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Re-radiation
#1
Alan Siddons sent this to me by e-mail.

This below is the entire presentation:
========================================================

"One is disturbed each day by verifiably untrue statements touted as incontrovertible facts about hot-button issues." -- Richard S. Lindzen

"CO2 absorbs in the infrared and reradiates heat downward, thus heating the earth." -- Richard S. Lindzen


Okay then, let's examine that particular "incontrovertible fact." You know how a solar oven works.

[Image: s1.gif]

By multiple reflections, the interior is exposed to more rays from the sun, so the food gets much hotter than it'd get otherwise. The operant principle is akin to how stage lighting works.

[Image: s2.gif]

In the zone where the beams intersect, the photon density is greater so more light is delivered. It may be obvious, but it's worth pointing out that the two beams pass through each other — they do not clash like the Light Sabers in Star Wars movies. Worth noting too is that whereas more light makes the target brighter, i.e., increases the amount of light being reflected, it also increases the amount being absorbed, thus heating the target. Which of course is how a solar oven works.

Now, you’ve been told that terrestrial infrared is re-radiated back to the earth’s surface and heats it. So let’s test this notion by turning a spotlight off and seeing if we can mimic a second spotlight with a mirror, which will provide re-radiation. After all, a mirror has no idea of what it’s reflecting — it could be visible light or infrared 'heat rays.' It makes no difference to a mirror.

Well, is it any surprise that nothing happens? Reflecting light back to the bright spot doesn’t make it brighter. (You can try this at home on a wall.)

[Image: s3.gif]

Close as you hold the mirror to the bright spot, there’s no effect. You might notice, though, that offsetting the mirror a bit can illuminate a zone that’s in shadow. In this case, light reflected from the bright spot brightens a darker area. But the mirror cannot make the bright spot brighter.

The lesson I draw from this real-world example is that radiant energy can only light something that has less radiance. Brighter illuminates darker. You can get a sense of this by omitting the spotlight altogether and just imagining a surface radiating light on its own. Holding a perfect mirror directly above it, you'd see something like this:

[Image: s4.gif]

There's no difference between them, and it shouldn't need explaining that the mirror image is not illuminating the very object that it's reflecting. But if the mirror isn't illuminating that object, the mirror isn't heating it either. To do that, the image in the mirror would have to be brighter, like the blazing image a solar oven provides. The same principle applies to conductive heat transfer: Higher temperature matter heats lower temperature matter. At equal temperature no heating occurs. Directing an object's own radiance back to it, then, doesn't do a thing and cannot make it more radiant. But from this proceeds another ramification.

A perfectly reflective 'white body' is the opposite of a blackbody. It can't be heated by radiative means. So suppose a mirror reflects a tiny amount of energy onto a white body.

[Image: s5.gif]

In the scenario above, 1 unit of light hits the surface yet a total of 1.1 units are reflected, thus creating energy out of nothing and contradicting the 1st law of thermodynamics. Since this is impossible, it demonstrates that second-hand reflected light cannot be regarded as part of the object’s "radiation budget." Indeed, this reflection is only the object's own radiance in another guise, not an additional source of light.

Now consider a blackbody.

[Image: s6.gif]

Without the mirror, a perfectly absorptive/emissive object would release one unit of light in response to one unit of irradiance. But if the blackbody absorbs another tenth from the mirror, then it's compelled to emit 1.1 units in all. To repeat, this is impossible. So this indicates that re-radiated infrared cannot be regarded as part of an absorbing object’s "radiation budget" either, or else the 1st law is wrong. This re-radiated infrared is only the object's own thermal emission in another guise, not an additional source of heat.

Putting these results together, one must conclude that "back-radiation" of any sort, whether visible or infrared, will simply not show up on an external radiometer because it represents no "extra" energy in the first place and it induces no illuminative or thermal effect. Re-radiated energy is neither reflected nor absorbed by a surface, therefore, because either outcome would point to additional energy that would register on a detector, signaling that more radiant energy is leaving the system than what is going in.

