R. W. Wood: Note on the Theory of the Greenhouse

This has been on my website http://www.wmconnolley.org.uk/sci/wood_rw.1909.html for some time now; but websites are so tedious to update. So I think I’ll copy it here; more may follow. And it will distract the squabbling children. Note that as of now, this is the maintained copy; the version on my website is now longer “live”.

R. W. Wood: Note on the Theory of the Greenhouse

The following text is from the Philosophical magazine (more properly the London, Edinburgh and Dublin Philosophical Magazine (and Journal of Science?); its name has morphed since), 1909, vol 17, p319-320. Cambridge UL shelfmark p340.1.c.95, if you’re interested.

I found this reference by reading “History of the greenhouse effect”, M. D. H. Jones and A. Henderson-Sellers, Progress in physical geography, 14, 1 (1990), 1-18. This, in its turn, I found from Jan Schloerer’s FAQ: Climate change: some basics.

I present the full text, although the second-to-last paragraph is (in my opinion) regrettable and wrong. See after the text for why I think its wrong.

XXIV. Note on the Theory of the Greenhouse

By Professor R. W. Wood (Communicated by the Author)

THERE appears to be a widespread belief that the comparatively high temperature produced within a closed space covered with glass, and exposed to solar radiation,
results from a transformation of wave-length, that is, that the heat waves from the sun, which are able to penetrate the glass, fall upon the walls of the enclosure and raise its temperature: the heat energy is re-emitted by the walls in the form of much longer waves, which are unable to penetrate the glass, the greenhouse acting as a radiation trap.

I have always felt some doubt as to whether this action played any very large part in the elevation of temperature. It appeared much more probable that the part played by the glass was the prevention of the escape of the warm air heated by the ground within the enclosure. If we open the doors of a greenhouse on a cold and windy day, the trapping of radiation appears to lose much of its efficacy. As a matter of fact I am of the opinion that a greenhouse made of a glass transparent to waves of every possible length would show a temperature nearly, if not quite, as high as that observed in a glass house. The transparent screen allows the solar radiation to warm the ground, and the ground in turn warms the air, but only the limited amount within the enclosure. In the “open,” the ground is continually brought into contact with cold air by convection currents.

To test the matter I constructed two enclosures of dead black cardboard, one covered with a glass plate, the other with a plate of rock-salt of equal thickness. The bulb of a themometer was inserted in each enclosure and the whole packed in cotton, with the exception of the transparent plates which were exposed. When exposed to sunlight the temperature rose gradually to 65 oC., the enclosure covered with the salt plate keeping a little ahead of the other, owing to the fact that it transmitted the longer waves from the sun, which were stopped by the glass. In order to eliminate this action the sunlight was first passed through a glass plate.

There was now scarcely a difference of one degree between the temperatures of the two enclosures. The maximum temperature reached was about 55 oC. From what we know about the distribution of energy in the spectrum of the radiation emitted by a body at 55 o, it is clear that the rock-salt plate is capable of transmitting practically all of it, while the glass plate stops it entirely. This shows us that the loss of temperature of the ground by radiation is very small in comparison to the loss by convection, in other words that we gain very little from the circumstance that the radiation is trapped.

Is it therefore necessary to pay attention to trapped radiation in deducing the temperature of a planet as affected by its atmosphere? The solar rays penetrate the atmosphere, warm the ground which in turn warms the atmosphere by contact and by convection currents. The heat received is thus stored up in the atmosphere, remaining there on account of the very low radiating power of a gas. It seems to me very doubtful if the atmosphere is warmed to any great extent by absorbing the radiation from the ground, even under the most favourable conditions.

I do not pretend to have gone very deeply into the matter, and publish this note merely to draw attention to the fact that trapped radiation appears to play but a very small part in the actual cases with which we are familiar.

Why is his second to last paragraph wrong?

Firstly, note that unlike the earlier paragraphs which describe the results of experiments, this paragraph merely expresses his opinion.

Second, although the troposphere is subject to convection, the stratosphere is not.

Third, in contradiction to his assertion about “the very low radiating power of a gas”, the troposphere is largely opaque to infra-red radiation, which is why convection is so important in moving heat up from the surface. Only in the higher (colder) atmosphere where there is less water vapour is the atmosphere simultaneously somewhat, but not totally, transparent to infra-red and thus permits radiation to play a part.


