Failure to duplicate Wood's 1909 greenhouse experiment

In 1909 the noted physicist R.W. Wood conducted an experiment aimed at demonstrating that the temperature of a greenhouse was not significantly influenced by the trapping of infrared radiation by its glass windows, but rather was the result of preventing its air from mixing with the cooler air outside the greenhouse. Wood observed less than a degree of difference between two boxes with respectively a glass and a rock salt window, see Wood's article along with comments by W.M. Connolley, to whom I am indebted for drawing Wood's experiment to my attention.

It is customary in physics to confirm experiments by repeating them before accepting them as definitive. I wrote earlier here that "Remarkably this custom has not been observed for this experiment, which has simply been accepted unquestioningly for the past hundred years as proof of Wood's suspicion." However some digging around in later issues of Phil. Mag. for 1909 turned up the article by Charles Greeley Abbot that I describe at http://boole.stanford.edu/Wood.

Like Abbot we had our doubts about Wood's experiment as soon as we saw it. First it is documented so sketchily as to make it impossible to duplicate faithfully. Second, it contradicts the understanding of a century of physicists who preceded Wood, starting with Fourier in 1824. Third, calculation of the quantity of heat that should be trapped by the glass window in Wood's experiment shows that fully two-thirds of the heat entering the box fails to be transmitted back through the window, raising the question of how that heat is able to leave the box without raising its temperature significantly. Wood makes no attempt to reconcile his one observation with this elephant in the room.

To get a better understanding of the situation we constructed not two but three boxes differing only in their windows. These are shown below, numbered 1 to 3 from east to west. The location is 37 degrees north of the equator and the photos were taken at about 2 pm on 11/28/09.


View from east.


View from above, with box numbers visible (1 to 3 from left/east to right/west).


View from west.

The boxes are made of corrugated cardboard and measure 12" × 12" × 6". The interior is padded with foam walls spray-painted with Krylon Ultraflat black paint for reasonable blackbody performance. Five sides of the exterior are wrapped with Reynolds aluminum foil to minimize heat transfer by radiation other than through the window in the sixth side. All joints are sealed with Scotch tape to minimize exchange of air between the interior and the exterior. Each box is instrumented with a temperature probe monitoring the air temperature at the center of the box and shielded from the sun with a small parasol.

The front opening of each box consists of a 12" × 12" window transparent to visible and near-infrared radiation, thereby allowing more than 85% of the incident sunlight to enter the box while confining the air within the box so as to prevent cooling by mixing with the outside air. Roughly 8% of the 15% not admitted to the box is attributable to reflection at the front and back window surfaces, while the rest consists of near-infrared insolation beyond 2.5 microns in the case of boxes 2 and 3 where the transmittance of glass and acrylic starts to fall off.

Box 1: Saran wrap Premium. This is 0.015 mm thick polyethylene film, which is largely transparent to blackbody radiation at 333 K (60 C). This serves as a modern-day counterpart to the rock salt window Wood used, polyethylene film not being available in 1909. Both materials serve to assess the effect on temperature of a window that passes infrared radiation while preventing convection to the outside.

Box 2: Window glass, 3/32" thick. This blocks over 90% of blackbody radiation at 333 K. Its thermal conductivity is approximately 1 W/m2/K, or R-1 in SI (European or RSI) units, relevant to temperature drop across the window, while its refractive index is close to 1.54, relevant to reflectivity at the front and back surfaces.

Box 3: Acrylic plate, 3/8" thick. Like glass, this also blocks over 90% of blackbody radiation at 333 K. Its thermal conductivity is approximately .25 W/m2/K, or R-4, while its refractive index is about 1.5.

The windows of boxes 2 and 3 are each instrumented with two K-type thermocouples measuring the temperature at the center of the two surfaces of the window, primarily to monitor the temperature drop across the window. These are shaded with the same parasol used to shield the interior probe, again to avoid spurious heating by direct insolation. It was deemed neither necessary nor practical to instrument Saran wrap in this way, the temperature drop being assumed to be negligible.

Very preliminary tests conducted on November 28, 2009 indicate that boxes 2 and 3 run respectively 15 and 20 degrees hotter than box 1, with a significantly larger drop across box 3's window than box 2's. While this is consistent with the expected differences obtained by theoretical considerations, it is so far removed from what Wood found as to raise the very interesting question of how he was unable to observe any significant difference between his two boxes.

