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.
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.
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.)
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.
I've gathered a considerable amount of relevant data and will report on these experiments in due course.
Stanford, CA 94305-9045