Visualizing the "Greenhouse Effect" – Atmospheric Windows

I believe Planck’s blackbody radiation law applies to “cavity” radiation. That is, it describes the amount of, directionality, and spectral distribution of electromagnetic energy radiated from a small “hole” in the surface of a cavity whose internal walls are at a uniform temperature. The amount of radiation is proportional to the area of the hole. This concept has been applied to the “surface” of a body. Specifically, for a differential area of the surface of a blackbody at a single temperature, the law gives the spectral distribution and the amount of energy radiated from that differential area into a differential solid angle. When discussing an atmosphereless earth, it’s reasonable to identify the transition between the earth and space as a surface–i.e., it exists in two not three dimensions. When considering a gaseous atmosphere, I’d appreciate it if someone could identify the “surface” to which Planck’s law can be applied. As I see it, Planck’s law does NOT apply to gases. Similarities may exist in the spectral nature of the radiation being emitted from a gas and the radiation being emitted from a differential area on a blackbody surface; but without a clearly defined surface, I don’t see how Planck’s blackbody radiation law can be applied. In particular, a part of Planck’s blackbody radiation law is the COSINE of the angle between (a) the direction of the solid angle into which the radiation is propagating and (b) the normal to the radiating surface. Would someone please tell define that “normal” for a gas?

“”””” mkelly says:
March 3, 2011 at 9:03 am
Phil. says:
March 3, 2011 at 6:48
“You have been shown data here which shows exactly how weakly N2 and O2 interact with IR, but it doesn’t agree with you preconceived belief so you ignore it. Your ‘idea’ contradicts a century of science on the structure of molecules and their interaction with radiation as well as the experimental basis for it.”
I must have missed that. The graph I saw never said anything about temperature. That is my specific question. If it was intended to say that those areas are the only frequency they emit/aborb under all temperatures I misunderstood the graph.
You seem so angry. I was not granting you permission to do anything I was trying to be polite/civil. If you chooe otherwise please let me know.
Adiabatic lapse rate accepts the fact that pressure causes heat. Less pressure less heat. So PV=nRT is valid. I know very little of Triton.
My heat transfer book shows insolation varys between 1063 w/m^2 at 90 deg to sun down to 41 w/m^2 at 5 deg. So if we are going to average then 55o w/m^2 should closer to what the surface receives. See what George said above. “””””
mk , only a casual reader here at WUWT would not be aware, that there is basically very little difference of opinion between what I understand about this radiative transfer, and what Phil says. I may put it a little differently; but I believe that Phil is much more conversant with this subject than I am; although I have no idea what he does.
I’m basically relying on concepts I learned in school nearly 55 years ago, along with a bit of late night book cramming; whereas it seems Phil is into this stuff every day.
I do a lot more hand waving, whereas Phil knows where to go get the goods.
But I’m a firm believer that there is no future in trying to deny that the absorption of LWIR radiant energy by GHGs is a fairly well understood process, by means of which the atmosphere is warmed. There are of course other processes that heat the atmosphere including solar radiative heating; but what we call the “greenhouse” effect; at least to the point that it heats the atmosphere, is demonstrably real, and fighting that is a poor choice of causes to die for.
I’m less sure of what subsequently happens, after that energy transfer heats the atmosphere, but it is obvious that as a consequencew some of the energy that would have escaped to space, is returned towards the surface, so the exit is at least delayed.
The way I like to think about the consequences of that delay, is that during that delay, the sun continues to input energy to the earth at its standard rate, and it is that incremental solar input during the extended exit time of the outgoing LWIR radiation that is the source of the net heating; not the returned LWIR itself.
And I have even offered an analagout situation of the water running into the bathtub, with the plug removed. The constant input rate stays fixed, but the outflow rate increases, as the water level rises in the tub, increasing the driving pressure.
If you now partialy restrict the drain opening (GHG interception), you now need a higher water pressure to create the same outflow rate you originally had; but it is the faucet that supplies the extra water to raise the level back to the stady state level; not the obstruction.
And what’s with the citation of the ideal gas equation of state; that only applies to a closed system; and the earth or any planetary atmosphere is anything but a closed system; so the ideal gas law, or any other equation of state, really adds no new understanding to the situation.
