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Re: [Phys-l] paint your roof?



If it's not coupling (radiating) to the air, and not coupling to your
hand, what *is* it coupling to?

I reckon that would be "outer space." I suppose the membrane has high
radiance in infrared as well as visible.

Astronomer hobbyists know this also happens to telescope objectives and
that's why we need "dew shields," especially in a humid area like Ohio.
If I am out at night in a field or lawn, high-humidity air is rising
from the field. The objective lens is radiating heat to space and is
colder than the ambient air. As the humid air comes into contact with
the lens, it can condense and fog the lens. A dew shield helps channel
the air past the lens (which is recessed by the shield) and helps
prevent the condensation.

On numerous cold nights when the ambient air temperature was in the
forties, I had the corrector plate on my Schmidt-Cassegrain telescope
frost over, even with a dew shield. I finally made a heater with series
carbon-film resistors sewn into elastic fabric and stretched around the
perimeter of the corrector end of the scope. It only requires a couple
watts or so to drive it. Just need it warm enough to prevent
condensation.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu