The preferred surface spectral response for sub-ambient sky cooling varies according to the amount of water vapor in
the atmosphere and the operating difference (Ta-Ts) between ambient and emitter surface temperatures. While all
good candidates average high emittance from 7.9 μm to 13 μm, where the atmosphere is most transparent (the IR "sky
window"), the preferred spectral response in the remainder of the Planck spectrum depends on a number of factors.
Emittances E in studies to date have been near the two extremes of a high E ~ 0.85 to 0.95, and an E value between 0.3
to 0.4 for surfaces which emit strongly only in the sky window. Cooling rates and ideal spectral properties vary with
operating conditions. The reasons behind this will be explained for select different coatings, using spectral densities for
emitted outgoing radiation, which is Ts dependent, and the incoming radiation that is absorbed, which is fixed unless
the atmosphere changes. Higher E surfaces always work best above and just below ambient but external factors that
reduce incoming radiation from the atmosphere, including very low humidity or heat mirror apertures, extend this
preference down to lower surface temperatures. Sky window spectrally selective coatings do not benefit as much
because they already absorb little incoming radiation, but always have the potential to achieve very much colder
temperatures if non-radiative heat gains are kept low.