Post by kiwistonewall on Sept 19, 2009 7:41:44 GMT
And apologies Radiant for my rudeness.
There are TWO types of radiation concerning us in the atmosphere.
1. Spectral absorption due to quantum transitions (either at high energy (UV) where we are talking about electrons being excited)
and at lower energies (infra-red) where we have absorption in various molecular motions. These spectra are precise, single frequency peaks, though experimentally these get broadened when measured.
2. "Blackbody" radiation due to movement of molecules. As molecules collide they accelerate and this causes emission of radiation (and absorption causes motion - in reverse). Because blackbody radiation is related to the probability distribution of molecule energy, the shape of the curve is the same as the energy probability distribution.
NOTE: This has nothing to do with "Blackbodies" which are ideal solids. We are talking about the SHAPE of the energy absorption/emission curve. Since the atmosphere is not optically dense at Earth blackbody radiation, some of the IR will escape, but a significant fraction will be absorbed. (Nor is the Earth an ideal Blackbody, but it too, like all matter gives off BB radiation.)
Do NOT confuse the two types. My analogy of scratches on a tinted window is apt.
The BB radiation from the Earth isn't very energetic. This is thermal energy. The biggest heaters of the atmosphere (apart from the sun) are convection and latent heat.
Once the atmosphere warms up, it will radiate over the Blackbody spectrum for its temperature.
Most older climate books say the atmosphere warms by convection and latent heat and cools by radiation.
So where do the greenhouse gases come in? Firstly, they will also participate in the ordinary thermal (blackbody) radiation process. They also absorb and emit at discrete wavelengths (broadened by doppler shift & collisions - more so at high pressure & temperature - so less in the upper atmosphere.
Any energy absorbed can then be emitted or thermalised.
But it is a two way street. Absorptivity = emissivity. (Good absorbers are good emitters & vice versa.)
The fact that SPECTRAL radiation is intense and blackbody radiation isn't does not mean that the absorption and emission of ordinary gas molecules isn't significant.
I don't think that anyone is remotely able to model the atmosphere in a true sense. That is why the models do not work.
Now it would be possible to calculate the theoretical emission of energy of air at a certain temp & pressure. I haven't found that anyone has done the math anywhere.
Unfortunately, I dumped all by Physical Chemistry & Data books a few years ago when I moved country, but it certainly is a computable problem.
But the atmosphere is a turbulent dynamic system where temperatures change by huge amounts at altitude - which massively moves the BB curve.
It doesn't sit still enough for us to get a handle on it!
Some final comments: The reason for the T^4 relationship is that the BB curve quickly grows in size as it moves to higher frequencies. Note that that applies ONLY to BB radiation, and not to discrete spectra - there is no such relationship applicable to discrete spectra.
Note also that various discrete spectra (C02, Ozone, water) will move in and out of the maximum peak of the BB curve as temperatures change.
All vary fascinating, dynamic, and not easy to explain, model or set equations to.! ;D
There are TWO types of radiation concerning us in the atmosphere.
1. Spectral absorption due to quantum transitions (either at high energy (UV) where we are talking about electrons being excited)
and at lower energies (infra-red) where we have absorption in various molecular motions. These spectra are precise, single frequency peaks, though experimentally these get broadened when measured.
2. "Blackbody" radiation due to movement of molecules. As molecules collide they accelerate and this causes emission of radiation (and absorption causes motion - in reverse). Because blackbody radiation is related to the probability distribution of molecule energy, the shape of the curve is the same as the energy probability distribution.
NOTE: This has nothing to do with "Blackbodies" which are ideal solids. We are talking about the SHAPE of the energy absorption/emission curve. Since the atmosphere is not optically dense at Earth blackbody radiation, some of the IR will escape, but a significant fraction will be absorbed. (Nor is the Earth an ideal Blackbody, but it too, like all matter gives off BB radiation.)
Do NOT confuse the two types. My analogy of scratches on a tinted window is apt.
The BB radiation from the Earth isn't very energetic. This is thermal energy. The biggest heaters of the atmosphere (apart from the sun) are convection and latent heat.
Once the atmosphere warms up, it will radiate over the Blackbody spectrum for its temperature.
Most older climate books say the atmosphere warms by convection and latent heat and cools by radiation.
So where do the greenhouse gases come in? Firstly, they will also participate in the ordinary thermal (blackbody) radiation process. They also absorb and emit at discrete wavelengths (broadened by doppler shift & collisions - more so at high pressure & temperature - so less in the upper atmosphere.
Any energy absorbed can then be emitted or thermalised.
But it is a two way street. Absorptivity = emissivity. (Good absorbers are good emitters & vice versa.)
The fact that SPECTRAL radiation is intense and blackbody radiation isn't does not mean that the absorption and emission of ordinary gas molecules isn't significant.
I don't think that anyone is remotely able to model the atmosphere in a true sense. That is why the models do not work.
Now it would be possible to calculate the theoretical emission of energy of air at a certain temp & pressure. I haven't found that anyone has done the math anywhere.
Unfortunately, I dumped all by Physical Chemistry & Data books a few years ago when I moved country, but it certainly is a computable problem.
But the atmosphere is a turbulent dynamic system where temperatures change by huge amounts at altitude - which massively moves the BB curve.
It doesn't sit still enough for us to get a handle on it!
Some final comments: The reason for the T^4 relationship is that the BB curve quickly grows in size as it moves to higher frequencies. Note that that applies ONLY to BB radiation, and not to discrete spectra - there is no such relationship applicable to discrete spectra.
Note also that various discrete spectra (C02, Ozone, water) will move in and out of the maximum peak of the BB curve as temperatures change.
All vary fascinating, dynamic, and not easy to explain, model or set equations to.! ;D