As anyone who’s ever paid an electric bill knows, cooling is costly. The second law of thermodynamics dictates that energy can’t spontaneously flow from a colder object to a warmer one, all else being equal. Cooling machines such as air conditioners and refrigerators therefore require an energy input to create and sustain an inside temperature lower than that of the outside air.
The second law is inviolate, but it allows for some counterintuitive effects. Earth’s atmosphere—greenhouse gases and all—is nearly transparent to IR radiation between 8 and 13 µm, which also happens to be the peak wavelength range of thermal radiation at typical terrestrial temperatures. If a material radiates especially strongly into the atmospheric transparency window, it can shed thermal energy directly to outer space, almost as if the atmosphere weren’t there.
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