Difference between IR and heat

[Buzz Bloom]

Are you saying that the CO2 in the atmosphere is not in local thermodynamic equilibrium?

Hi Andrew:

Thanks for your post.

The answer to your quoted question is "no". I am not saying that. What I am saying is that this low temperature equilibrium is not affected by the IR radiation absorbed from the Earth. The CO2 does not get warmer by absorbing these IR photons. The CO2 is not in thermodynamic equilibrium with these photons. The explanation for this is also in your quote from the "Einstein Coefficients" article:

For local thermodynamic equilibrium, the radiation field does not have to be a black-body field, but the rate of interatomic collisions must vastly exceed the rates of absorption and emission of quanta of light, so that the interatomic collisions entirely dominate the distribution of states of atomic excitation.
​

I have underlined the important part. Unfortunately I do not have the research skills to calculate the relevant Einstein coefficients from which the average time between a CO2 molecule's absorbing and re-emitting a photon. I also do not have the skills to calculate the mean free path of a moving CO2 molecule to determine the average time between a CO2 molecule's absorbing a photon and colliding with another molecule. I asked, via email, a knowledgeable physics professor at MIT about this, and he confirmed that the average re-emitting time is much less than the average colliding time.

Regards,
Buzz

 

[willem2]

For example see https://en.wikipedia.org /wiki/Greenhouse_effect . This article says:

The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface and the lower atmosphere, it results in an elevation of the average surface temperature above what it would be in the absence of the gases.​

That is, the bonds of the molecules absorb radiant energy at specific wavelengths, and then re-radiate this absorbed energy at the same wavelengths. This has nothing to do with the radiant energy being converted to kinetic energy or heat. Half of the re-radiated IR goes upward, and half goes downward.

That quote doesn't mention mention re-radiation at the same wavelength at all.

Here is another article that describes the coefficients for re-radiation.

https://en.wikipedia.org/wiki/Einstein_coefficients​

I can't compute from this how often re-radiation happens. can you?

I discussed the multiple repetitions of re-radiation and absorption in post #28 of

https://www.physicsforums.com/threads/cause-of-greenhouse-effect.835599/page-2 .​

The Einstein_coefficients article also explains that the spontaneously re-emitted photon has the same energy as the absorbed photon.

But it does not tell how often this happens.

The article I cited in my post #27 described the radiation detected from the atmospheric CO2 has the same spectrum as the CO2 IR absorption spectrum. It is not a thermodynamic Planck spectrum.

That's because at the wavelengths where the radiation is absorbed the radiation that finally makes it out will be emitted from a higher altitude where it's colder.

Another thing is that most of the heat transport from the surface to the atmosphere is in the form of convection, including latent heat of water vapour. This will actually warm the upper atmosphere, and the heat will have to be radiated away. If the atmosphere can radiate, is must also be able to absorb. See Kirchhofs law of radiation.

 

[Buzz Bloom]

Hi WIllem:

Thanks for your post. I will list quotes from your post and respond to them one at a time.

That quote doesn't mention mention re-radiation at the same wavelength at all.

That is correct. This Wikipedia article does not discuss the spontaneous re-emission of photons at the same wavelength. This explanation of the greenhouse gas mechanism is currently not widely written about, and has not yet made it into Wikipedia's greenhouse effect articles. That is why I cited the Einstein Coefficients article. I also previously cited a post (#29 - gave wrong number previously) in a closed thread, "Cause of Greenhouse effect" in which I cited a recent news article about detecting the ID radiation from CO2 in the atmosphere:

http://dailysciencejournal.com/researchers-measure-green-house-effect-for-the-first-time/21420/​

The following is the article in Nature which the above news article discusses.

http://www.nature.com/nature/journal/v519/n7543/full/nature14240.html​

I can't compute from this how often re-radiation happens. can you?

No I cannot. I confessed to my inadequacies in my post which you commented on.

Unfortunately I do not have the research skills to calculate the relevant Einstein coefficients from which the average time between a CO2 molecule's absorbing and re-emitting a photon. I also do not have the skills to calculate the mean free path of a moving CO2 molecule to determine the average time between a CO2 molecule's absorbing a photon and colliding with another molecule. I asked, via email, a knowledgeable physics professor at MIT about this, and he confirmed that the average re-emitting time is much less than the average colliding time.​

I will email this professor again, and ask for his permission to quote him here from our correspondence.

But it does not tell how often this happens.

Correct. The application of Einstein's contribution (regarding the coefficients named after him) to the greenhouse gas mechanism was not worked out during Einstein's lifetime. My response to your previous quote above, is also relevant to this quote.