In view of these discrepancies it is remarkable that there has been no previous attempt to duplicate Wood's results, and disconcerting that they have been the basis for a century of claims that infrared trapping plays no significant role in greenhouse warming.

We plan to refine the experiment and these measurements in order to improve both their accuracy and meaning as well as gaining insight into how Wood's observation might have come about, at which time it will be appropriate to release more detailed temperature readings. For example double glazing in the form of a second window of Saran wrap a centimeter from the first may well raise the temperature of box 1 by insulating the window better. Meanwhile suggestions for improving the methodology can be emailed to the author's surname using hostname cs.stanford.edu.

Double glazing experiment 12/3/09

We repeated the previous week's experiment with boxes 1 and 3 increased from single glazing to double glazing. The main concern with box 1 had been that its thermal resistance was negligible. We dealt with this by adding a second Saran wrap window with a 6 mm (1/4") air gap between the two windows, a standard for some greenhouses. Air has 40 times the thermal resistance or R-value as glass, and 10 times that of perspex, so this gap would correspond to a ten-inch glass slab. To be sure that there was no convection we divided this thin layer of air into 2" x 2" cells with 1/4 " strips of acetate cut from blank overhead transparency slides.

Each layer of material has two reflective surfaces. Assuming a nominal refractive index of 1.5 for all materials, this entails a reflectivity of 4% at each surface. Two layers of Saran wrap will therefore reflect 16% of the incoming solar radiation, as well as 16% of the outgoing thermal radiation at 20 times the wavelength.

To minimize the differences between the windows we added a second perspex window to box 3, of the same 12" x 12" x 3/8" dimensions as the first, with a negligible air gap between them. Being negligible, there is no additional thermal resistance. Nevertheless the gap is many wavelengths of light whence the reflections from the adjacent surfaces combine incoherently and therefore additively by intensity, for a total of 16% as for the double-glazed Saran wrap window.

At 2 pm on 12/03/09 the box with the double-glazed Saran wrap window reached 65 C while the box with the two 3/8" perspex sheets reached 80 C. (We subseqently recorded 82 C with the latter.) During the previous hour the former climbed rapidly to its limiting temperature while the latter was much slower.

Had we settled for an early measurement we would have concluded that box 1 was the hotter. However box 3 continued to rise and eventually overtook box 1 and went on to become 15 C higher.

These temperatures were measured about 2/3 of the way to the front (window) side of the box. To explore the variability from front to back we retracted the probes by pulling most of the probe out of the box, taking the precaution to insulate thermally the resulting exposed part of the probe. Box 1 decreased to 62.6 degrees while Box 3 decreased to 76.3 degrees. This decreased the difference between these two boxes from 15 degrees to a little over 13 degrees, still a substantial difference. This is consistent with the hypothesis that these similarly sized boxes exhibited similar convection patterns. We attributed the decrease in this case to the fact that the boxes were aimed upwards towards the sun and that hot air rises, as a simplification of the more complex route no doubt followed by the convection in each box. Had the insolation come from below it is an interesting question whether the window end or the black-bottom end of the box would be hotter.

Conclusion

The conclusions we draw from these experiments are as follows.
  • The greater thermal mass (actual mass times specific heat) of perspex relative to that of air slows down the heating of the former, but is immaterial once thermal equilibrium is reached.
  • The high thermal resistance of the double-glazed saran window of Box 1 did not result in Box 1 heating up the way Box 3 did. Hence relatively little of the incoming insolation energy is leaving via conduction through the 6 mm air gap. Moreover the 2" x 2" cells should prevent any significant convection. This leaves radiation as the only remaining conduit. Box 1 is therefore cool because its window is transparent to infrared radiation.
  • In equilibrium Box 3 is significantly hotter than Box 1, which we attribute to the blocking of infrared radiation by the 3/4" total of perspex, forcing the incoming energy to leave the box by conduction through the perspex (since convection through the perspex is impossible).

