Guest post by Ira Glickstein
The Atmospheric “greenhouse effect” has been analogized to a blanket that insulates the Sun-warmed Earth and slows the rate of heat transmission, thus increasing mean temperatures above what they would be absent “greenhouse gases” (GHGs). Perhaps a better analogy would be an electric blanket that, in addition to its insulating properties, also emits thermal radiation both down and up. A real greenhouse primarily restricts heat escape by preventing convection while the “greenhouse effect” heats the Earth because GHGs absorb outgoing radiative energy and re-emit some of it back towards Earth.
Many thanks to Dave Springer and Jim Folkerts who, in comments to my previous posting Atmospheric Windows, provided links to emission graphs and a textbook “A First Course in Atmospheric Radiation” by Grant Petty, Sundog Publishing Company.
Description of graphic (from bottom to top):
Earth Surface: Warmed by shortwave (~1/2μ) radiation from the Sun, the surface emits upward radiation in the ~7μ, ~10μ, and ~15μ regions of the longwave band. This radiation approximates a smooth “blackbody” curve that peaks at the wavelength corresponding to the surface temperature.
Bottom of the Atmosphere: On its way out to Space, the radiation encounters the Atmosphere, in particular the GHGs, which absorb and re-emit radiation in the ~7μ and ~15μ regions in all directions. Most of the ~10μ radiation is allowed to pass through.
The lower violet/purple curve (adapted from figure 8.1 in Petty and based on measurements from the Tropical Pacific looking UP) indicates how the bottom of the Atmosphere re-emits selected portions back down towards the surface of the Earth. The dashed line represents a “blackbody” curve characteristic of 300ºK (equivalent to 27ºC or 80ºF). Note how the ~7μ and ~15μ regions approximate that curve, while much of the ~10μ region is not re-emitted downward.
“Greenhouse Gases”: The reason for the shape of the downwelling radiation curve is clear when we look at the absorption spectra for the most important GHGs: H2O, H2O, H2O, … H2O, and CO2. (I’ve included multiple H2O’s because water vapor, particularly in the tropical latitudes, is many times more prevalent than carbon dioxide.)
Note that H2O absorbs at up to 100% in the ~7μ region. H2O also absorbs strongly in the ~15μ region, particularly above 20μ, where it reaches 100%. CO2 absorbs at up to 100% in the ~15μ region.
Neither H2O nor CO2 absorb strongly in the ~10μ region.
Since gases tend to re-emit most strongly at the same wavelength region where they absorb, the ~7μ and ~15μ are well-represented, while the ~10μ region is weaker.
Top of the Atmosphere: The upper violet/purple curve (adapted from figure 6.6 in Petty and based on satellite measurements from the Tropical Pacific looking DOWN) indicates how the top of the Atmosphere passes certain portions of radiation from the surface of the Earth out to Space and re-emits selected portions up towards Space. The dashed line represents a “blackbody” curve characteristic of 300ºK. Note that much of the ~10μ region approximates a 295ºK curve while the ~7μ region approximates a cooler 260ºK curve. The ~15μ region is more complicated. Part of it, from about 17μ and up approximates a 260ºK or 270ºK curve, but the region from about 14μ to 17μ has had quite a big bite taken out of it. Note how this bite corresponds roughly with the CO2 absorption spectrum.
What Does This All Mean in Plain Language?
Well, if a piece of blueberry pie has gone missing, and little Johnny has blueberry juice dripping from his mouth and chin, and that is pretty good circumstantial evidence of who took it.
Clearly, the GHGs in the Atmosphere are responsible. H2O has taken its toll in the ~7μ and ~15μ regions, while CO2 has taken its bite in its special part of the ~15μ region. Radiation in the ~10μ region has taken a pretty-much free pass through the Atmosphere.
The top of the Atmosphere curve is mostly due to the lapse rate, where higher levels of the Atmosphere tend to be cooler. The ~10μ region is warmer because it is a view of the surface radiation of the Earth through an almost transparent window. The ~7μ and 15μ regions are cooler because they are radiated from closer to the top of the Atmosphere. The CO2 bite portion of the curve is still cooler because CO2 tends to be better represented at higher altitudes than H2O which is more prevalent towards the bottom.