Summing up, re-radiated light has as much chance of enhancing surface energy as you have of standing in a bucket and lifting it off the ground. In other words, it can't be done. Following a principle similar to conductive heat transfer, and also similar to electrons needing a lower electrical potential in order to move, radiant energy can exert an effect only on something that is radiating less. Radiant energy is like water; it can only flow downhill.

The principle I’m invoking is very simple: A body cannot be illuminated by its own light.

I chose a mirror in this essay because it puts back-radiation on the strongest possible footing. The radiant energy reflected off a mirror goes entirely in one direction, unlike re-radiating gases which emit in all directions, and this reflected energy is many times greater than a gas is able to absorb/reemit. Yet even a mirror adds no radiance to the object whose radiance it is reflecting. This alone is sufficient to prove that re-radiation from trace gases cannot heat the earth.

Alan
It is our attitude toward free thought and free expression that will determine our fate. There must be no limit on the range of temperate discussion, no limits on thought. No subject must be taboo. No censor must preside at our assemblies.

–William O. Douglas, U.S. Supreme Court Justice, 1952
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#2
I'm (edit to correct poor grammar) saving that one. Well done. I couldn't see the images provided however.
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#3
(06-25-2010, 08:28 PM)Goose52 Wrote: I saving that one. Well done. I couldn't see the images provided however.

Ok.

I will try the images again from another place.

They make all the difference to see them!
It is our attitude toward free thought and free expression that will determine our fate. There must be no limit on the range of temperate discussion, no limits on thought. No subject must be taboo. No censor must preside at our assemblies.

–William O. Douglas, U.S. Supreme Court Justice, 1952
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#4
Can anyone see the images?
It is our attitude toward free thought and free expression that will determine our fate. There must be no limit on the range of temperate discussion, no limits on thought. No subject must be taboo. No censor must preside at our assemblies.

–William O. Douglas, U.S. Supreme Court Justice, 1952
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#5
I can see them now. What a splendid and simple presentation. Thank you.
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#6
Yes dear I see them.

Quote:Summing up, re-radiated light has as much chance of enhancing surface energy as you have of standing in a bucket and lifting it off the ground. In other words, it can't be done. Following a principle similar to conductive heat transfer, and also similar to electrons needing a lower electrical potential in order to move, radiant energy can exert an effect only on something that is radiating less. Radiant energy is like water; it can only flow downhill.

Actually, another way of looking at this is the coefficient of energy, which is at it's perfect beat with a rating of "1". Realistically, it is just hoping for too much to expect to achieve that anywhere. And when we actually discover the way to get it's maximum efficiency, we will have discovered the perfect energy source.

In other words, you cannot keep expecting light energy, reradiated, to keep adding to the coefficient. It just doesn't work that way in real life. I don't know if I am making sense here, but I learned this in science class, back in the fifth, or sixth, grade.
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#7
I'm not so sure I'm in agreement with this.

Quote:In this case, light reflected from the bright spot brightens a darker area. But the mirror cannot make the bright spot brighter.

Since it "brightens a darker area", we can get an idea of the area it is effecting. Just because we can not "see" it on the bright spot does not mean that it isn't hitting that bright spot and having a measurable effect, albeit not visible.

Remember, we can't "see" (with the naked eye) what is happening in the IR range. I suspect that instruments measuring IR could detect the difference.

Just making an observation.
I know you think you understand what you thought I said,
but I'm not sure you realize that what you heard is not what I meant!


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#8
(06-26-2010, 05:58 AM)JohnWho Wrote: I'm not so sure I'm in agreement with this.

Quote:In this case, light reflected from the bright spot brightens a darker area. But the mirror cannot make the bright spot brighter.

Since it "brightens a darker area", we can get an idea of the area it is effecting. Just because we can not "see" it on the bright spot does not mean that it isn't hitting that bright spot and having a measurable effect, albeit not visible.

Remember, we can't "see" (with the naked eye) what is happening in the IR range. I suspect that instruments measuring IR could detect the difference.