* Note sur la th̩orie de la serre, par R.W. Wood РFrench translation, refers back to my web page.
* Someone really really dumb wrote a paper saying that because the greenhouse effect is a misnomer, it couldn’t possibly warm the earth. Mercifully I’ve forgot who. Can someone remind me?
* What appears to be the journal website

16 thoughts on “R. W. Wood: Note on the Theory of the Greenhouse”

  1. I analysed this experiment very closely.

    The important fact, in my analysis, is that the glass plate placed above the rock-salt is at ambient temperature, and therefore itself radiates considerable energy in the IR spectrum. As the temperature difference between the glass plate over the box with a rock-salt lid and the glas lid of the other box is small, any “greenhouse effect” would also be small.

    A repeat of the experiment in which both boxes where kept in a vacum to prevent ambient air from cooling the lids of the two boxes, and in which the glass plate used to screen IR radiation from the sun (and atmosphere) was refrigerated so as to not itself be a significant source of IR would, in my view, show a much greater temperature in the box with the glass lid.

    [closely: but not quite closely enough. Read it again: first the two enclosures were tested without a plate above either. In this situation, the salt plate keep[s] a little ahead of the other. Then,the sunlight was first passed through a glass plate. IOnce this is done, There was now scarcely a difference of one degree between the temperatures of the two enclosures. So the presence of the glass plate above both enclosures is not particularly important.

    However, should you do the experiment so please report the results -W]


  2. While convection in the stratosphere is not enough to maintain the adiabatic lapse rate in the presence of solar heating of Ozone and direct radiation loss to space (and in fact has a reverse lapse rate), there still is some convection. This can be seen by the CO2 concentration tracking reasonably soon as it increases at lower levels. The stratosphere convection is mainly driven by local bulging from day to night and strong upthrusts from strong uprising storm clouds causing lateral currents and some mixing.


  3. Leonard, while there are turbulent currents in the stratosphere that do mix gasses fairly thoroughly, I do not think it is correct to call them convection, which is a particular type of air flow driven by temperature differences. Those eddy currents can mix gasses through the stratosphere in a matter of weeks, but are sufficiently slow that radiation is still the dominant form of energy transfer in the stratosphere.


  4. It seems to me that at the heart of every discussion about the “heat-trapping” properties of the atmosphere, there is a basic confusion about what heat is, and what radiation is.

    Heat is not the same as infrared radiation, although infrared radiation causes heating. By even using the word “heat” or “warming”, the confusion is embedded right from the start, so let’s take it down a layer of definition.

    The interaction of light (electromagnetic radiation) with matter (when it does interact and is not merely passing through) causes two things to happen.

    The first is that the internal energy of the matter is raised. This kind of energy is essentially kinetic – the matter has more internal “vibration”. This is called “heat”.

    What then happens is that the heated matter then re-radiates electromagnetic radiation – in all directions, sharing the heating effect with other matter.

    Of course, this is an extremely simplistic picture, and I haven’t included all the elements.

    Sunlight comes through the atmosphere and most of it reaches the surface of the Earth, where it causes heating and re-radiation. The detail is that the radiation coming back out from the Earth is a different “colour”. As this re-radiation reaches the atmosphere on its way back out, the situation is different than when the radiation was on its way down.

    Sunlight radiation doesn’t interact much with the molecules of gas in the atmosphere, but the re-radiation does. For every individual gas molecule the same thing happens – heating and re-radiation – in all directions. It’s the “in all directions” thing that is the most important in being able to claim that the atmosphere “traps” the Earth’s radiation. A considerable amount of the radiation that was heading back out to Space gets bounced back to the Earth again, where it gets radiated back and forth repeatedly. Because the radiation “hangs around” longer than simply heading back out to Space immediately, the Earth is warmer than it would otherwise have been.

    I think it was only once I understood this that I was able to grasp the Greenhouse Effect.

    The next stage of understanding is that out there, in the stratosphere, up where the air is more clear, the probability of the radiation interacting with a gas molecule reduces because there’s less gas up there – which means that more radiation leaves than gets bounced back.

    For me, it’s that simple, and I don’t need to invoke any other concepts or physics. I don’t need to use any terms derived from evidence from centuries of experiments. All the other effects stem from these two – the “radiation-trapping” ability of the lower atmosphere and the “net radiation loss” effect of the upper atmosphere. It does rely on an understanding of the modulation change of the sunlight radiation when it interacts with the Earth’s surface. The key principle is that whatever “absorbs” electromagnetic radiation must also “re-emit”.