I don’t know whether Phil and I agree on everything; he doesn’t post a lot of extensive writings; probably has better things to do (so do I); but I think we likely agree on a whole lot more than we might disagree; and in the past I have learned that I should take note of such disagreements, because it has proven to be me that is usually wrong.
I’m far less interested in the role of CO2 than it may seem, because I’m fairly convinced that it is H2O that is in full control of the situation (specially clouds) so my interest in the details of CO2 is more curiosity than concern. I certainly do want to understand the inner Physics of the CO2 absorption bands; and more importantly the thermal radiation from gases; but I believe the answer to earth’s Temperature comfort range lies in the totality of the properties of the H2O molecule; not the CO2 molecule.

Myrrh says:
March 3, 2011 at 4:47 am
You said “So, radiation is included, not just conduction, which means that it is impossible for a cooler molecule to transfer heat to a hotter one because that would violate the 2nd Law.”
You based this on the following definition of the 2nd law: ““When two isolated systems in separate but nearby regions of space, each in thermodynamic equilibrium in itself, but not in equilibrium with each other at first, are at some time allowed to interact, breaking the isolation that separates the two systems, and they exchange matter or energy, they will eventually reach a mutual thermodynamic equilibrium.”
Myrhh, you have read the 2nd law and have drawn the wrong conclusion. The 2nd law considers “two isolated systems in separate but nearby space, not in equilibrium with each other,” as you have pointed out. It then says that when they are allowed to interact they “exchange matter or energy. . .” This is the crucial point THEY ie both bodies – hotter and cooler – exchange energy. But the crucial conclusion from the 2nd law, is that as a result they “will eventually reach a mutual equilibrium.” This is because the hotter body is transmitting at a far greater flux density than the cooler body.
The GHG hypothesis describess a cooler atmosphere radiating to a warmer surface, which some, including you, say is impossible because it violates the 2nd law. But we have seen that the 2nd law says both bodies will exchange energy, and they will eventually equilibriate – if they are isolated systems. If the Earth was not continually recharged with energy from the sun, it would cool down to the temperature of the atmosphere, and both of them would continual to cool as they equilibriate with space.

davidmhoffer
I said
The thermodynamic meaning of heat implies the ability to do work in the given situation.
This applies to the radiation moving from the hotter surface to the colder surface.
It does not apply to the radiation moving from the colder surface to the warmer surface.
You said
I’m very interested in how this works Bryan. How is it that radiation going in one direction is able to do work, but radiation going in the other direction is unable to do work?
I say
Its really a very simple practical test.
I can give you several examples of a higher to lower temperature situation doing work like a steam engine and so on.
You cannot give me one example of low temperature to high temperature situation doing work.

Tim Folkerts says:
March 2, 2011 at 9:18 pm
I didn’t mean “possibly”: I was thinking of two macroscopic chunks of material pressed together. This would put millions of atoms from the two pieces in contact, which will all be colliding many times per second with atoms from the other object. There WILL be many collisions here. There will be millions of microscopic transfers of energy ever second. It is just the NET flow of energy for large collections of energy that is governed by the 2nd Law, not the transfers during individual collisions.
So you say. But at the meeting point of these two is the environment they are in, the 2nd Law says heat does not flow spontaneously from colder to hotter. I’m the one saying there should be a ‘possibly’ in there. That atoms in matter, waves in radiations, etc. might be able to do such things doesn’t mean that they can actually do such things. This “NET” flow of energy fitting 2nd Law, is imaginary. You may think you’ve calculated it all out “statistically” as an explanation, but that’s saying nothing at all, you’ve just added that to the Law. Average, your NET, is an artificial construct. It doesn’t acutally exist nor is it included in the Law as the Law stands.
I think, this is of the same ilk as using ideal gas laws to describe real gas molecules. The ideal gas is imaginary, just as your Net is imaginary. The real gas is real. Ideal gas laws describe imaginary gases, they do not describe real gases. Just so, your Net flow of energy is an “ideal” assumption of what is happening, it does not actually describe what is happening or what is capable of happening.