Just making an observation.

I think the point made was there is no actual INCREASE from the reflection of the light,just spread out over a larger area.Thus no measurable increased Visible light is occurring.

You can not get additional energy from it.
It is our attitude toward free thought and free expression that will determine our fate. There must be no limit on the range of temperate discussion, no limits on thought. No subject must be taboo. No censor must preside at our assemblies.

–William O. Douglas, U.S. Supreme Court Justice, 1952
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#9
I used to have large telescopes with concave mirrors in them.One time with a 11" Celestron F10 Schmidt Cassegrain,I burned up the thick plastic cap that overs the 2" hole at the other end of the telescope from the secondary reflector.

It was possible because visible light of the sun was being concentrated by the two mirrors to a small area causing the damage.If it was spread out then no damage would be possible because then the surface area would have the same energy spread out in a larger area (diffusion).But the total amount of visible light reflected would be the same.

I was viewing the sun with a solar filter at the time and was breaking down the scope to put it away.The reason why the 2" cap was put on.I had not yet capped the other end of the scope.It was possible to set the house on fire if left unattended long enough!
It is our attitude toward free thought and free expression that will determine our fate. There must be no limit on the range of temperate discussion, no limits on thought. No subject must be taboo. No censor must preside at our assemblies.

–William O. Douglas, U.S. Supreme Court Justice, 1952
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#10
(06-26-2010, 05:58 AM)JohnWho Wrote: I'm not so sure I'm in agreement with this.

Quote:In this case, light reflected from the bright spot brightens a darker area. But the mirror cannot make the bright spot brighter.

Since it "brightens a darker area", we can get an idea of the area it is effecting. Just because we can not "see" it on the bright spot does not mean that it isn't hitting that bright spot and having a measurable effect, albeit not visible.

Remember, we can't "see" (with the naked eye) what is happening in the IR range. I suspect that instruments measuring IR could detect the difference.

Just making an observation.

JW, let's say the initial concentration of light, put out a 75% efficiency ratio, compared to the total amount of energy emitting from the sun. That would leave a 25% efficiency remaining. Now, that other result would be another portion of the remaining 25%, probably 10%. But it will never reach that perfect 100% efficiency. That is why there is a slight increase in light, but nothing akin to the original.
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#11
What I'm trying to say is this:

Suppose the incoming light or radiation is "X".

In this case, something less than X is reflected back. We'll call this SLTX.

It then is re-reflected by a mirror in the example. What is reflected by the mirror is even less than SLTX (ELTSLTX), but still measurable.

So, the primary surface is now receiving light or radiation of X + ELTSLTX.

Perhaps we can't, in the example, actually see the new total of X + ELTSLTX, but it is certainly there (and some of this total is radiated back as well).


In relation to our climate, however, what is the salient point here?

Is it that the surface of the earth may be slightly warmer or is it that the atmosphere might be a little warmer?

More specifically, the question becomes whether the small amount of CO2 in the atmosphere makes any appreciable difference in this concept AND whether the extremely slight possible increase in the CO2 level each year makes any measurable difference in this concept?

Another question I might have that must have been discussed before is how much of the Sun's incoming radiation reaches the earth's surface directly? Some must be absorbed by the atmosphere on the way in (down), just as some gets absorbed on the way out (up). The parts of the atmosphere that are involved in this process will then re-radiate in all directions, do they not?

Just speaking about atmospheric CO2, does it absorb all of its possible amount from the incoming solar radiation? If so, then how is it "trapping" heat when, in fact, it is really stopping the heat from reaching the surface in the first place?

Just wondering.
I know you think you understand what you thought I said,
but I'm not sure you realize that what you heard is not what I meant!


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#12
(06-25-2010, 06:07 PM)Sunsettommy Wrote: Alan Siddons sent this to me by e-mail.

