"Ash Wyllie" wrote in message
...

Look up _radiational cooling_ . Space is in the single digits absolute,
and
the atmosphere is pretty much transparent to radiation. Objects will
radiate
energy trying to heat up interstellar space, and cool in the process.

Ash, I'd like to expand a bit on your statements.
In the 3-5 and 8-12 micrometer region of the electromagnetic spectrum,
atmospheric transmission is, like you said, close to 100% transparent. Outer
space, in those bands, is to photons like a Hoover is to dust in the
carpet - it just sucks heat right out.
The rate at which the transfer of photons occurs is dependent on, among
other things, the emmissivity of the surface and the thermal conductivity
from the mass to that surface. The perfect case is a gaussian emmissivity of
one, and since perfection is difficult, the closest you can come is a bunch
of nines behind the decimal point. A pure white specular surface is at the
opposite end of the scale.
One good example to back up your statement is in the winter time. We
have all felt the phenomena when the sky is clear that the nights are much
colder than when the sky is overcast. That is because interstellar space is
sucking the heat off the planet's surface. The adjacent atmosphere loses a
lot of its energy to the surface and its temperature decreases also.
Please, don't confuse the cloud cover effect with the so-called
"greenhouse" effect.
I always used the term "radiative cooling" (not radiational) to keep in
tune with NASA's glossary:
(http://eobglossary.gsfc.nasa.gov/Lib...seg=q&segend=s)
radiative cooling Cooling process of the Earth's surface and adjacent
air, which occurs when infrared (heat) energy radiates from the surface of
the Earth upward through the atmosphere into space. Air near the surface
transfers its thermal energy to the nearby ground through conduction, so
that radiative cooling lowers the temperature of both the surface and the
lowest part of the atmosphere.

Oh, did I mention that I spent the better part of two decades doing infrared
measurements using a Michelson Interferometric Spectrometer? [Don't try
saying that with more than two drinks under your belt.] A lot of that dealt
with radiative transfer.