Just as an ideal gas has no volume etc. and so cannot actually describe what is happening to a real gas which has volume etc., so your Net transfer because ‘energy flows in every direction and collisions happening between them transfer energy doesn’t mean that the cooler are actually transfering that energy to a hotter.
The transfer of energy at EACH collision MUST be from the hotter to the colder to be according to the Law. You’re fudging it by creating this idea of a Net Transfer when the Law says it applies to all energy too.
I think this is part of the problem where AGWScience gets lost to the point where it thinks, as the energy graphic depicts here, that shortwave light energies can heat the earth and are now calling it Thermal Energy. Physical limitations in actual reality are jettisoned to this imaginary Net Transfer just as Carbon Dioxide is treated as an ideal gas without volume etc. capable of doing impossible things like defying gravity to spread in the atmosphere without work being done even though its heavier than air because it bounces off all the other molecules in the atmosphere ‘diffusing’ as if an imaginary ideal gas in a jar.
If, heat does not always flow spontaneously from the hotter to the colder in all matter and energy as the Law states and you can prove it doesn’t as you claim it doesn’t, then you have falsified the Law.
You cannot say that the Law doesn’t apply to your particular set of energies or matter just because you think it shouldn’t.

“”””” davidmhoffer says:
March 3, 2011 at 1:18 pm
Bryan;
You cannot give me one example of low temperature to high temperature situation doing work.>>>
So as I understand it, being unable to answer the question I asked, having no response even to suggest that the question I asked didn’t represent the issue in some manner, not even a hint that the issue posed is not representative, you now retreat into that most devastating of classic debate challenges:
Oh yeah? Give me an example. “””””
David, it doesn’t take very much savvy, to grasp the notion that once a flock of Photons from any source are launched into the regions of space, they have no recollection of their origin, nor any knowledge about where they are going.
It might be a billion years before they encounter anything that they can take any notice of. At that point they are too senile to recall that they came from a colder place than whatever this new thing is, so they are just going to land on it anyway.
The point is that this is a one way open system.
The second law in the form it was stated by Clausius, makes it clear that it applies to closed cyclic systems; where the means even exists for the energy to go in either direction. Then it becomes clear, that although each end can communicate with the other and even transport energy (in the form of photons shall we say) , the reverse is also possible, but the net result is that, in the absence of some other effect (as Clausius put it) namely the doing of work on the system, there can only be a NET transfer of energy in one of those directions; which Clausius asserted, is the direction from the hotter (higher Temperature) source to the colder (lower temperature) sink.
Like you I am amazed that this very simple concept is unable to be grasped by so many. In an open non-cyclic system, the energy can go via radiation at least, any darn place it wants to.
In the case of the downward (or back radiation if you like), it is no different from paying for your groceries at the cash register.
You buy $7 worth of Negra Modello (specially after reading this); and you hand the teller a hundred dollar bill, and (s)he gives you back $93.
You are still losing money; just not as fast as if the teller gave you no change back. You perhaps still rely on your employer to keep supplying you with more money in your pay check; he is the one making you not poor; not the teller who simply gives you back some change.
Likewise it is the sun with its continuous pay check, that is keeping you warm, and the GHG “insulation” is simply slowing the rate at which you go broke.

http://knowledgedrift.wordpress.com/2011/03/03/why-the-co2-greenhouse-gas-debate-doesnt-matter/
I have provided the most comprehensive rebuttal possible to this debate in clear English, using the radiative properties of CO2 as calculated by the IPCC, extrapolated forward with the addition of twice as much CO2 as human’s have added to the atmosphere in our entire history so far added on top of what is already there, which at the highest consumption rates for fossil fuel we have ever achieved will require another 150 years to accomplish, and shown conclusively the catastrophe that awaits us. Having been spending far too much time in this thread and on this topic, I can with confidence suggest the following:
1. a whole bunch of people will scratch their heads and ask….that’s it? that’s what we’re beating each other over the head with physics text books about?
