Alfred Falk wrote in
:

Salty Thumb wrote in
news:rNeXc.1129$Cc.449@trnddc07:

does anybody know if there is a word or measurement that describes
that efficiency at which a substance traps longer waved solar
radiation?

For example glass is pretty good at trapping heat, so that may be
rated at 100 "Scovillage" units or possibly it could be described as
having a "Maxwell" rating of .5 celsius per second per cubic meter.
Polycarbonate also used for green/glass houses may have a similar
values while brick or other opaque materials would be close to zero.
What would the real term for Scovillage or Maxwell be?

I would be surprised if there were such a term because the greenhouse
effect is not a property of the material alone. The obvious (to me,
at least) variables a
1) The spectrum of the incoming radiation. For an
incandescent
source such as the sun this is mostly determined by the source
temperature. (The "surface" of the sun is about 6000 K) But
atmospheric absorption figures in this as well.
2) The spectrum of outgoing radiation. This is mostly
determined
by the temperature reached by surfaces inside the greenhouse.
3) The transmission coefficient of the glass at every
wavelength.

Also, it is not meaningful to talk about the greenhouse effect with
opaque materials as there is no radiation passing through. (Unless
you want to talk about x-rays and gamma rays.)


I didn't like my chances, either, but I thought I'd ask anyway. To
clarify, I'm looking for a term that enables comparisons of different
materials for use in a greenhouse application. The term/measurement
doesn't necessarily have to have mathematical precision, but if there is
a directly measurable value that's good too. Something like R- values
for building insulation in the US, or PR/+ ratings for microprocessors.

If the E in low E (emissivity) glass is quantifiable (even with some
fudging allowed) and not just some vaccuous idea, then that's probably
exactly what I'm looking for.

Seems polycarbonate is widely used, but is that because of superior
trapping or a combination of trapping, structural strength, price and/or
other?. I'd also like to see how Lexan, plain plexiglass, clear PVC and
one other clear plastic (don't remember off hand) compare solely from a
heat trapping / heat retention perspective.