That is a good explanation, as far as it goes. However, it seems there is something else going on. The ~7μ and ~15μ radiation emitted from the bottom of the Atmosphere is absorbed by the Earth, further warming it, and the Earth, approximating a “blackbody”, re-emits them at a variety of wavelengths, including ~10μ. This additional ~10μ radiation gets a nearly free pass through the Atmosphere and heads out towards Space, which explains why it is better represented in the top of the Atmosphere curve. In addition, some of the radiation due to collisions of energized H2O and CO2 molecules with each other and the N2 (nitrogen), O2 (oxygen) and trace gases, may produce radiation in the ~10μ region which similarly makes its way out to Space without being re-absorbed.
There is less ~15μ radiation emitted from the top of the Atmosphere than entered it from the bottom because some of the ~15μ radiation is transformed into ~10μ radiation during the process of absorption and re-emission by GHGs in the atmosphere and longwave radiation absorbed and re-emitted by the surface of the Earth.
Source Material
My graphic is adapted from two curves from Petty. For clearer presentation, I smoothed them and flipped them horizontally, so wavelength would increase from left to right, as in the diagrams in my previous topics in this series. (Physical Analogy and Atmospheric Windows.)
Here they are in their original form, where the inverse of wavelength (called “wavenumber”) increases from left to right.
Source for the upper section of my graphic.
Top of the Atmosphere from Satellite Over Tropical Pacific.
[Caption from Petty: Fig. 6.6: Example of an actual infrared emission spectrum observed by the Nimbus 4 satellite over a point in the tropical Pacific Ocean. Dashed curves represent blackbody radiances at the indicated temperatures in Kelvin. (IRIS data courtesy of the Goddard EOS Distributed Active Archive Center (DAAC) and instrument team leader Dr. Rudolf A. Hanel.)]
Source for the lower section of my graphic.
Bottom of the Atmosphere from Surface of Tropical Pacific (and, lower curve, from Alaska).
[Caption from Petty: Fig. 8.1 Two examples of measured atmospheric emission spectra as seen from ground level looking up. Planck function curves corresponding to the approximate surface temperature in each case are superimposed (dashed lines). (Data courtesy of Robert Knutson, Space Science and Engineering Center, University of Wisconsin-Madison.)]
The figures originally cited by Dave Springer and Tim Folkerts are based on measurements taken in the Arctic, where there is far less water vapor in the Atmosphere.
[Fig. 8.2 from Petty] (a) Top of the Atmosphere from 20km and (b) Bottom of the Atmosphere from surface in the Arctic. Note that this is similar to the Tropical Pacific, at temperatures that are about 30ºK to 40ºK cooler. The CO2 bite is more well-defined. Also, the bite in the 9.5μ to 10μ area is more apparent. That bite is due to O2 and O3 absorption spectra.
Concluding Comments
This and my previous two postings in this series Physical Analogy and Atmospheric Windows address ONLY the radiative exchange of energy. Other aspects that control the temperature range at the surface of the Earth are at least as important and they include convection (winds, storms, etc.) and precipitation (clouds, rain, snow, etc.) that transfer a great deal of energy from the surface to the higher levels of the Atmosphere.
For those who may have missed my previous posting, here is my Sunlight Energy In = Thermal Energy Out animated graphic that depicts the Atmospheric “greenhouse effect” process in a simlified form.
I plan to do a subsequent posting that looks into the violet and blue boxes in the above graphic and provides insight into the process the photons and molecules go through.
I am sure WUWT readers will find issues with my Emissions Spectra description and graphics. I encourage each of you to make comments, all of which I will read, and some to which I will respond, most likely learning a great deal from you in the process. However, please consider that the main point of this posting, like the previous ones in this series, is to give insight to those WUWT readers, who, like Einstein (and me :^) need a graphic visual before they understand and really accept any mathematical abstraction.
Dave Springer;
re photons and mass
technical omission on my part. Never occurred to me to delve into it as my off the top of my head assumption was that the increase in mass would be completely insignificant. At some point one has to consider that one is splitting hairs already split and split again. E=MC^2 requires that anything travelling at the speed of light have a mass approaching zero for the amount of energy we are talking about on a per photon basis. In fact (now that you jogged my memory) if an object accelerates it gains energy stored as momentum and so its mass must increase accordingly. Any mass of any significance can’t be accelerated to the speed of light without consuming enough energy to build a small galaxy. So the ONLY thing that can achieve the speed of light is a photon because its mass is a limit approaching zero.
I looked at the light sail a long long time ago and came to the same conclusion. The reflected photon must be at a lower wavelength.
