Wood, 1909, continued

Many years ago I transcribed R. W. Wood: Note on the Theory of the Greenhouse onto my personal website (yes, it really was that long ago; and really it was transcribed even earlier from a free-hosting site). As this seems a good excuse, I'll copy it to the end of this post as a reference.

But the immeadiate reason for this post is DC, who points out Vaughan R. Pratt's Wood's 1909 greenhouse experiment, performed more carefully. I think I am or was uneasily aware that this exists, though I can't recall reading it. We will not simply dismiss him because he is emeritus.

Pratt's first and I think major complaint is that Wood "superimposed a glass plate on the salt window". This is based on Wood's statement that "the sunlight was first passed through a glass plate". As far as I can tell Pratt thinks Wood's glass plate was directly on top; which would indeed be a problem. But I think that Wood put some distance between the two, and I think that isn't a problem¹. As to the rest... I find my mind bounces off it. Perhaps I'm getting old; I certainly find that I don't care about these struggles as I used to. Or perhaps Pratt's stuff is badly written. Unlike Wood's, it wasn't AFAIK published.

None of this, of course, has any particular relevance to the atmospheric greenhouse effect which we all care about, apart from the regrettable similarity of name.

Notes

1. I think Pratt's attitude to Wood smacks of "our ancestors were idiots because they knew less than us". This is almost invariably false. I sometimes veer close to this - see my notes on Aristotle's physics for example - but I think I don't fall in.

Refs

* Dumb America.

* The Worst Economists in the World.

* COUNTDOWN BEGINS ON BRIDGE COLLISION CLIMATE LINK.

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

(originally at http://www.wmconnolley.org.uk/sci/wood_rw.1909.html)

The following text is from the Philosophical magazine (more properly the London, Edinborough and Dublin Philosophical Magazine), 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 pretent 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 experiments described earlier, 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.