Editing Radiative cooling (section)

== Terrestrial radiative cooling ==

=== Mechanism ===
Infrared radiation can pass through dry, clear air in the wavelength range of 8–13&nbsp;μm. Materials that can absorb energy and radiate it in those wavelengths exhibit a strong cooling effect. Materials that can also reflect 95% or more of sunlight in the 200 nanometres to 2.5&nbsp;μm range can exhibit cooling even in direct sunlight.<ref>{{Cite journal|last=Lim|first=XiaoZhi|date=2019-12-31|title=The super-cool materials that send heat to space |journal=Nature|volume=577|issue=7788|pages=18–20|doi=10.1038/d41586-019-03911-8|pmid=31892746|doi-access=free}}</ref>

=== Earth's energy budget ===
{{further|Earth's energy budget}}
The Earth-atmosphere system is radiatively cooled, emitting long-wave ([[Infrared radiation|infrared]]) radiation which balances the absorption of short-wave (visible light) energy from the sun.

Convective transport of heat, and evaporative transport of latent heat are both important in removing heat from the surface and distributing it in the atmosphere. Pure radiative transport is more important higher up in the atmosphere. Diurnal and geographical variation further complicate the picture.

The large-scale circulation of the [[Earth's atmosphere]] is driven by the difference in absorbed solar radiation per square meter, as the sun heats the Earth more in the [[Tropics]], mostly because of geometrical factors. The atmospheric and oceanic circulation redistributes some of this energy as [[sensible heat]] and [[latent heat]] partly via the mean flow and partly via eddies, known as [[cyclone]]s in the atmosphere. Thus the tropics radiate less to space than they would if there were no circulation, and the poles radiate more; however in absolute terms the tropics radiate more energy to space.

=== Nocturnal surface cooling ===
Radiative cooling is commonly experienced on cloudless nights, when [[heat]] is radiated into [[outer space]] from Earth's surface, or from the skin of a human observer. The effect is well known among [[amateur astronomy|amateur astronomers]].

The effect can be experienced by comparing skin temperature from looking straight up into a cloudless [[night sky]] for several seconds, to that after placing a sheet of paper between the face and the sky. Since outer space radiates at about a temperature of {{cvt|3|K|C F|lk=on}}, and the sheet of paper radiates at about {{cvt|300|K|C F}} (around [[room temperature]]), the sheet of paper radiates more heat to the face than does the darkened cosmos. The effect is blunted by Earth's surrounding atmosphere, and particularly the water vapor it contains, so the apparent temperature of the sky is far warmer than outer space. The sheet does not block the cold, but instead reflects heat to the face and radiates the heat of the face that it just absorbed.

The same radiative cooling mechanism can cause [[frost]] or [[black ice]] to form on surfaces exposed to the clear night sky, even when the [[ambient temperature]] does not fall below freezing.

=== Kelvin's estimate of the Earth's age  ===
{{further|Age of the Earth}}
The term ''radiative cooling'' is generally used for local processes, though the same principles apply to cooling over geological time, which was first [[William Thomson, 1st Baron Kelvin#Age of the Earth: geology and theology|used by Kelvin]] to estimate the age of the Earth (although his estimate ignored the substantial heat released by radioisotope decay, not known at the time, and the effects of convection in the mantle).

=== Astronomy ===
Radiative cooling is one of the few ways an object in space can give off energy. In particular, [[white dwarf]] stars are no longer generating energy by fusion or gravitational contraction, and have no solar wind.  So the only way their temperature changes is by radiative cooling.  This makes their temperature as a function of age very predictable, so by observing the temperature, astronomers can deduce the age of the star.<ref>
{{cite journal
 |last1=Mestel |first1=L.
 |date=1952
 |title=On the theory of white dwarf stars. I. The energy sources of white dwarfs
 |journal=Monthly Notices of the Royal Astronomical Society
 |volume=112 |issue=6
 |pages=583–597
 |bibcode=1952MNRAS.112..583M
 |doi=10.1093/mnras/112.6.583 
|doi-access=free
 }}</ref><ref>{{Cite web|url=http://old.physics.upatras.gr/UploadedFiles/course_149_4311.pdf |title=Cooling white dwarfs |publisher=Physics Department, University of Patras}}</ref>