and
2. An argument will immediately break out between Phil, George, Ira and who knows who else that I should have shown error bars. Followed by someone claiming I have the sign wrong, it should be cooling, not warming. And a troll carefully explaining in great detail that I didn’t do the log calcs correctly, he doesn’t know how to do them himself but his calculator does and mine are wrong. A slew of PNS groupies going see? We TOLD you nothing would happen if you didn’t listen to us, but I don’t really pay attention, I’m not really listeng to them, another troll carefully explaining that if we convert to coal to gas plus burn all the oil plus shut down all the nuclear reactors it will happen in 90 years…and there ought to be one comment from Khartoum complaining that my instructions on how to build a quinzie should have included the fact that it generates its own heat source from within and melts almost instantly, and is there any way to harness this commercially because it appears to be perpetual motion, free energy, maybe we can use it to heat the planet because there’s an ice age coming and it turns out the CO2 is pretty much useless.

to George E. Smith
You wrote: “Reed, in the “Cavity” case you cite, the radiation inside the cavity, that is presumed to have black walls (non thermally conducting), is of course isotropic. There is no preferred direction inside the cavity.”
The walls do not have to be ‘black’.
Cavity radiation is an interesting phenomenon. In a completely enclosed isothermal cavity, it makes no difference what material the interior walls are composed of. The internal radiation in the cavity will conform precisely to blackbody radiation at the temperature the material is at.
Even if the walls were composed of N2, it will emit blackbody radiation. Thus N2 can indeed emit all wavelengths under certain conditions. And if you inject any photons into this hypothetical N2 constructed radiation cavity, of any wavelength, it will all be absorbed by the N2 molecules within. 100% absorption regardless of wavelength.
How a gas, atom, molecule, etc responds to thermal radiation, or emits thermal radiation, is dependent on the conditions of the environment it is placed within.

Ira,
nice job on the explanation. The important parts were correct and well stated.
concerning those with problems on radiative matters,
stefan’s law is often shown in an abbreviated fashion, P/a = epsilon * sigma * T^4. The full version is P/a = epsilon * sigma * (Tb ^4 – Ts^4), where Tb is the temperature of the body and Ts is the temperature of the surroundings (assuming epsilon – the emissivity – is the same for all).
Except for the sun, the surroundings of Earth are pretty much the microwave background at 2.7K.
If one has a BB at 300K and they place it inside a box at 300K, there will be radiation absorbed and emitted by the BB continually but the BB will remain at 300K. If the box were cooled to 2.7K, approximately zero so far as radiation rates are concerned, the the BB will cool down eventually to 2.7K and it will take a fairly short amount of time to cool to 290K. If the box were cooled only to 290K, the time it takes to cool the BB from 300K to 290K will be somewhat longer than the time it takes to cool down in the first case. In neither case is there a NET flow of energy from the colder box to the warmer BB.
Once the BB reaches the temperature of the box,there will no longer be a net flow of energy in either direction yet there will be radiation being emitted and absorbed by box and BB.

The one area in which it seems that I differ from what Phil has said, is that regarding the emission of a (frequency) continuum spectrum of thermal (due to Temperature) radiation by neutral gases; say N2, or O2, or how about a mono-atomic gas like Argon.
I understand (to some extent) how individual atoms can radiate specific frequency spectral lines, as a result of energy level transitions of an electron in that atom. (used to teach it in fact). I also understand to about the same extent, how those spectral lines can have a “fine structure” producing several lines instead of one; due to refinements first proposed by Arnold Sommerfeld and others. I even understand how some atoms have even a hyperfine structure to their atomic spectra; that is a result of the specific structure in the nucleus of that atom, rather than its electron configuration.
I have a quite rudimentary understanding of how molecules that have atoms connected together, which are capable of undergoing internal molecular vibrations and other oscillatory motions, can also emit line spectra that generally fall into the Infra-Red frequency range, because of the masses of the oscillating components, and the strengths of the binding forces that hold them together. I’m less happy with my understanding of how it happens that those oscillations also have some “fine structure” that manifests itself in bands of closely spaced narrow lines; that have some intrinsic line width that is a result of Heisenberg’s Uncertainty Principle, and the lifetimes of the excited states that lead to those intrinsic line widths. The longer the lifetime of the excited state, the narrower is the frequency spread of the emitted photon. dE.dt > h/2pi or something like that.