Dave Springer: THANKS. I second all you wrote (except for what I consider an unnecessary swipe at the great majority of our fellow citizens who work at honest jobs and pay their taxes and serve in the military, and so on. Many of them, perhaps most, have more common sense than many of my fellow PhDs. :^)
Indeed, I hope that our US Ethanol adventure, which, so far, has only raised food prices and reduced the average MPG on my Prius, while saving little actual energy, may set in place an infrastructure for turning waste biomass from food industries and restaurants and, eventually, homes, into usable fuel, along with plants that may be produced on land unsuitable for other agriculture.
Joel Shore;
Spin it anyway you want Joel. You highly recommended a book which you haven’t read most of, skimmed sections you admit you would need to spend considerable time on to understand, and still want your opinion of the science to be credible with that book as your evidence.
Are you in the field of climate science? Because then presenting evidence not yet evaluated, nor even understood, as proof of a prior conclusion would be standard procedure. I hope none of your students are going into fields where they might be designing things to life and death standards in the real world.
davidmhoffer: Well, I may recommend books that I have only read part of. (I didn’t know that one wasn’t qualified to judge a book as being worthwhile until one had read every word of it.) But at least I don’t pontificate on things that I apparently know very little about, as you seem to do here: http://knowledgedrift.wordpress.com/climate-humour-page-the-climatologist-and-follow-the-money-series/the-physicist-and-the-climatologist-follow-the-money/ (Unless the humorous part is supposed to be imagining a physicist with a limited enough knowledge to actually say what that physicist says.) Perhaps you could actually learn something yourself from Ray’s book?
By the way, since you are so worried about credibility, exactly why are we supposed to believe that your opinion on the science is credible…in fact, more credible than most of the scientists in the field and all the major scientific bodies on the planet? At least I am not making that sort of claim.
…But, at least we agree on igloos.
Joel Shore;
You never change much do you.
1. Which part of “humour” do you not understand? If the punchline didn’t get a laugh out of you, then that’s your problem, a LOT ofother people did. Are you seriously going to try and discredit me by pointing out that a joke I wrote isn’t technicaly accurate?
2. On credibility, just as I suggested to Oliver, if you have a serious well thought out criticism of anything I’ve presented, then say what it is. No sarcasm, no words in my mouth, no ridiculous exageration, no quotes out of context, just the issues. If my opinion differs from that of the bulk of the people in the field then show me where I’m wrong. That’s the flaw in this ridiculous debate is that when confronted with factual explanations of known science, people like you respond with that most cutting of discrediting remarks “oh yeah? well if you’re smart, how come most of the experts disagree with you?”
I don’t know Joel, how come? On what do they disagree and provide a scientific explanation showing that I am wrong and they art right on that specific issue. No 3rd hand references to proxy data, no arm waving about the consensus of this or that, no well you should read this book to see how wrong you are…Just the specifics of the issue.
And no, I don’t think it is credible to recommend a book I’ve only read a portion of and don’t understand. In fact, I don’t even think it is credible to recommend a book I have read and do understand unless I have read several other books on the same topic and have some wider perspective as to how that book lines up with what is generaly available. See Spot Run was the best book I ever read…until I read my second book.
Joel Shore
You said in an earlier post that you taught your students about the second law.
What kind of students?
Physics majors, engineering, climate science or some other background.
What text book did you advise them to read as back up material?
Bryan: It is a course in the introductory algebra-based physics sequence taken by majors from the biological, medical, and environmental sciences, engineering technologists, and the like. We use a standard introductory textbook ( http://www.amazon.com/College-Physics-Strategic-Approach-Workbooks/dp/0321602285/ref=sr_1_1?ie=UTF8&qid=1300189907&sr=8-1 )
davidmhoffer,
Good job skewering Joel Shore as a poseur. I’m enjoying your posts pointing out his appeal to an un-read authority. Joel never has had much credibility here, but I’ll give him one thing, he’s doggedly persistent in the face of contrary evidence that debunks his beliefs. That’s the mark of a True Believer. A Jehovah’s Witness couldn’t do it any better.
Condolences to his students, though.
Joel thanks for the reply.
I still find it odd that you would prefer an expression like;
‘the walls of the igloo heat the eskimo’
to
‘the walls of the igloo insulate the eskimo’
I would have failed my physics exam with such confused use of the word HEAT.
Does this not cause a lot of trouble when you go on to analyse the Carnot Cycle?