That's because at the wavelengths where the radiation is absorbed the radiation that finally makes it out will be emitted from a higher altitude where it's colder.

I am sorry, but this makes no sense to me. I think you may have misunderstood the news article from the Daily Science Journal cited above, or what I said about it. The temperature of the CO2 gas is irrelevant. The radiation from any thermal source will have a Planck spectrum. See

https://en.wikipedia.org/wiki/Planck's_law​

The IR spectrum detected from atmospheric CO2 is NOT a Planck spectrum. It is the absorption spectrum for CO2.

Another thing is that most of the heat transport from the surface to the atmosphere is in the form of convection, including latent heat of water vapour. This will actually warm the upper atmosphere, and the heat will have to be radiated away. If the atmosphere can radiate, is must also be able to absorb. See Kirchhofs law of radiation.

Ignoring the effect of clouds to keep this simple, the upper atmosphere temperature of CO2 is a result of low atmosphere heating by conduction from contact with the Earth's surface, and the transfer of this heat to higher atmosphere by convection. This average steady state equilibrium flow of heat into the atmosphere is balanced by removal of the heat in the form Planck spectrum radiation. The Planck spectrum involves the removal of the kinetic energy of a gas during molecule collisions. This is entirely unrelated to absorption and re-emission of photons, which is the phenomenon discussed in the Einstein Coefficients article.

Regards,
Buzz

 

[Andrew Mason]

Hi Andrew:

You are quite mistaken regarding your quote above. Thermodynamics is about the transfer of heat. When a mirror reflects photons, there is no heat exchanged. A very cold mirror can reflect IR energy back to a hot source without violating any laws of thermodynamics. This reasoning also applies to GH gasses. GH gasses re-emitting half of their absorbed IR photons back to Earth are not transferring heat and are not violating thermodynamic laws.

There is a transfer of EM IR radiation energy from the Earth surface to certain of the gases in the atmosphere. Some of this energy is emitted by these molecules back toward the Earth and absorbed by the earth. Let's assume that the emission of IR photons by the gas molecules in the atmosphere is not blackbody radiation following Planck's law (i.e it does not depend on the thermodynamic state of the emitting gas). Nevertheless, the absorption by the Earth of the radiation from these gas molecules has the same effect on the internal energy of the Earth's surface (ie. its temperature) as if heat flow had occurred into the Earth from an external source.From a thermodynamics perspective, when the Earth surface radiates infrared energy through the atmosphere and into space, there is negative heat flow (Q<0) with respect to the earth. When some of that radiation energy is absorbed by CO2 gas molecules in the atmosphere and then re-emitted so that some of that energy returns to the Earth surface, a positive heat flow (Q>0) with respect to the Earth occurs.

The OP seems to believe that the proponents of this theory think that the magnitude of the positive Q exceeds that of the negative Q. That seems to be the basis for his criticism of the theory, which of course is nonsense.

So I disagree with your statement that the laws of thermodynamics do not apply. You may be right that the temperature of the CO2 in the atmosphere does not determine its capacity for absorption and emission of IR. But it is possible that an individual CO2 molecule can emit a photon back to the Earth of higher energy than it absorbed (ie. from the earth). This can occur because those molecules have a thermodynamic temperature ie. kinetic energy - and are continually exchanging kinetic energies through collisions.

The laws of thermodynamics prevent a net flow of heat from the CO2 to the Earth (second law). That is to say that statistical laws prevent those higher energy re-emissions from predominating and cause a net positive heat flow from the CO2 to the earth. In other words, the magnitude of the heat flow from the Earth will always be greater than the magnitude of the heat flow to the Earth produced by the reemitted IR photons from the CO2.

AM

 

[willem2]

Ignoring the effect of clouds to keep this simple, the upper atmosphere temperature of CO2 is a result of low atmosphere heating by conduction from contact with the Earth's surface, and the transfer of this heat to higher atmosphere by convection. This average steady state equilibrium flow of heat into the atmosphere is balanced by removal of the heat in the form Planck spectrum radiation. The Planck spectrum involves the removal of the kinetic energy of a gas during molecule collisions. This is entirely unrelated to absorption and re-emission of photons, which is the phenomenon discussed in the Einstein Coefficients article.

My point here, is that if the upper atmosphere radiates away black body radiation, it must also absorb it. See the wikipedia on kirchhofs law

[QUOTE} With this definition, a corollary of Kirchhoff's law is that for an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, the emissivity is equal to the absorptivity
[/QUOTE]
The page about Einstein Coefficients gives no quantitative information about the likelyhood of re-emission at the same frequency.

 

[Buzz Bloom]

The OP seems to believe that the proponents of this theory think that the magnitude of the negative Q exceeds that of the positive Q. That seems to be the basis for his criticism of the theory, which of course is nonsense.

