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Thursday, December 16, 2010

Eli Rabett on Judith Curry.ja's fabulous ability to unintentionally mislead the curious with her appalling lack of knowledge of the physics of IR absorption

[We all love Eli!]

It's a Turnstile

by Eli Rabett, Rabett Run, November 30, 2010
One thing that Judith Curry's blog has done is attract a number of newbies (well, at least new to Eli) with a number of questions that have simple answers once Eli takes a few days to think about them. It's not that your average climate blogging bunny does not know the answer, but rather that the explanation has to be on a level that the naive skeptic can grok. Grokking for anyone born after 1970 is understanding something at a soul deep level.

An Analog version of this is
I can’t believe that IR absorbtion directly heats the air, for it’s pretty transparent and close to an ideal gas.

Yet, Curryja wrote…………. “Then through the equipartition of energy, the absorption of radiation causes the molecule to move faster and bump into the other molecules like N2, O2, and make them move faster. …”

My “built-in-doubter” is triggered.

Absorption of a photon I thought merely bumped an electron from one orbital shell to another, raising the molecule’s potential energy not its kinetic (temperature). Just as winding a clock tightens its mainspring instead of warming the mechanism. That’s why there’s spectrometer lines…….
Well, that energy is released later by re-radiating another photon when the electron returns to its natural orbit
There are important, in the sense that many have them, misconceptions here. First that the atmosphere is transparent in the IR. The atmosphere is transparent in the visible, but, there are many regions of the IR where absorption is high, including those regions where CO2 and H2O absorb. A handy dandy number to carry about is that at the surface the average distance light can travel in the CO2 spectral region is 10 meters (or about 35 feet for you unethical customary unit users).

The second is the impression that the energy stays in the molecule that absorbs it, to be re-radiated later. This is not so, it is quickly degenerated to thermal motion (translation, zipping about) via collisions. This process is called thermalization and requires about a microsecond at atmospheric pressure. So where does the emission come from the bunnies ask?

Well, there is a considerable amount of thermal energy at room temperature, and even much lower. True this average energy is low compared to even the lowest vibrational excitation of CO2 (which would be ~1000 K), but it is enough that a small, but significant fraction of the CO2 molecules are excited to levels which can emit in the IR (about 6% at room temperature).

A third often stated misconception is that CO2 is such a small fraction of the atmosphere that it cannot absorb or maintain any significant amount of energy. This is where Eli the wonder bunny has thought of a great answer:

Think of CO2 as a turnstile through which energy passes from the ground to the atmosphere. A large crowd can pass through a few turnstiles. 

UPDATE: Upon reflection (and a pointed comment by one who does not wish to be named, always welcome at RR), this is even better than Eli thought. Having to go through turnstiles slows up rushing crowds. If the CO2 is playing the role of a turnstile, and the poor innocent photons have to get through a series of them it will really slow up the rate of emission to space.
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