23:00