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Re: "Greenhouse" effect. Was: light is slowing down



Donald E. Simanek wrote:

On Sat, 7 Feb 1998, LUDWIK KOWALSKI wrote:

I remember reading somewhere that a small greenhouse box in which glass was
replaced by an IR-transparent crystal had essentially the same temperature
for two substances. It is the absence of convection, they said, that is the
main cause of high temperatures in green houses.
Ludwik Kowalski


That was another one Alistair Frasier brought up last Thursday when he
visited our university. The "greenhouse effect" he said operates in the
astmosphere, but not in greenhouses. Textbooks have been repeating the lie
about how greenhouses operate. But what I didn't know was that (according
to Allistair, it was physicist Robert W. Wood who first nailed this error
*nearly a century ago*.) Don't textbook writers ever learn?

Yes, they do. My meteorology textbooks of recent years still call the
effect the greenhouse effect but explain that the rise in temperature in
a greenhouse is mainly due to suppressed convection. Robert Wood did
show around the turn of the century that the greenhouse effect was not
due to the lack of transmission of IR through glass. Robert Greenlear
presented data several summers ago at an AAPT meeting from a simple
student research project that showed that the major contributer to the
rise in temperature in a greenhouse was the suppressed convection.


Wood made a small greenhouse from rock salt panels. Temperature was
measured inside, and compared to another greenhouse with glass panels. He
also made a glass greenhouse of the same size with louvered glass panels
to permit convection. Conclusion: It is the suppressed convection which is
responsible for the temperature within greenhouses. Indeed, many
greenhouses today use plastic coverings.

Fraser's site, "Bad Science" has a discussion of greenhouse effect.

http://www.ems.psu.edu/~fraser/BadScience.html

-- Donald Simanek

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| Roger A. Pruitt, PhD |
| Department of Physics |
| Fort Hays State University |
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