I don’t quite understand how those multiple lines appear, as that seems to be taught in chemistry, rather than in Physics classes; so I’m working on that.
Now so far as I am aware, all of the preceding, applies to atoms or molecules essentially independently of the phase of the matter, except to the extent that that state alters the physical parameters of the interactions.
The ability of molecules to interact with Electromagnetic Fields, evidently is highly dependent on the “antenna” characteristics of the particular molecule. The classical “radio” antenna of Hertzian waves is a structure where charges can move around due to electric currents in the antenna, and thereby set up intertwined electric and magnetic fields; which it turns out are pretty much always perpendicular to each other. Per Maxwell’s equations, if those electric currents are not stationary, so the electric charges have a constant velocity, then the antenna will radiate energy away from itself, in a manner that depends on the dimensions of the antenna, and the rates at which the currents are varied (the frequency). This is all taught in “RadioPhysics”; one of the things I actually studied. The most basic antenna is the electric dipole, consisting of separated electric charges of opposite sign. Well atoms and molecules contain separated electric charges; electrons and protons. In some atoms or molecules, these charges are symmetrically disposed, so that the net positive charge and the net negative charge, both have the same center of charge; and we tend to say the atom (or molecule) has no “dipole moment”; that being the charge at each end times the separation. So molecules like N2, or O2, are considered to have no net dipole moment, so they make really lousy antennas, either for transmitting or receiving. So we describe them as being “not Infra-red active”.
Some molecules like H2O for example which is not at all symmetrical; there’s that 104 degree elbow, have a significant dipole moment; so it is very IR active.
Well although I don’t understand much of the detail of this I do grasp the general concepts.
So now what about “black body” or thermal radiation; EM radiation fields that have a continuous frequency spectrum, with a very defined spectral distribution that depends only on the absolute Temperature of the material, and doesn’t depend at all on the nature of the material. (remember that true black body radiation is a theoretical concept that doesn’t actually exist anywhere, but which is closely approximated by some real systems)
There is nothing in the theory of BB radiation which contains the Physical properties such as bond strengths and the like of any actual material. Its origin is considered to be due to the acceleration of electric charges, as explained in Maxwell’s equations.
Well any isolated neutral atom or molecule getting along in outer space is subject basically to nothing but the gravitational force; which is far and away the weakest of all the Physical forces of nature; so it doesn’t do a heck of a lot for any isolated neutral atom or molecule; which continues on in a straight line at a constant velocity according to Newton’s Laws (or maybe Einstein’s)
So Maxwell says such a free roaming particle does not radiate EM energy; well if it did it would apparently evaporate itself, since there is nothing around to resupply it with energy.
But now, if we put a whole lot of similar neutral particles close to each other, and bestow on them a Temperature which is not zero K , then we expect these particles to have on average an energy kT per degree of freedom of which there are at least three in ordinary three d space.
In such an assemblage of particles, collisions occur between particles, so the amount of energy of any particle constantly changes in a completely unknown, and unknowable fashion; although it has statistical properties which can be prescribed from the concept of Temperature.
Well now we have a system, which doesn’t have any defined structural state like a molecule or even an atom; but it does contain electric charges which must undergo acceleration during the interaction of two particles in a collision.
According to Maxwell, during the time when that interaction is occurring, and acceleration of the charge is happening, the particles must radiate EM waves, which will result in lowering the average energy of the particles (if those photons escape), so the material will cool, unless some source of energy supply is present to resupply the particles with energy.
I see no reason why two atoms (or molecules) in such a collision would not radiate according to Maxwell’s equations for electromagnetism.
Now in the case of a gaseous material, the density of the particles is quite low. Avogadro’s number of atoms or molecules, in 22.4 litres of the gas at STP.
Now because of the gaseous phase of the material the particle density is low compared to what it is in the liquid or solid phase of the same material.
Consequently the number of particles radiating from some very thin layer (perhaps a mono-molecular layer) will be quite small, so the strength of the radiation will be likewise small. This presumably means the thin layer sample will have a very low value of emissivity; not unlike that of a single atomic layer in a gold film, would have a low emissivity.