This below is the entire presentation:
========================================================

"One is disturbed each day by verifiably untrue statements touted as incontrovertible facts about hot-button issues." -- Richard S. Lindzen

"CO2 absorbs in the infrared and reradiates heat downward, thus heating the earth." -- Richard S. Lindzen
Lindzen would seem to be a scientist who has studied this subject and yet he can make this statement? I had no idea that the radiation from gas molecules was affected by gravity, yet this is what he seems to be implying. Why should the radiation only be downwards? If this were true then there would be no radiation upwards to space. The atmosphere would lose no heat and we'd fry. The reason for top-of-atmosphere radiation to space has to be that the air is so thin that there's little chance for upward radiation to be re-absorbed by air molecules and so escapes to space.

Much closer to the surface, gases still radiate in all directions; there is no explanation why they should do otherwise. Any such radiation is almost certain to be re-absorbed. If the atmosphere is "largely opaque" to long-wave infrared, and it's generally accepted that it is, then it has to be opaque in all directions. Down-going radiation will be absorbed just as upgoing radiation from the surface was absorbed.

I don't buy the "saturation" idea either. Saturation might be observed in the laboratory in a container centimetres long containing a rarefied gas, but in the real world, they have to travel tens of kilometres to reach space. It isn't logical.

There are two schools of thought as to the mechanism of "re-radiation". One is that the GHG (I use the term only because others do; I don't subscribe at all to the "greenhouse" theory) molecules absorb the IR in the molecular bonds and re-radiate later, thus losing their acquired "extra" energy. The problem with this theory is that they don't convert the acquired energy to kinetic energy, and so don't heat up. The re-radiated energy is only weakly absorbed by non-GHGs so the atmosphere
would lose more energy to space than is observed. In effect the enrgy would "bounce around" between GHG molecules with some of it reaching the surface.

However, one point seems to have escaped most scientists and those discussing the science. While nitrogen and oxygen absorb (and therefore radiate) very weakly in longwave IR, their concentrations are tens of times that of water vapour, thousands of times more than CO2. For example, the absorption of nitrogen is around 3000 time less than CO2, but it's 2000 times more abundant in the atmosphere, so it's effect is around 2/3 that of CO2. Oxygen IS included in the climate models, yet nitrogen is not.

The other mechanism is that they absorb and then convert the extra bond-energy to motion, so heating up, and radiate IR at the corresponding increased level. Unfortunately for both mechanisms, the probability that the energised molecules will collide with other air molecules is very close to 1; they'll give up their extra energy by collision, so warming the entire atmosphere.

On the subject of the conundrum of a cooler atmosphere heating a warmer surface, I've discussed it elsewhere.
Ernest Rutherford: "If your experiment needs statistics, you ought to have done a better experiment."
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#13
Not entirely irrelevant to this thread is the post I just added to the "Do Global Energy budgets make sense. ???" thread here. The "back radiation" is shown as totally absorbed by the surface - where is the albedo effect?
Ernest Rutherford: "If your experiment needs statistics, you ought to have done a better experiment."
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#14
(06-26-2010, 06:43 PM)MostlyHarmless Wrote:
(06-25-2010, 06:07 PM)Sunsettommy Wrote: Alan Siddons sent this to me by e-mail.

This below is the entire presentation:
========================================================

"One is disturbed each day by verifiably untrue statements touted as incontrovertible facts about hot-button issues." -- Richard S. Lindzen

"CO2 absorbs in the infrared and reradiates heat downward, thus heating the earth." -- Richard S. Lindzen
Lindzen would seem to be a scientist who has studied this subject and yet he can make this statement? I had no idea that the radiation from gas molecules was affected by gravity, yet this is what he seems to be implying. Why should the radiation only be downwards?

I'm curious - does Lindzen say "only downward" or is he simply saying that some reradiation is downward?

Just asking.
I know you think you understand what you thought I said,
but I'm not sure you realize that what you heard is not what I meant!


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#15
(06-30-2010, 11:50 AM)JohnWho Wrote: I'm curious - does Lindzen say "only downward" or is he simply saying that some reradiation is downward?

