Wednesday, February 09, 2011

Thoughts From the Chum Tank

Eli has been hanging about a bit in the chum tank. Contrary to expectation, there was work and thought to be done, in particular the upside downwelling radiation bit, so here, quickly, are the bottom lines:

a) the only radiation striking the surface from excited H2O and CO2 in the atmosphere comes from very close to the surface, because the mean distance a photon that can be absorbed by these greenhouse gases travels at normal pressures and temperature is a few tens of meters at best.

b) if you want to think about what happens to such photons in the atmosphere think of diffusion of the energy, or if you will of a drunkard's walk. The bottom line is that the emission from the surface is effectively blocked.

c) the best analogy is radiation trapping, where light at resonant frequencies cannot get through the absorbing media even though there is emission to go along with the absorption, and the only net absorption/emission occurs at the border of the absorbing gas. Collisions are not explicitly dealt with but the model is useful for thinking about what is going on in the atmosphere, however, because of the de-excitation by collisions, self-reversal is not observed in the atmosphere.

There is a book and a computer model. In the case of the greenhouse effect radiation does not have to be transferred from the inside of the atmosphere, but is thermally excited at the top (radiation to space) and at the bottom (downwelling). Science of Doom is getting into this, but it comes down to the idea that the downwelling radiative warming of the surface comes from very close to the surface


4 comments:

Anonymous said...

So who's warming whom? Does the atmosphere warm both from the top down and from the bottom up? If so which is stronger? Why are nights (I've heard) warming faster than days?

Pete Dunkelberg

Dallas said...

Nights, particularly nights in relatively dry climates, are where the blocking of outgoing radiation would have the most impact at the surface. High humidity masks the impact of co2. Eli is right on here. Except for short distances, there is no change in the direction of heat (IR radiation) flow. The second law of thermodynamics has not been repealed.

No analogy will be perfect, the combination of the blanket and the tub analogies is pretty good. I like my co2 is a traffic cop analogy, though it does need some work.

David B. Benson said...

Eli, are you enjoying your year so far?

Hank Roberts said...

Applause for "quickly, here are the bottom lines"
Short clear posts are less entertaining, but most helpful.

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I have made this letter longer than usual, because I lack the time to make it short (Je n'ai fait celle-ci plus longue parceque je n'ai pas eu le loisir de la faire plus courte)~Blaise Pascal, Lettres Provinciales (1656-1657), no. 16.