But I don’t see any reason why each of them won’t radiate a thermal continuum spectrum, that depends only on the Temperature. Of course establishing the Temperature of such a sparse sample would be rather difficult; well not to worry; I am sure Mother Gaia, knows exactly what the Temperature is, and she will see that the requisite amount of radiation is forthcoming.
The above is to me a plausible explanation of how black body like radiation arises; regardless of what the nature of the source material is. I’ve never ever seen any formal physics denunciation of the very notion that gases simply refuse to radiate, because they are gaseous.
At what point does an H2O molecule leaving the solid surface of a block of ice, or the slightly denser perhaps surface of a liquid sample, suddenly decide to stop radiating a thermal spectrum ? Simply doesn’t make any sense to me, but I can appreciate that the emissivity may very well reflect the lower particle density of the source material.
So personally, I believe that ANY material that has a finite non-zero Temperature can and does radiate a continuum thermal spectrum dependent only on the Temperature (the spectrum that is).
Yes the allowable average energies per degree of freedom, of the radiating particles apparently do have to be quantized; but nothing in the Planck derivation of the BB radiation law, is there any assumption of some energy level structure to the radiating system; such as occurs with either atomic or molecular line, and band spectra.
Evidently in this belief, Phil and I apparently do not agree (I think). I’m still working on establishing a solid basis in my mind for that belief. I’m tending to believe that the aha explanation of the mystery lies in the fact that the negative electric charge is associated with a light particle of mass about 511 keV; while the positive charge is associated with the proton mass of about 938 MeV, a ratio of about 1836.
While those “charge clouds”, maybe concentric in an isolated particle (between collisions), I suspect that they separate during collision so there is a definite electric dipole moment for the duration of the collision interraction.
And I have to say, that my quantum mechanics, is not yet up to snuff, to explain the details of how this works; but I am increasingly confident that it does work, and that the collision dynamics is what makes it all possible.
If I have this all totally screwed up, i suppose Phil or one of the many resident PhD Physicists who frequent WUWT, can straighten me out. I may have to throw myself on the mercy of Prof Will Happer at Princeton to get some steerage on this. Meanwhile the books will have to do; but If I figure it all out I will let you know.
There is one aspect of this that is perhaps not so difficult to understand. The molecular band spectra (say of CO2) consist of very many pretty much equally spaced (in frequency) narrow lines. In practice, these lines are “temperature broadened” due to the Doppler effect, and the velocity distribution of the molecules, which of course has a Maxwell-Boltzmann distribution function. They also are “pressure broadened” as a consequence of the local gas pressure; which can be explained as a simple consequence of the mean time between collisions. An excited (CO2) molecule may have a lifetime (in isolation) that can be many milliseconds; but the collisions in the lower atmosphere occur many thousands of times faster than that, so the int5rinsic lifetime gets catastrophically shortened due to a collision which promptly terminates the excited state, and thermalizes the energy. As a result the line width increases per the Heisenberg dE.dt > h/2pi relationship, when the lifetime is cut short. The question is under what atmospheric conditions do thes broadened fine structured line spectra merge into a continuous blocking band. I don’t know (yet) but I’m sure it is known so I’m not worried.
So what about my collision induced dipole moments for neutral atom radiations. Well the interaction time of two atoms or molecules in collision, is very much faster by many orders of magnitude, than the mean time between molecular collisions that result in pressure broadening of molecular absorption lines; and that means that the uncertainties in the emitted photon energies become extremely large.
As a result the discrete quantized energies per degree of freedom, that were required to abolish the Ultra-Violet catastrophe, and yield the Planck function, result in extreme Heisenberg broadening of the intrinsic line width; which is why those merge completely into a continuous spectrum; even though the particle energy states are quantized.
So I don’t have any problem at all, accepting a completely continuous thermal spectrum of emission resulting from a system of quantized equipartition energies of colliding molecules. That to me is as plain as daylight. The specific details of all of that are as yet not plain as daylight to me at all; but I am slowly getting the hang of it.