Just asking.
The emphasis would appear to be on the word "downward". I'd suggest he means there's a "net downward" radiation. To me this seems intuitively wrong. See the last page of this thread for my take on it. He appears to be agreeing that the "energy budget" diagrams are essentially correct, which that thread dissects.
Ernest Rutherford: "If your experiment needs statistics, you ought to have done a better experiment."
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#16
Perhaps, in the scheme of things, it is only the downward reradiation that is important.

The upward reradiation essentially would have gone in that direction anyway.

The sideways reradiation may warm nearby CO2 (in this case) which, in turn, will reradiate in all directions, again some downward.

I agree that logically any reradiation would go in all directions. But only that which goes down would either add to the surface temperature or slow down the cooling of the surface temperature the way I understand it.

I, too, am not in agreement with Lindzen's statement:

"CO2 absorbs in the infrared and reradiates heat downward, thus heating the earth."

because it misleads somewhat (else we wouldn't be having this discussion).

Especially since, I believe, in some cases it may not actually "heat the earth", it may only slightly slow down the cooling of the earth.
I know you think you understand what you thought I said,
but I'm not sure you realize that what you heard is not what I meant!


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#17
(06-30-2010, 07:47 PM)JohnWho Wrote: The sideways reradiation may warm nearby CO2 (in this case) which, in turn, will reradiate in all directions, again some downward.
That sideways re-radiation effectively cancels out. Don't forget that when bodies (molecules in fact) radiate, they are losing energy. There can be no net gain in any direction within the bulk of the atmosphere. The molecules are merely exchanging energy. We are not dealing with a surface here,
Quote:I agree that logically any reradiation would go in all directions. But only that which goes down would either add to the surface temperature or slow down the cooling of the surface temperature the way I understand it.
It can't warm the surface as the surface is radiating more energy upwards than the atmosphere can radiate downwards. But where does that downward radiation come from? Consider this:
^
--------------------------------
v^
--------------------------------
^ v
--------------------------------
v

This represents three thin layers (horizontal lines) with upward ^ and downward v radiation. Each up-or-down arrow represents half of the radiation from a layer. You can see that the middle layer is receiving as much upwards as downwards. It therefore neither gains nor loses net energy. The upper layer has an unbalanced flow upwards and the lowest an unbalanced flow downwards. For a "vertical" layer on the other hand,, there is always another "vertical" layer to the left and right. Flows are balanced and therefore cancel out.

This picture does not chime with the energy flow diagrams, nor with the concept (implied therein) of the "top-of-atmosphere" only radiating to space, yet the entire atmosphere radiating downwards to the surface

You should read the threads The Greenhouse Effect - A Few Cracked Panes and Do Global Energy budgets make sense. ???.
Ernest Rutherford: "If your experiment needs statistics, you ought to have done a better experiment."
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#18
There is a series of posts on the "Hockey Schtick" discussing (at great length) the supposed myth of "back radiation" from a cooler body to a hotter one. The illustrated examples are supposed to prove that the greenhouse effect is a myth also. The arguments by Alan Siddons and Claes Johnson "proving" that cold bodies do not emit any radiation towards hotter bodies are simplistic and flawed. From the latest post AGW is Science Fiction Hiding Behind False Computer "Models":

Quote:The Sun heats the Earth and the Earth heats the atmosphere
...heat energy CANNOT flow from Cold to Warm objects

No one has said that it can, to my knowledge.

Quote:If Back Radiation actually reached and heated the Earth as Trenberth shows, then Parabolic Mirror Solar Ovens would produce heating Day and Night

I agree with that.

Quote:IT IS “NOT POSSIBLE” FOR BACK RADIATION FROM A COLDER ATMOSPHERE TO HEAT UP A WARMER EARTH

No one, certainly not Kiehl & Ttrenberth have claimed this. What they are saying is that a cooler body (or surroundings) slows the cooling of a nearby body (earth's surface). If you place a hot body (nice cuppa char) in a refrigerator, it will cool down faster than if you leave it on your kitchen worktop. If you leave it next to the freshly boiled kettle, it will cool down slower still. The cooling rate depends on the temperature difference between the cuppa and its surroundings. The first and second laws of thermodynamics define the process, and the Stefan-Boltzmann law quantifies it for emitted radiation.

