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Re: [Phys-l] paint your roof?

Woolf, Lawrence wrote:
-----Original Message-----
From: Brian Whatcott
Sent: Wednesday, May 27, 2009 5:47 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] paint your roof?

We have only heard that the reflective roofing material is colder than the ambient night air, and this should be true for the best reflectors AND the worst, surely?
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Suppose you have a metal reflector. It will be highly reflective in the visual and the IR. So it will have very low IR emissivity, so it will not efficiently cool at night.
Your assertion appears to hang on the qualifier "efficiently"

I would rather think (wrongly??) that the specular surface will cool just as surely as
a black body, but on a longer time constant.
During a clear night with the coolest air temperature around the pre-dawn hour that's a substantially long interval compared with either time-constant, at least to my impression.

Brian W