Bryan: I don’t think I talked about the igloo problem myself except to say that in that problem the heat flow is still from the hotter object to the colder object. I did endorse davidmhoffer’s basic point about the igloo…i.e., that you would be warmer with the igloo than without it…but that doesn’t mean that I endorsed exactly his phrasing in all of his posts (although, frankly, I couldn’t find the point where he had the phrasing that you claim he had).
I agree that it is potentially confusing to say that the walls heat the eskimo. It is better to say that the walls keep the eskimo warmer than he would be without them, just as the greenhouse gases cause the earth’s surface to be warmer than it would be without them.
davidmhoffer says:
Well, I guess I am confused about the humor then. I thought the joke was supposed to be that the physicist defends the correct way of thinking about the problem until it is pointed out that he could get lots of grant money by saying otherwise, as the climatologist has done. Now, you are telling me that the physicist was wrong and the climatologist right all along? I agree that is essentially the case…but I don’t see how it works for you.
Well, if you are talking about this thread, I think that you have basically been correct (except for the little riff where you talked about why you don’t believe AGW is significant). But, I had no bone to pick with you in this thread until you turned around and attacked me in regards to the comment that I made about Ray’s book. I was simply defending myself from this attack. Why you felt it necessary to attack me in this way, I don’t know.
If you are asking about where your “physicist” is wrong in the thread that I linked to, then I would say he is wrong in arguing that the surface temperature of the earth does not depend on the atmospheric composition. Clearly, it does…as the earth, and to a much larger degree Venus, have surface temperatures much larger than they would have in the absence of an IR-active atmosphere.
As this is not the first discussion of the role of radiation in heat transfer, it’s clear it’s a topic of interest and the science is not settled for everyone.
Had this exchange taken place in person, Joel’s minor gaffe would be barely a hiccup.
Whether or not a dichroic lightbulb reflects IR at itself and thereby increases its temperature and efficiency is more interesting than somebody’s book club choice.
“”””” Oliver Ramsay says:
March 15, 2011 at 8:41 am
As this is not the first discussion of the role of radiation in heat transfer, it’s clear it’s a topic of interest and the science is not settled for everyone.
Had this exchange taken place in person, Joel’s minor gaffe would be barely a hiccup.
Whether or not a dichroic lightbulb reflects IR at itself and thereby increases its temperature and efficiency is more interesting than somebody’s book club choice. “””””
Well once again the explanation is quite wrong; just as is the atmospheric case, with GHGs.
Incandescent light bulbs emit thermal radiation in the visible frequency range, so we can see it, and the higher the Temperature of the filament, the more light it puts out. As a result of all that emitted thermal radiation, the filament would naturally (and rapidly) cool down if you shut off the power to the filament.
The “Dichroic” coating, that reflects the non visible energy being emitted from the filament, has the effect of slowing the exit of the long wave emissions, and also raising the Temperature of the residual gas in the lamp (a necessary ingredient). Meanwhile the filament is still connected to the electricity source, so it continues to input energy at a high rate, so the Temperature of the filament would increase. The Tungsten filament, has a positive Temperature coefficient of resistance, so the higher filament Temperature raises its resistance further, which will reduce the current in that filament, and hence reduce the electrical input power required to maintain a given filament Temperature.
It is the hotter filament running off lower electrical power input, that gives the lamp its higher efficiency. In all of these cases, it is the primary source of energy input; the electrical input, or the solar energy input, or your body chemical energy input, that is the source of the increased Temperature; not the return of some energy that was trying to escape, and let things cool down.
The atmospheric “back radiation” (hate that term) does NOT heat the surface to a higher Temperature; it is the input of solar energy that raises the Temperature of the surface; just as it is your own body heat that keeps you warm when you put a blanket between you and a colder “heat sink”.
Smokey,
Thanks for the compliment, but…Jehovah’s Witness?
Not a fair comparison! I’m as avid a student of religous texts (though not religious) as I am of climate. The JW kids who knock on your door are just kids. I’ve several friends who are JW Elders who know their stuff, and while I may disagree with them, I’ve found that their beliefs are tied to specific passages and logic applied to them. I can respect that a lot more than certain ministers in certain houses of worship in my area who just get angry when asked “where in the book does it actually SAY that?” because they often don’t know, or its a trick question because I already know that what I asked isn’t in the book at all. Try that with an Elder and you’ll get an informed response.