Hi Andrew:

Thanks for your post.

I do not know what the OP thinks, but my thought is that the mechanism you describe has nothing to do with the greenhouse effect, which involves CO2 and other greenhouse gases. The conduction -> convection path of heat, is followed by thermal radiation from the atmosphere of which about half goes up and escapes from the earth, and half goes down back to earth. This mechanism involves the entire atmosphere, which is about: 78% nitrogen, 21% oxygen and 1% everything else. CO2 is part of this 1%, about 0.036% of the atmosphere. It is entirely a thermal process, unlike the greenhouse mechanism, which is not thermal at all.

Regards,
Buzz

 

[Buzz Bloom]

My point here, is that if the upper atmosphere radiates away black body radiation, it must also absorb it. See the wikipedia on kirchhofs law

Hi willem:

Thanks for your post.

The quote above is entirely correct about the upper atmosphere needing to absorb the black body energy it radiates. You are mistaken that the form of this energy is absorbed radiation. The absorption starts with conduction into the lowest atmosphere, and then by convection to the upper atmosphere.

You also said:

The page about Einstein Coefficients gives no quantitative information about the likelyhood of re-emission at the same frequency.​

The following are relevant quotes from the paper.

A photon with an energy equal to the difference E2 - E1 between the energy levels is released or absorbed in the process. The frequency ν at which the spectral line occurs is related to the photon energy by Bohr's frequency condition E2 - E1 = hν where h denotes Planck's constant.​

Spontaneous emission is the process by which an electron "spontaneously" (i.e. without any outside influence) decays from a higher energy level to a lower one. The process is described by the Einstein coefficient A21 (s−1) which gives the probability per unit time that an electron in state 2 with energy E2 will decay spontaneously to state 1 with energy E1, emitting a photon with an energy E2 − E1 = hν.

Due to the energy-time uncertainty principle, the transition actually produces photons within a narrow range of frequencies called the spectral linewidth.​

I suppose one might interpret this language as a bit careless insofar as it does not explicitly say that the E1 and E2 in these two quotes are the same energy levels of an atom or molecule. The "likelihood of re-emission at the same frequency" (or possibly two frequencies both within the same spectral linewidth) is 100%.

Regards,
Buzz

 

[willem2]

The quote above is entirely correct about the upper atmosphere needing to absorb the black body energy it radiates. You are mistaken that the form of this energy is absorbed radiation. The absorption starts with conduction into the lowest atmosphere, and then by convection to the upper atmosphere.

Kirchhoffs's law for radiation, states that any material that radiates at a certain wavelength must also absorb at that wavelength.

The Idea here is that there's a net input of energy from non-radiation sources, there must be net radiation radiating this energy away. But this implies that radiation must also be absorbed at the same wavelengths where it is radiated. There will of course be less radiation absorbed than radiated away, because one side (space) is very cold and there's no incoming IR from that side.

 

[Buzz Bloom]

Kirchhoffs's law for radiation, states that any material that radiates at a certain wavelength must also absorb at that wavelength.

Hi Willem:

Thanks for your post:

I would much appreciate it if you could cite the source with the text that your interpret with the quote above, and also include a quote of the context. With my admittedly inadequate research skills, I could not find by searching the internet anything that approximates this language.

My guess is that you misinterpreted something you read about Kirchhoff's law for radiation. I believe the law means that:

any material that radiates at a certain wavelength must also be able to absorb at that wavelength.​

Regards,
Buzz

 

[willem2]

My guess is that you misinterpreted something you read about Kirchhoff's law for radiation. I believe the law means that:

any material that radiates at a certain wavelength must also be able to absorb at that wavelength.​

That's certainly what I meant also. I got that quote from the wikipedia page about Kirchhoff's law of radiation.
Since the atmosphere does emit IR radiation, it must also be able to absorb it. There's plenty of radiation about. Why doesn't this happen? You still have given no reason why it doesn't happen.

 

[Buzz Bloom]

That's certainly what I meant also. I got that quote from the wikipedia page about Kirchhoff's law of radiation.
Since the atmosphere does emit IR radiation, it must also be able to absorb it. There's plenty of radiation about. Why doesn't this happen? You still have given no reason why it doesn't happen.

Hi willem:

Thanks for your post. Sorry if I misunderstood your previous question.

It does happen. I confess that the concepts in my elaboration below are guesses, since I haven't studied anything in detail about this particular physical phenomenon.

