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Re: [Phys-l] paint your roof?



On May 27, 2009, at 2:03 PM, curtis osterhoudt wrote:

... I've never been very comfortable with the "radiating to space" explanation (though it may be totally right and my intuition fails me).

I've noticed that a lot of people seem to be uncomfortable with that explanation despite its fundamental correctness. Of course, it's not completely correct because the atmosphere is not completely transparent. Indeed, on cloud covered nights the sky is basically opaque and, as a result, radiative loss to the sky is not as big an effect. (More correctly, while radiation to the sky is just as big an effect, depending only on the temperature of the radiating object, it is countered by massively larger radiative input from the clouds above at temperatures > 273 K rather than the piddling input from deep space at 3 K.) Moreover, there are lots of competing and complicating processes including convection in the air, conduction both to the air and through supports to the ground, and radiative coupling to something like the 2 pi steradians that aren't open "sky." As a result the effect isn't always so apparent. But most people do know that, in midwinter, clear skies make for much colder nights.

In short, I have a hard time seeing how metals (or whatever) can sense a nice big heat sink out there *somewhere* and end up at a lower temperature than the surroundings.

They don't have to sense anything. They just radiate as they always do. But when you have 2 pi steradians radiating back to you at only 3 K, the radiation balance is way out of whack!

Do they continuously suck heat out of the air and re-radiate it to the vacuum?

Basically, yes.

Taking that to the extreme, if I put a piece of metal into a "transparent" container and put it under vacuum, the metal would eventually reach the temperature of the background radiation in the universe

Right. It's easiest to do the thought experiment by imagining a warm object placed in deep space.

, unless something is coupling to the metal (conductively, convectively, or radiatively) and warming it back up. It wouldn't just hold on to its heat energy. At which distance do I need to put an insulator to keep the communication from taking place? Around Earth's atmosphere? Outside the galaxy? If so, what relativistic effects contribute to the phenomenon? I'm not sure I follow all that, but you can do the thought experiment pretty well
Is it because conductors have such a near continuum of phonon and electronic modes that they can "talk" this way? Is the communication affected by the Casimir effect?

Now you're beyond my pay grade!

John Mallinckrodt
Cal Poly Pomona