Radiative cooling

• 1. Introduction

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  Radiative cooling is the process by which a body loses heat by thermal radiation. Every physical body, as Planck's law describes, spontaneously and continuously emits electromagnetic radiation. That quiet, invisible emission turns out to be one of nature's most useful tools. Long before anyone coined the term, farmers in India and Iran were freezing water on warm nights using nothing but a shallow clay tray, a bed of hay, and a clear sky. The same principle that forms frost on a car roof in autumn drives the cooling of white dwarf stars across billions of years. And in 2014, a scientific breakthrough made it possible to use this ancient mechanism in full daylight, opening a path toward cooling buildings without any electricity at all. What is actually happening when a body radiates its heat away? Why does a cloudless night feel colder than a cloudy one? And how close are we to covering rooftops in materials that can air-condition a building using only the void of space?

• 2. How the Physics Works

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  Infrared radiation passes through dry, clear air most freely in the wavelength range of 8-13 micrometres. Materials that absorb energy and radiate it back out in those same wavelengths exhibit a strong cooling effect. The atmosphere is unusually transparent in that window, meaning heat emitted at those frequencies escapes to space rather than being trapped close to the ground. Materials that can also reflect 95% or more of sunlight across the 200 nanometre to 2.5 micrometre range can achieve cooling even under direct sunlight. Two distinct tricks are needed, then: emitting in the right infrared band, and bouncing solar energy away before it can turn into heat. Outer space itself sits at roughly 3 K, making it an extraordinarily cold heat sink. By contrast, Earth's atmosphere, laden with water vapour, radiates back at something much closer to room temperature. That difference is what the best radiative cooling materials are designed to exploit.

• 3. Frost, Stars, and Kelvin

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  On a cloudless night, heat radiates upward from Earth's surface and from human skin into the dark sky above. Amateur astronomers notice the effect keenly. A simple demonstration involves holding a sheet of paper between the face and the open sky: the paper, radiating at roughly 300 K, actually warms the face compared with the frigid apparent temperature of open space. The sheet does not block cold; it reflects the face's own radiated heat back toward it and re-emits what it absorbs. The same mechanism can cause frost or black ice to form on exposed surfaces even when the surrounding air temperature has not yet fallen below freezing. At geological scales, Kelvin used this same principle to estimate the age of the Earth, though his calculation did not account for heat released by radioisotope decay or for convective mixing in the mantle. White dwarf stars present a purer case: no longer generating energy through fusion or gravitational contraction, and without a solar wind, they lose temperature only by radiating. Their age can be read directly from their temperature, because the cooling curve is so predictable.

• 4. Ice Before Refrigeration

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  Before mechanical refrigeration existed, communities in India and Iran developed reliable methods for making ice using nocturnal radiative cooling alone. In India, the apparatus was a shallow ceramic tray holding a thin layer of water, set outside with a clear view of the night sky. The bottom and sides were insulated with a thick bed of hay. On a calm, clear night, with the air only modestly above freezing, convective heat gain from the surrounding air stayed low enough that the water lost more heat upward by radiation than it gained from its surroundings, and ice formed. In Iran, the approach was grander in scale: large flat ice pools built on a bed of highly insulative material, surrounded by tall walls. The walls protected the water surface from convective warming, the insulative pool walls guarded against heat conducted from the ground, and the broad flat expanse of water allowed both evaporative and radiative cooling to proceed together. These two traditions represent early engineering of the same infrared atmospheric window that modern photonic materials are designed to exploit.

• 5. Daytime Cooling Breakthrough

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  In 2014, researchers developed the first daytime radiative cooler, built around a multi-layer thermal photonic structure that selectively emits long-wavelength infrared radiation toward space while reflecting sunlight. Under direct sunlight, it achieved temperatures 5 degrees Celsius below the ambient air. That was a proof of concept. Later work produced paintable porous polymer coatings whose internal pores scatter sunlight to give solar reflectance values of 0.96-0.99 and thermal emittance of 0.97; under direct sunlight those coatings kept surfaces 6 degrees Celsius below ambient and delivered cooling powers of 96 watts per square metre. In 2015, silvered polymer films with a solar reflectance of 0.97 and thermal emittance of 0.96 were reported to remain 11 degrees Celsius cooler than commercial white paints under a mid-summer sun. A 2017 design embedded resonant polar silica microspheres randomly in a polymer matrix, producing a material translucent to sunlight but with infrared emissivity of 0.93 in the atmospheric transmission window; backed with silver, it delivered a midday cooling power of 93 watts per square metre and was suited to high-throughput roll-to-roll manufacturing.

• 6. Buildings and Rooftops

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  Cool roofs combine high solar reflectance with high infrared emittance to simultaneously reduce heat gain and increase heat loss. Traditional building surfaces such as paint, brick, and concrete already carry thermal emittances of up to 0.96, which lets them passively radiate heat to the sky at night. White cool-roof paint coatings reach solar reflectances of up to 0.94 and thermal emittances of up to 0.96. Their reflectance comes from optical scattering by dielectric pigments embedded in polymer resin; their thermal emittance comes from the resin itself. Common white pigments such as titanium dioxide and zinc oxide absorb ultraviolet radiation, which is why the solar reflectances of such paints do not exceed 0.95. A full-scale experimental campaign in Arganda del Rey, Spain, tested an active hydronic system combining sky-exposed radiators, thermally active ceiling panels, and thermal energy storage. It achieved average cooling potentials of up to 84.9 watts per square metre and reduced indoor temperatures by up to 8.7 K against a control cell, while maintaining comfort with outdoor temperatures approaching 40 degrees Celsius. A numerical model validated with data from a residential demonstrator assessed in Madrid and Rome showed seasonal energy efficiency ratios up to 35 times higher than conventional air conditioning, with seasonal energy performance improvements of 6.2% using a commercial daytime radiative cooling material and 10.3% with an ideal broadband emitter.

• 7. Heat Shields and Space Telescopes

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  High emissivity coatings that facilitate radiative cooling are also applied in reusable thermal protection systems for spacecraft and hypersonic aircraft. In these heat shields, a high emissivity material such as molybdenum disilicide is applied over a thermally insulating ceramic substrate. Total emissivity values in the range 0.8-0.9 must be maintained across a wide range of high temperatures. Planck's law dictates that as temperature rises, the peak of radiative emission shifts to shorter wavelengths and higher frequencies, which shapes material selection for different operating regimes. Effective heat shields must also provide damage tolerance; some designs incorporate self-healing capability through the formation of a viscous glass layer at high temperatures. The James Webb Space Telescope takes a different approach: its large reflective sunshield permanently blocks radiation from the Sun, Earth, and Moon, keeping the telescope structure in shadow so it cools purely by radiation, reaching an operating temperature of roughly 50 K. That operating temperature is critical to the telescope's infrared science, and the fact that it is sustained by passive radiative cooling rather than by mechanical refrigeration points toward just how far this principle reaches, from ancient ice pools in India and Iran to the coldest operating scientific instrument humanity has ever sent into space.