Oliver Ramsey;
Whether or not a dichroic lightbulb reflects IR at itself and thereby increases its temperature and efficiency is more interesting than somebody’s book club choice.>>>
What has dichroic got to do with it? Or a light bulb?
Put an object radiating a steady ten watts inside the igloo. Start it off at the ambient temper of the igloo. Over time, the temperature of the radiating object will rise in comparison to an identical object outside the igloo.
“”””” davidmhoffer says:
March 14, 2011 at 11:19 am
Dave Springer;
re photons and mass
technical omission on my part. Never occurred to me to delve into it as my off the top of my head assumption was that the increase in mass would be completely insignificant. At some point one has to consider that one is splitting hairs already split and split again. E=MC^2 requires that anything travelling at the speed of light have a mass approaching zero for the amount of energy we are talking about on a per photon basis. In fact (now that you jogged my memory) if an object accelerates it gains energy stored as momentum and so its mass must increase accordingly. Any mass of any significance can’t be accelerated to the speed of light without consuming enough energy to build a small galaxy. So the ONLY thing that can achieve the speed of light is a photon because its mass is a limit approaching zero.
I looked at the light sail a long long time ago and came to the same conclusion. The reflected photon must be at a lower wavelength. “””””
Well E=mc^2 simply equates mass and energy; it certainly doesn’t imply that something is travelling at the speed of light.
Remember that the energy of a real particle travelling at a velocity (v) has an energy of mv^2 /2 ; NOT mv^2 .
The (c) is simply the velocity of Maxwellian Electromagnetic waves; = 1/sqrt (mu-naught x epsilon-naught)
You may recall that particle Physicists like annav use a system of units where c = h-bar = 1 , so she equates energy and mass as simply the same thing in two forms, and since h-bar is also one, then energy and frequency are also the same thing in that system of units.
Thank you Joel Shore for what you say on March 14, 2011 at 6:00 am under the title: “O H Dahlsveen says”
I notice from your writing that you don’t think I understand much about “how radiative balance works” which means that I am wrong in saying that if the “Energy Flow Plan” shows 324 W/m² of extra “magical” radiation as being radiated towards the Earth’s surface, as is preferred by AGW believers, I am, according to you, automatically wrong in assuming that the same amount of radiation must also be radiated in all other directions. That must mean, whether you and Trenberth’s plan like it or not, that at least 324 W/m² are also directed towards space or in other words 648 W/m² in total must have been generated by GHGs. The plan however does not show that and I think it is very strange that you say ”— if this diagram was so trivially wrong in the ways you think it is, other scientists in the field would notice that?”
Please tell me Joel, do you know of any other ‘un-shielded’ substance, apart from GHGs, which radiate energy in one direction only? If you do then let me know and I will consider withdrawing the word “magical”.
You further say; “The rule is that the energy flows must balance in the end.” -I agree with that Joel, but then you go on “They do…and this is because the surface receives additional radiation (the so-called “back radiation”) from the atmosphere.” – Joel, it is ‘at the top of the atmosphere’ where incoming and outgoing radiation must balance. So please tell me where radiation from GHGs are hiding the energy. The plan shows quite clearly that W/m² in = W/m² out. Furthermore it also shows GHG radiation as being in perpetual motion between the surface and the GHGs. – That, Joel is why I do not agree with anyone who agree with AGW theories.
Since you ask this next question; “Ah…How do you think the atmosphere gets to be at a temperature such that it emits radiation if not by absorbing some too?” – I shall tell you Joel, in a few simple words how I “think the atmosphere gets to be at a temperature such that it emits radiation if not by absorbing some too?”” as you put it. – Radiation to and from GHGs is not necessarily needed at all. As the Sun’s shortwave radiation strikes the Earth some electro magnetic energy is absorbed by the surface-atoms & molecules which respond by increasing their ‘kinetic heat’. I am certain we both know that the atmosphere is held in place by gravity which means the atmosphere and the planet’s surface are in contact which further means that the surface’s kinetic heat is transferred to the atmosphere by conduction. (It must do so according to The Zeroth Law of Thermodynamics, which is the law that every-one in this debate seems to ignore.) – Subsequently any particular ‘air pocket’ warms up and expands until it becomes lighter than adjacent air, at which time convection takes place. For each 1000 feet of altitude or height the air-pocket gains it looses (on average) 2 °C by adiabatic expansion. Bear in mind that the opposite (heat gain) is applicable to descending air. (Foehn winds or Chinooks)
I’m just curious about something. Do any of the experts here happen to know the average global (direct plus diffuse) clear-sky solat irradiance measured at the surface, or the average clear-sky measured solar reflected? I only ask because it seems to have a direct bearing on the “global energy budget” diagram.
