Dienstag, 31. Januar 2017

How the radiative process in the atmosphere is working

Here a very simple explanation of the process:
The earths surface is radiating IR rays towards space, but only 15-30% are leaving undisturbed (through a so-called atmospheric window). At certain wavelengths they are not absorbed by CO2 & Co.
If one radiative active molecule (water vapor, CO2, methane) is hit by an IR ray, it gets added some plus energy. Every molecule is vibrating and hitting other molecules and transporting energy to others – much quicker than they re-emit radiation. Through steady radiation that part of the atmosphere gets heated up a small bit.
At the same time all radiative molecules above 0 Kelvin are emitting IR radiation – at a certain wave length and in an average direction. Which means 50% up / 50% down. The Backward radiation from the lowest layer of the atmosphere goes down to the surface, the other IR rays are going up to the next layer of the atmosphere. There the above mentioned game starts again, and so on.
We must consider that nearly everything above 0 Kelvin (-273°C) is radiating – also the earths surface, the ocean surface and the clouds. The earths surface is always radiating more than the atmosphere or the clouds. So there is always a net transfer of heat towards space.
Only O2 and N2 – the 99% of the atmospheres molecules – are not radiating and not absorbing IR rays. But they can receive and distribute heat energy by contact with neighboring molecules. Additionally, moving masses of air can transport heat – mostly upwards.
Clouds (water droplets) are different from water vapor und are even more absorbing. In fact, they behave like a blackbody. Like a black matte painted sheet metal they are absorbing all IR radiation throughout the whole wavelength spectrum. And they are emitting radiation the same way in all directions.
On the top of the troposphere (about 5 to 10 kilometers above ground) the atmosphere gets thinner and there is nearly no water vapor. So CO2 is radating mostly undisturbed towards space, as the distance between the molecules is large.
These are just the rules how radiation is working. I have not written anything about amounts and figures. There are also a lot of variables and additional regulating effects.
In short: IR radiation from the surface is not reflected by CO2. Its a bit more complicated.

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