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Radiative cooling is the process by which a body loses heat by thermal radiation.

Terrestrial radiative cooling

Earth's energy budget

In the case of the earth-atmosphere system it refers to the process by which long-wave (infrared) radiation is emitted to balance the absorption of short-wave (visible) energy from the sun.

The exact process by which the earth loses heat is rather more complex than often portrayed. In particular, convective transport of heat, and evaporative transport of latent heat are both important in removing heat from the surface and redistributing it in the atmosphere. Pure radiative transport is more important higher up. 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 cyclones 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.

Radiative cooling on Earth's surface at night

Radiative cooling is commonly experienced on cloudless nights, when heat is radiated into space from the surface of the Earth, or from the skin of a human observer. The effect is well-known among amateur astronomers, and can personally be felt on the skin of an observer on a cloudless night. To feel the effect, one compares the difference between looking straight up into a cloudless night sky for several seconds, to that of placing a sheet of paper between one's face and the sky. Since outer space radiates at about a temperature of 3 kelvins (-270 degrees Celsius or -450 degrees Fahrenheit), and the sheet of paper radiates at about 300 kelvins (room temperature), the sheet of paper radiates more heat to one's face than does the darkened cosmos. The effect is blunted somewhat by Earth's surrounding atmosphere which also traps heat. Note that it is not correct to say that the sheet "blocks the cold" of the night sky; instead, the sheet is literally warming your face, just like a camp fire warms your face; the only difference is that a campfire is several hundred degrees warmer than a sheet of paper, just like a sheet of paper is several hundred degrees warmer than the deep night sky.

The same radiative cooling mechanism can sometimes 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.

Radiative cooling can explain why the earth is not heating up in accordance with numerous climate computer models, despite nearly 20 years of tweaking. The predictions of the computer models are based on many variables, but one factor either missing completely or not fully understood or modeled correctly, is the quantum of heat energy radiated back into space. This will vary according to the latitude, local weather conditions, and cloud cover, for example. One can appreciate the sheer difference in effective radiative heat loss when there is no cloud cover, versus a lot of cloud cover. The green-house model assumes that heat can't escape into space, because carbon dioxide somehow traps that radiative heat, (despite carbon dioxide being heavier than air and like other gases, found in decreasing concentrations ( molecules per cubic metre) the higher the altitude Ref http://www.biomind.de/realCO2/) and that clouds aren't a serious factor ( probably because even after decades of weather forecasting experience, cloud cover models are desperately unreliable) and mathematically impossible to predict and write in a manner suitable for inclusion into climate change models. However, in highly populated industrial areas of Europe and China for example, there is considerable mixing of carbon dioxide at various altitudes, with factors such as jet airliner traffic also confusing measurements. There appears to be a lack of carbon dioxide studies at places where it is remote from industrialization and jet aircraft, however, since all gases have lower pressure at higher altitudes, the molecular concentration of carbon dioxide by weight will also be lower, therefore the effect of reflecting or absorbing energy from the sun or radiated back up from the earth will be complicated to measure scientifically. This also explains why there are no upper atmospheric "hot spots" which were predicted by the many climate models being trialed around the world. Satellite images show that this feature is clearly an incorrect assumption and climate modellers will have to tweak the models again as this theoretical driver forcing does not confirm the proposition.

Kelvin's estimate of the age of the Earth

The term radiative cooling is generally used for contemporary processes, though the same general principles apply to the cooling of the planet over geological time, which was first used by Kelvin to estimate the age of the Earth (though you cannot neglect the fission heat source for this purpose, so his answer was wrong).

Applications

Architecture

Cool roofs combine high optical reflectance with high infrared emissivity, thereby simultaneously reducing heat transfer from the sun and increasing heat removal through radiation.

Nocturnal ice making

In India before the invention of artificial refrigeration technology, ice making by nocturnal cooling was common. The apparatus consisted of a shallow ceramic tray with a thin layer of water, placed outdoors with a clear exposure to the night sky. The bottom and sides were insulated with a thick layer of hay. On a clear night the water would lose heat by radiation upwards. Provided the air was calm and not too far above freezing, heat gain from the surrounding air by convection would be low enough to allow the water to freeze by dawn.

Spacecraft

Spacecraft travel through vacuum, and therefore have to shed any excess heat by radiation.

See also

• Optical solar reflector, used for thermal control of spacecraft
• Passive cooling
• Radiative forcing
• Stefan-Boltzmann law
• Terrestrial albedo effect
• Urban heat island
• Urban thermal plume

References

1. "Lesson 1: History Of Refrigeration, Version 1 ME". Indian Institute of Technology Kharagpur. Archived from the original (PDF) on 2011-11-06.

• Thermodynamics
• Atmospheric radiation