O H Dahlsveen says:
Yes…You are wrong although the reason is a bit subtle, so let me explain why. For an individual emission event, the radiation will be isotropic. However, when you consider the sum total of all such events in an “optically-thick atmosphere”, it does not end up being isotropic when the IR radiation initially enters the atmosphere from below. To understand this, let’s take an extreme example and assume that the atmosphere is so optically thick that most of the radiation suffers its first absorption and re-emission in the first 100 meters near the surface. Then, for the subsequent re-emission, many of the photons emitted toward the surface make it back toward the surface without being absorbed again (because they are getting emitted so close to the surface). However, very few that are emitted upwards can make it out of the atmosphere without an additional absorption. After this additional absorption, again some will go down and some up.
In the end, what you have mathematically has some name like a “random walk first passage problem” and in such a problem the amount of radiation that makes it back to the surface before ever managing to escape from the atmosphere is (in the optically thick limit) much more than half of the radiation emitted by the surface.
Mind you, it is not surprising that you might not be familiar with this result and I don’t fault you for being confused on the point. However, I do fault you for jumping to the conclusion that you must be right and the scientists who have thought way more about this than you (and have checked it against actual measurements) are wrong. That is not a very mature way to approach a scientific issue.
Yes, radiative balance between the Earth, sun, and space will be maintained even as you ramp up GHGs (although if you ramp them up quickly enough, you can be a little out of balance…on the order of fractions to a few W/m^2…because of the large thermal inertia of the oceans). However, the question is how this radiative balance is maintained and the answer is that it is maintained by an increase in the steady-state surface temperature. So, in the end, after greenhouse gas levels stabilize, the earth is going to be back in radiative balance but at a new higher temperature. After all, even Venus is in radiative balance with the sun and space.
And, I don’t understand your point about perpetual motion. This is a dynamic system with energy constantly being supplied from the sun. Noone has ever claimed that “perpetual motion” is impossible in an open system where energy is constantly being supplied.
Fine…So, you have just explained how the lapse rate gets established in an atmosphere heated from below (because most of the solar energy passes through the atmosphere). However, if you think that somehow proves that you can get a surface temperature higher than radiative balance would imply without invoking greenhouse gases then you will be disappointed. If the atmosphere were transparent to IR radiation, then the average temperature of the surface would, be radiative balance, have to be what the solution of the Stefan-Boltzmann Equation says it has to be for incoming and outgoing radiation to balance. Yes, the temperature might decrease as you move up in the atmosphere…but the boundary condition provided by radiative balance would be on the surface temperature in that case (because that would be where the radiation escaping to space is being emitted from).
(If the earth’s atmosphere was undergoing gravitational collapse then you could have a constant source of energy but I don’t think you are seriously trying to argue that this is the case?)
George E. Smith says:
March 15, 2011 at 9:26 am
It is the hotter filament running off lower electrical power input, that gives the lamp its higher efficiency. In all of these cases, it is the primary source of energy input; the electrical input, or the solar energy input, or your body chemical energy input, that is the source of the increased Temperature; not the return of some energy that was trying to escape, and let things cool down.
I have to disagree George, it is the presence of the dichroic coating that causes the filament to be hotter, without that coating the filament will be colder.
O H Dahlsveen: If my explanation of the random walk and how it works still confuses you, here is a much simpler random-walk case that you can simulate yourself:
Consider the x-axis. Start at zero and then jump to 1 (that is analogous to the original emission from the surface and then the first absorption by the atmosphere). On each following step, choose a random number (or flip a coin) and jump backward and forward with equal probability (that corresponds to a photon being emitted up or down in the atmosphere with equal probability). Continue doing this until you either get back to zero or you get to some pre-determined number (such as 10) and then stop (that corresponds to either having the radiation return to earth or having it escape the atmosphere). If you do this many times, you will see that you end up back at 0 much more often than you end up at the pre-determined number, and the larger the pre-determined number that you choose (which corresponds to making the atmosphere optically thicker), the greater will be the imbalance.