The law is usually misquoted with just a single temperature term, but this is a special case where the "surroundings" are at absolute zero, and so the second term becomes zero and can be omitted. The law simply defines radiation flow as the difference between the fluxes from two adjacent bodies, or a single body and its surroundings. Two bodies means two fluxes. Try to prove that a flux from the cooler body doesn't exist, has no effect at all, and you're denying that law.

"Thought experiments" done using diagrams showing light reflected from a surface to a mirror then back to the surface (as at the start of this thread) are totally irrelevant. The light is not being radiated by the surface or the mirror, but being reflected. To radiate white light, the illuminated surface would have to be at 5-6000°K.

Neither are "thought experiments" involving so-called "white bodies", again as in this thread. In this case, the diagram is totally misleading - note that there are two arrows shown. One arrow shows 1.1 units reflected from the surface past the mirror, yet there is a mysterious second arrow showing light hitting the mirror. Any light reflected from the surface must be the sum of the two arrows.

[Image: s5.gif]

If it's a perfect mirror, then it's intercepting 0.1 units from the surface (leaving 0.9 to bypass the mirror) and reflecting 0.1 units back. There is no net flow of light energy between the mirror and the surface. The radiation (light in this case) fluxes cancel out. That's why you can't increase the illumination of a surface with light reflected from a mirror illuminated by that surface. However, the surface still reflects 1 unit from the source.

From Why Greenhouse Theory Violates the 2nd Law of Thermodynamics:

Quote:"energy from the hotter body has left that body - it therefore cools down. The smaller amount of energy from the cooler body cannot fully replace that lost energy; it just slows the rate of cooling."

I seem to recognise that statement - it's from The Greenhouse Effect - A Few Cracked Panes - I can modestly report.

The "thought experiment" discussed therein involving a heated radiator and a colder body is relevant, but the description and analysis of the processes involved are incorrect.

Quote:I always see that weasel word: Toward. No doubt a cooler object does radiate "toward" a warmer object. The question is what happens as a result.

[Image: radiator.gif]

Quote:Say you have a blackbody plate (think of an electric heater) radiating 1000 W/m² toward another plate which, because of distance, absorbs half of that intensity, i.e., 500 W/m². At equilibrium, the receiving plate thus radiates 250 W/m² toward the 1000 W/m² plate. Question: Does the 1000 W/m² plate thereby rise to 1250 W/m²? If so, then, by raising the radiator’s temperature without adding more energy, you’ve disproved the first law of thermodynamics. Effectively, you’ve made the radiator heat itself. Moreover, now at 1250 W/m², the radiator will heat the other plate still more, absorb another dose of back-radiated energy, and will reach 1562 W/m². And so on, ad infinitum.


Either radiative heat transfer obeys the 2nd Law or it does not. If the 2nd Law does hold, then light can only transfer energy to something that is radiating less. This would mean that the physics of radiative forcing is fictitious.

"Question: Does the 1000 W/m² plate thereby rise to 1250 W/m²?" - no it doesn't. Alan Siddons is confusing radiation with heat content and therefore temperature. To increase the radiation from the heated plate to 1250 W/m² requires that its temperature be raised to that which will radiate that amount. We don't know the specific heat of the heated plate, nor its size. Using the figures of 1000 and 500 gives 91.3°C and 33.3°C, with the net flow of 500 W/m² from hot to cold, as chance would have it, so the system would stabilise at that. Note that the remaining 500 W/m² from the heated plate is bypassing the cooler plate entirely - it plays no part in the system.

So his thought experiment doesn't violate the second law, you don't get "runaway warming", and the second plate has an influence on the heated plate. There's no substitute in any thought experiment for actually doing the sums.
Ernest Rutherford: "If your experiment needs statistics, you ought to have done a better experiment."
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