Thermal radiation — where it appears
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
The cold reservoir overhead
On a clear, still night a car roof can grow frost while the air around it is several degrees above freezing. Nothing in the air is cold enough to freeze water. The roof is radiating its heat upward to a sky whose radiation back is that of something twenty degrees colder than the air — part of it the cold upper atmosphere, part of it space itself, seen through a window in the infrared where the air is transparent. The clear sky is a cold reservoir available everywhere, and it can be used: to make frost, to cool buildings below the air temperature even in sunshine, and in principle to run an engine at night.
The thin foils that stop radiant heat
In a vacuum, heat can cross a gap only as radiation, and a single sheet of foil a few hundredths of a millimetre thick, hung in the gap and touching nothing, halves it. Two sheets cut it to a third, thirty to a thirty-first. The foil does not need to be thick or cold or clever; it needs only to be shiny, because a sheet that absorbs little also emits little, and each one becomes a radiator in its own right, floating at a temperature between its neighbours. The golden blankets on spacecraft and the silvering inside a vacuum flask are the same arithmetic of resistances in series.
Named alongside it
The objects these essays reach for when they reach for this one.
EmissivityAtmospheric windowCarnot efficiencyFrostHeat engineKirchhoffs lawMultilayer insulationRadiation shieldRadiative coolingStefan boltzmannStefan boltzmann lawThermal resistance