When an atmospheric molecule interacts with another atmospheric molecule, a thermal photon will/may be emitted, and possibly two photons, one from each molecule. This would be characterized as stimulated emission. Such photons are subsequently likely to be absorbed by a similar molecule, and then soon after, spontaneously re-emitted. These re-emitted photons and then also likely to be absorbed by a similar molecule.

This emission -> absorption -> re-emission -> absorption ->... sequence will continue perhaps many times until instead of a photon:

(1) being absorbed, it will finally (a) hit the Earth, or (b) escape into space; or
(2) being re-emitted, the excited molecule will hit another molecule.​

The number of photons that hit the Earth in this scenario is about the same as the number which escape into space. This is because such a re-emitted photon is equally likely to be headed downward as upward.

The next scenario is the normal (non-greenhouse) effect which increases the Earth's temperature from what it would be with no atmosphere. Following the heating of the atmosphere by conduction-convection, all of these emissions/re-emissions and absorption of the thermal black-body photons are non-thermal, in that a molecule's being excited from its absorption of a photon does not make the gas warmer. However, when two molecules collide, and no photon is emitted, the energy of the excited state will add to the kinetic energy of one or both of the colliding molecules, and this will make the atmosphere warmer. So, both the thermal and non-thermal re-emitted photons that hit the Earth return some of the energy from the Earth back to the Earth. This is NOT a violation the thermodynamics law about a cold body not being able to heat a warmer body. The warmer Earth is in thermal equilibrium with the cooler atmosphere, and there are net exchanges of heat from the Earth warming the atmosphere, with some of that heat being returned to the Earth, reducing what would otherwise be a warmer atmosphere.

When a greenhouse molecule absorbs a photon, it also is involved in a similar scenario of repeated re-missions and absorptions. Although it is rare for an excited molecule to collide before it re-emits a photon, it can happen. When it does, the small number of cases when it does results in some slight warming of the atmosphere.

So far I have ignored clouds. When a photon from Earth, or re-emitted from a water vapor molecule, hits a water droplet in a cloud, and is absorbed by a water molecule, the excited molecule is much more likely to collide before it re-emits a photon. This is because the molecules in a liquid drop are much closer together than they are in a gas. Therefore, these photons heat the water droplets, which in turn by conduction thermally warm the atmosphere gasses.

I hope this answers your question.

Regards,
Buzz

 

[Herman Trivilino]

The notion that heat is only energy transfer is is an incorrect interpretation that has become en vogue recently.

So, now it's an argument over the meaning of a word? Fine. Physicists use the word heat to mean a transfer of energy between objects because of a difference in temperature between those objects. You won't be able to convince us that it's become en vogue recently when it's been that way for a very long time with no serious discussion of any reason to change it.

Consult the introductory college-level physics textbooks. Look at the new ones. Look at the ones that were in use 60 years ago. They will support what we're telling you.

Review the physics education literature. They will reveal that this definition is alive and well and is indeed the one in use, and has been for at least the last 60 years.

Internal energy is what you appear to be confusing with heat. Understandable in view of the way the terms are tossed around in less formal circumstances.

 

[Andrew Mason]

Hi Andrew:
I do not know what the OP thinks, but my thought is that the mechanism you describe has nothing to do with the greenhouse effect, which involves CO2 and other greenhouse gases. The conduction -> convection path of heat, is followed by thermal radiation from the atmosphere of which about half goes up and escapes from the earth, and half goes down back to earth. This mechanism involves the entire atmosphere, which is about: 78% nitrogen, 21% oxygen and 1% everything else. CO2 is part of this 1%, about 0.036% of the atmosphere. It is entirely a thermal process, unlike the greenhouse mechanism, which is not thermal at all.

I am not talking about convection or conduction.

We are concerned with the mechanism by which CO2 absorbs IR in the frequency range close to the peak of the IR spectrum emitted by the Earth and then emits IR radiation in the same frequency range isotropically, about half of which is directed back toward the Earth surface. CO2 does this because of its molecular structure. N2 and O2 are transparent to this IR radiation. That must mean that O2 and N2 do not absorb or emit IR radiation in this range.

But this does not mean that the radiation emitted back to the Earth by the CO2 does not result in heat flow into the earth. It does. The thermal effect of this radiation on the Earth must obey thermodynamic laws. That is all I am saying. The OP stated that the greenhouse mechanism can't be correct because it would violate thermodynamic laws. No one is saying that the greenhouse mechanism involves heat flow into or out of the atmospheric CO2. It doesn't. But it does involve heat flow out of and back into the earth.

AM

 

[Buzz Bloom]

No one is saying that the greenhouse mechanism involves heat flow into or out of the atmospheric CO2.

Hi Andrew:

I apologize for misunderstanding your post #36. I agree with everything in your last post, #43.

Regards,
Buzz