    • Boxless experiment, 1/03/10

    A central tenet of the atmospheric greenhouse effect is that a significant fraction of the heat arriving at the surface of the Earth is back radiation from the atmosphere, which increases in temperature with increasing amount of greenhouse gases. It is therefore natural to ask whether the same effect can be seen in glass in a way independent of convection. With the boxes of the above experiment, while the radiation is easily calculated the convection on both sides of the window is not so obvious. Can convection be eliminated?

    One way to do this would be to conduct the experiment in outer space, where there is only radiation and no convection because no air. On Earth one could do it in an evacuated bell jar, but the glass of the jar itself already blocks radiation while being subject itself to convection on its outside.

    I therefore came up with the following "poor man's vacuum chamber." I instrumented a one-square-foot sheet of 3/8" thick perspex (acrylic) with thermocouples at front and back, then enclosed it completely in saran wrap with a 1/4 inch air gap on each side, fully sealed. (As noted above Saran wrap is completely transparent to visible light and mostly transparent to infrared, being only 15 microns thick.) I covered a similarly sized sheet of black-painted aluminum with saran wrap on one side, also with a fully sealed 1/4 inch air gap. I then stacked the perspex and aluminum with a 1 inch air gap between them open to the outside air, and aimed the stack at the Sun, with the perspex between the Sun and the aluminum. Lastly I put a fan off to one side, several feet away, which sent a stiff breeze between the cardboard and the perspex.

    The following shows the setup. The front and back thermocouples are visible in the first photo.

    The results, obtained between 12:30 pm and 1:15 pm on Jan. 3, 2010, were as follows. All temperatures are in Centigrade.

    I started the experiment with a sheet of white cardboard (with no saran wrap of its own) in place of the aluminum. The perspex took 20 minutes to rise from 20 C at front and back to 31.7 C at front and 33.3 C at back, where it stabilized.

    I then replaced the cardboard by the saran-wrap-insulated black aluminum sheet. The perspex then took 25 minutes to further rise to 42.4 C at front and 47.4 C at back, where it stabilized. (The aluminum itself was also instrumented and reached 78 C. I did not instrument or measure the white cardboard, assuming it would heat only negligibly.)

    Conclusion

    With the white cardboard, more sunlight passed through the perspex than with the black metal because the cardboard reflected most of the sunlight back through the perspex while the black metal absorbed it. However more infrared passed through the perspex with the metal than with the cardboard because the metal converted the sunlight to heat which it then reradiated through the perspex as infrared. The perspex blocked none of the sunlight and therefore did not warm significantly on that account relative to how much it warmed on account of the infrared. This effect is a good analogy to the mechanism by which the Sun first heats the surface of the Earth, which then radiates infrared to heat the atmosphere.

    The saran wrap served two functions: (i) to isolate the window and the metal from each other convectively, and (ii) to allow air to be blown forcefully between them without the breeze cooling down either the window or the metal significantly, further improving the convective isolation of the window from the metal.

    The saran wrap adds considerably to the setup's albedo, or reflection. Altogether the inbound followed by outbound radiation passed through saran wrap six times, three in each direction: one on each side of the window, and one over the black aluminum sheet. And it passed through the perspex twice, once inbound and once outbound. Each of these layers has two surfaces, making a total of 16 interfaces between air and substances with a refractive index of around 1.5. Each such interface reflects about 4% of the normally incident radiation, making the total reflection 64% when accumulated naively (i.e. ignoring reflection of reflected radiation). The inbound 32% of that tends to attenuate the warming, while the outbound 32% enhances it by adding to the trapping effect. The latter presumably helps the perspex reach 33 C when the back plate is white cardboard.

    Current work

    More recently I have been studying Wood's experiment more closely to find out whether there might have been some rational explanation of his failure to observe warming resulting from trapping of heat, which is at odds with the results of others performing similar experiments starting with Horace De Saussure in 1767. To this end I've replaced the saran wrap with an optical quality salt window, and instrumented the boxes with multiple TMP-05B chips, tiny thermometers the size of a matchhead, to better understand the onset and distribution of warming throughout the box and across the window. Since salt windows of the kind typically found in optics laboratories are small for reasons of mechanical strength, the boxes are sized accordingly.

    I've gathered a considerable amount of relevant data and will report on these experiments in due course.

    Vaughan Pratt
    Professor Emeritus
    Stanford University
    Stanford, CA 94305-9045