George E. Smith says:
March 15, 2011 at 9:26 am
“It is the hotter filament running off lower electrical power input, that gives the lamp its higher efficiency. In all of these cases, it is the primary source of energy input; the electrical input, or the solar energy input, or your body chemical energy input, that is the source of the increased Temperature; not the return of some energy that was trying to escape, and let things cool down.”
———————-
Thank you, George.
It seems one could have the dichroic filter behind or in front of the filament, transmitting visible and reflecting IR, or vice versa.
Is the principal purpose to reduce heating of the illuminated area and fixture or, indeed, to maintain gas temperature?
“”””” Phil. says:
March 15, 2011 at 1:13 pm
George E. Smith says:
March 15, 2011 at 9:26 am
It is the hotter filament running off lower electrical power input, that gives the lamp its higher efficiency. In all of these cases, it is the primary source of energy input; the electrical input, or the solar energy input, or your body chemical energy input, that is the source of the increased Temperature; not the return of some energy that was trying to escape, and let things cool down.
I have to disagree George, it is the presence of the dichroic coating that causes the filament to be hotter, without that coating the filament will be colder. “””””
Well Phil, I don’t disagree that without the dichroic mirror, the filament will be colder; I think I even said so; but it is so because the dichroic mirror prevents the escape of some radiation, so the filament can’t cool as fast.
If you were to coat the lamp instead with a broad band high reflectance coating, so that none of the radiation could escape; why not put a vaccum around the whole thing as well so the bulb can’t cool by conduction/convection. The filament would get hotter yet at even lower input current (at the same Voltage).
Sans the electrical energy input, it isn’t going to get very hot at all.
I agree that the dichroic coating works to make the lamp more efficient; but it is the elctricity that is being converetd to radiant energy that is visible, the coating simply stops it from unnecessarily cooling by radiation of “useless” non visible frequencies.
It’s the same as dichroically coated solar thermal collectors. The sun provides the energy to heat the surface and thereby the working fluid (water or whatever inside ); the coating simply stops the emission of energy in the form of LWIR radiation that would allow the surface to cool.
I think we may have to arm wrestle on this one Phil .
George E Smith;
Remember that the energy of a real particle travelling at a velocity (v) has an energy of mv^2 /2 ; NOT mv^2 .>>>
George, its been a long time since I worked my way through E=MC^2. Here’s my recollection as it pertains to this discussion:
The energy E in a given mass is = MC^2
BUT
Since we can CHANGE E by accelerating the mass from a velocity of 0 to a velocity of say 1/2 C, the energy in the mass has now increased by 1/2 M V^2 where V= 1/2 C
Without doing the arithmatic, the point is, E contained in the mass went up a whole big bunch. For the equation to balance, since C is a constant, my understanding was always that Mass increased accordingly.
Since the mass keeps on increasing, the amount of energy required to accelerate it to the speed of light goes outa sight..so to speak…and thus the only things that can actually GO the speed of light are things like photons which have a mass of near zero in the first place.
From my copy of Einstein’s theory from which I cannot cut and paste the forumuls just the text:
“This expression approaches infinity as the velocity v approaches the velocity of light c. The velocity must therefore always remain less than c, however great may be the energies used to produce the acceleration”
Jim Masterson say on March 13, 2011 at 10:09 pm: “There are problems with Kiehl and Trenberth 1997, but the points you raised aren’t the ones. The 66 W/m² is the net flow from the surface or 390 W/m² – 324 W/m² = 66 W/m²”
I do understand what you are saying as we are both saying the net flow of radiation from the surface is 66 W/m². (390 – 32 4= 66) However you further say: “You can’t subtract the 40 W/m² from the 66 W/m² because the amount escaping through the atmospheric window comes from the total surface radiation of 390 W/m² or 390 W/m² – 40 W/m² = 350 W/m²”
I cannot see where else it can be subtracted from but from the 66 W/m² surface radiation as that is the only IR long-wave radiation that can possibly contain wave-lengths which GHGs cannot absorb. The term “atmospheric window” means simply “passage through GHGs for IR wave-lengths which cannot be stopped by GHGs” – That must therefore mean that the circuit set up by 324 W/m² from GHG back- radiation is closed in and cannot escape from the earth system.
However the questions that bug me still remain:
1) Where do the 324 W/m² of “back radiation come from? – Or in your case
350 W/m²?
2) Why does radiation from GHGs (in the Energy Flow Chart) only flow in one direction. (towards the Earth)?