    [I prefer the even simpler: the earth is warmer with a GHE because it is warmed by two sources: the sun, and the atmosphere. But very few people are comfortable with that -W]


  5. A picture or animation is worth a thousand words, but most diagrams are useless, I find.

    Here, there is an attempt to explain that sunlight is a different bunch of wavelengths (yellow) than Earth radiation (red), but there is a lot of confusion otherwise, and things don’t seem to add up properly :-


    There is no distinction between the non-radiative transfer of heat and the transfer of radiation that causes heating.

    There is no explanation that “absorption” must result in “re-emission”.

    There is a confusion between what the atmosphere is and what clouds are (which are part of the atmosphere, aren’t they ?).

    I find this kind of diagram worse than useless.

    [Labelling the OLR as “outgoing solar” isn’t very helpful either

    And this one is worse ! :-

    [I don’t think there is much wrong with those pix. They just don’t explain the GHE, but they aren’t trying to, I assume; they are trying to explain the energy budget -W]


  6. My understanding is (but the article doesn’t say) that the additional glass plate is at some distance from the box, intercepting only sunlight and little else:

    “In order to eliminate this action the sunlight was first passed through a glass plate.”

    But there is another thing: the back radiation from the sky. The rock salt plate will let all of that through… in effect we have a small greenhouse standing on the floor of the big one 🙂

    No, this doesn’t explain the finding, but it means that the effect we are looking for is quite a bit smaller than one might naively think.


  7. Tom @3,
    I thought that is exactly what I said in 2. However, small currents are not conduction or radiation, so they are convection. I did say they were weak and could not dominate.


  8. Leonard, the most common understanding of the term “convection” is the circulation of air or a fluid brought about by gravity acting on changes in air density as a result of heating or cooling. It is that process which dominates in the troposphere, but which is for all practical purposes absent in the stratosphere.

    Checking a few dictionary definitions, I find that any circulation of the air, and even diffusion can be called “convection”, so you are strictly speaking correct. However, even so I think if you are using “convection” in its more general sense, rather than the paradigmatic sense I describe above, you should be very clear to avoid confusion.


  9. @jo abbess #5

    Actually the first diagram you link to is quite “transparent” (pun intended) to the physics of energy (im)balance it is trying to convey. Radiative and non-radiative energy transport are indeed both indicated. Effects wrt clouds and atmosphere are clearly distinguished. That absorption of radiation results in both re-radiation and conversion to translational kinetic energy via collisional de-excitation (and all of the other microphysical processes that occur) is well known to atmospheric physicists. As a matter of fact, this is one of the better such diagrams I have seen. Of course, one shouldn’t mistake this cartoon with the detailed physical models of our atmosphere and climate.


  10. A simple energy balance can do wonders to explain the GHE. Since heat is a form of energy:

    If: (Sun Energy In) = (Earth Radiation Out)
    Then: there is no warming.

    Once this balance is disturbed, then a net heating, (or cooling!) will occur. Of course there are multiple components determining the value of either side of the equation, the concept remains the same.

    [You’re wrong. Outgoing radiation = Incoming, whenever the system is in equilibrium. As it (nearly) is now. With the GHE, incoming = outgoing -W]

    Also, it is important to note that our existence relies completely on the GHE. Other planets who do not have and atmosphere, and therefore no GHE, are pretty cold!


  11. I forgot to add the coefficient which implies there is a ratio between the two energies. when the coefficient moves outside of the inhabitable range we’ll fry or freeze!


  12. More thoughts on the Wood box experiment, it seems to me that, if the box is “perfect” — i.e., perfectly insulated, the glass plate perfectly opaque in thermal infrared, and no other “leaks”, like conduction through the glass plate –, then, when placed in sunlight it should heat up without limit, as all the time energy is coming in that has no way out. Or rather, until the inside of the box starts glowing deep red, which being in the visible range, passes through the glass 🙂

    Obviously temperatures don’t even get close to this before reaching equilibrium. What this tells me is that the “leaks” must be significant and swamp any radiative contribution to cooling the box.

    Where Wood may well correct is that a traditional greenhouse but with rock-salt panes instead of glass would work just as well (but OTOH a “perfect”, hermetic greenhouse with panes made of some thermally non-conducting glass would nicely serve as a Solar oven). The relevance to the Earth atmosphere is of course nil, except to somewhat undermine the metaphor.

    [I think that is right. As I understand it (aka I recall a casual conversation once), the plastic covering often used in modern large greenhouses is actually transparent, or more so, in the IR -W]


  13. Hats off to William M. Connolley for having the integrity to draw everyone’s attention to Wood’s paper on the greenhouse even though he disagreed with Wood’s conclusion.


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