(Why Wood and Nahle were correct and Pratt was in error.)
The famous experiment by Robert W. Wood, at John Hopkins University, with two carton boxes/greenhouses, in 1909, is being mentioned everywhere, and on many websites,* as simple experimental evidence proving the fallacy of the greenhouse gas effect theory (GHE).
According to the GHE theory, the small greenhouse with a glass cover had to reach a temperature of nearly 15°C higher than the other small greenhouse with a salt rock (halite) ceiling. This is because salt rock is a material which is “neutral” to infra-red, while glass can theoretically “trap” almost 80-85% of infra-red outgoing from the heated bottom of the greenhouse, and significantly increase the temperature, by “backradiating” the infrared (IR) waves.
Nothing of that took place, and both greenhouses showed almost the same temperatures inside, with a discrepancy of “scarcely one degree”. For years this experiment was sufficient to dispel giving any scientific ground to the greenhouse gas effect theory. But several decades later, many GHE advocates “forgot” this experiment.
One hundred years on, in 2009, Professor Vaughan Pratt of Stanford University (Palo Alto, California) tried to replicate the Woods experiment using more modern materials (plastic plates and foils, along with the “old” glass plates).** Pratt came to the conclusion that Wood’s experiment was in error, because according to Pratt’s surveys the glass and acrylic greenhouses showed temperatures 15°C and 20° C higher than the one inside the other small greenhouse with a thin polyethylene film cover.
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Thereafter, in 2011, Professor Nasif Nahle of Monterrey, Mexico performed his own very accurate repeat experiment using four small greenhouses under strict peer-reviewed control.*** Nahle came to the conclusion that Wood’s experiment was totally correct. Nahle's findings were that in the three small greenhouses having covers of different materials (glass and plastic polymers) and upon one hour of solar exposure, the temperature differences were scarcely in the range of 1° to 1.5° C (as in the Wood’s experiment a century before). Nahle saw that the other “holed” greenhouse - more exposed to cooling convection and environment temperature - showed a lower temperature. This was compelling proof that a greenhouse is heated merely by the blocking of air convection with the outside environment and not by any specious mechanism(s) such as “backradiation” or the “trapping” of longwave outgoing infra-red radiations.
However, this article was written to address a “gap”; many sources and commentators mentioned the experiments above, but curiously nobody – as far as reasonably known – sought to explain (by rigorous physical and mathematical analysis) why Wood and Nahle were right yet Pratt was wrong.
Furthermore, it is usually said that Wood’s experiment is “easily replicable”, even by high school children. Although this assertion could be true, at least for the low cost and availability of the materials, it takes a lot of skill and attention to detail to carefully assemble them and conduct the experiment, as we shall see.
But it is always necessary for any researcher/physicist to try and turn into physical laws and mathematical formulas any physical experiments in order to get rigorous and scientific understanding – as well as acquire deeper insight of it.
Moreover, a technical physics analysis of those experiments, can better clarify the real mechanisms of heat transmission between soils and atmosphere, or solid and gaseous materials.
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