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Fefetltl

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- TL;DR Summary
- Question about the physical sense of air emissivity instead of well define closed surface object's emissivity

Hello guys :)In the frame of finding a physical model for the temperature of Earth's surface, talking about the very "idealized" two-layers model of atmosphere, I ask you now the question to the other physicists or engineers: does it make sens to associate an emissivity to a layer of air (+ some gases in few quantity) ?Why my answer is clearly noThe emissivity is defined between two medias with a discontinuous variation of refractive index, i.e the wall of a oven and the surrounding air, a hot resistance surface in a boiler and the surrounding water etc.

Mathematically speaking we should write ##\epsilon = 1 - t - r##, with ##r## reflection (scattering + specular), ##t## transmission, at thermal equilibrium, Kirchhoff's law.

When you consider now a layer of air with continuous refractive index ##n(z)## at height ##z##, then the layer just upper this one at height ##z + d z## with ref. index ##n(z + d z)##. The transmission between these infinitely close layer is ##1## and the emissivity is clearly ##0##. This is clearly ##0## because you don't have an abrupt interface in atmosphere: it is a continuously varying media (see exponential decrease of gas density and so the same for optical index squared). But for the two layers model, they put that the "upper layer" of atmosphere ##\epsilon## is equal to ##0.78## ... just to adjust the Earth's surface temperature at ##288## K... practical but does not make any physical sense.For you, does the emissivity of a layer of air exist?I ask you the question, because of course I can be wrong. I asked also in another forum, no one can answer...

Mathematically speaking we should write ##\epsilon = 1 - t - r##, with ##r## reflection (scattering + specular), ##t## transmission, at thermal equilibrium, Kirchhoff's law.

When you consider now a layer of air with continuous refractive index ##n(z)## at height ##z##, then the layer just upper this one at height ##z + d z## with ref. index ##n(z + d z)##. The transmission between these infinitely close layer is ##1## and the emissivity is clearly ##0##. This is clearly ##0## because you don't have an abrupt interface in atmosphere: it is a continuously varying media (see exponential decrease of gas density and so the same for optical index squared). But for the two layers model, they put that the "upper layer" of atmosphere ##\epsilon## is equal to ##0.78## ... just to adjust the Earth's surface temperature at ##288## K... practical but does not make any physical sense.For you, does the emissivity of a layer of air exist?I ask you the question, because of course I can be wrong. I asked also in another forum, no one can answer...