Radiative Cooling
Certain materials naturally lose heat by emitting infrared radiation through the atmosphere into the cold of space — cooling themselves without electricity, refrigerants or moving parts.
The natural process by which surfaces emit thermal infrared radiation and cool themselves — sometimes below the surrounding air temperature.
It offers a way to cool buildings, cities and communities without electricity, refrigerants or greenhouse-gas emissions — a rare climate solution that adds nothing to warming.
A new generation of engineered materials can now cool below ambient in full sun. That opens the door to passive cooling at scale in a rapidly warming world.
Radiative Cooling Explained
A short, on-camera introduction by a working researcher in the field — the clearest starting point before we go deeper into the physics.
Any object whose temperature is above absolute zero glows in the infrared.
Any object whose temperature is above absolute zero continuously emits thermal radiation. The warmer it is, the more it emits — and the shorter the wavelengths it emits at. This is why thermal cameras can see people in the dark: we are, quite literally, glowing.
Why do some materials radiate heat better than others?
Radiative cooling is not a coating trick — it is a property of the material itself. To understand why some surfaces can cool below the surrounding air while others simply bake in the sun, we have to go down to the scale of atoms and molecular bonds.
Every warm surface glows — but each material glows in its own colors.
- 1Sunlight strikes the surface and is absorbed by the material's atoms and molecules.
- 2That absorbed energy sets the atoms vibrating — stretching, bending and rotating the bonds that hold the material together.
- 3Vibrating charges radiate electromagnetic waves. For everyday temperatures, those waves fall in the thermal infrared.
- 4The exact wavelengths a material emits are set by its bonds and crystal structure. Some bonds vibrate right inside the 8–13 µm atmospheric window; others do not.
- 5Materials whose vibrations line up with the window can send their heat straight through the atmosphere to the cold of space. Materials whose vibrations lie outside the window lose their radiation to the atmosphere itself, which sends much of it back down.
Why not every white surface can cool the way PDRC does.
A white wall reflects sunlight, but reflection alone is not enough. To cool below air temperature under the sun, a surface must also emit strongly in a narrow infrared band. That requires deliberate engineering of the bonds, pigments and micro-structure of the material — not just its color.
The strong Si–O stretching vibration lies right inside the atmospheric window, which is why silica-based coatings and glass films are so widely used in engineered radiative coolers.
Oxides such as SiO₂, Al₂O₃ and certain perovskites have lattice vibrations that emit strongly across large portions of the window — but their solar reflectance must be engineered separately.
Many polymers happen to have vibrational modes inside the window. Structuring them as porous or hierarchical films boosts both reflectance and emissivity in the right bands.
Metals reflect visible light but are poor infrared emitters — a bare metal roof stays hot because it cannot radiate its heat away efficiently.
Titanium-dioxide paints reflect sunlight well but also absorb some UV and near-infrared, and their binders emit unevenly across the window. That is why an ordinary white surface warms in the sun instead of cooling below air temperature.
Modern passive daytime radiative coolers combine carefully chosen pigments, porous structures and binders so the whole material system reflects >95% of sunlight and emits >95% inside the atmospheric window.
The performance of a passive daytime radiative cooler comes from the whole material system — pigments, binders, porosity and thickness working together. No single ingredient is responsible; it is the way the system is designed that determines whether a surface can quietly send its heat to space.
A wavelength range where heat can reach space.
Think of the atmosphere as a wavelength-selective filter rather than a solid barrier. Greenhouse gases absorb most of the infrared radiation Earth emits and re-radiate it in all directions — including back toward the surface. But between roughly 8 and 13 µm, atmospheric absorption is much weaker, so infrared emitted in this range travels much more directly to space. This range is what scientists call the atmospheric window.
Below ~8 µm and beyond ~13 µm, greenhouse gases and water vapour absorb most outgoing thermal radiation and re-emit it in all directions.
Between 8 and 13 µm the atmosphere transmits over 70% of upward infrared on a clear, dry day.
Water vapour absorbs more strongly within the 8–13 µm range, reducing net radiative heat loss. Dry, high-altitude sites are ideal; humid nights weaken the effect.
Why some materials cool better than others.
Passive radiative coolers combine two properties that rarely appear together in nature: very high reflectance across the solar spectrum — ultraviolet, visible and near-infrared — and very high thermal emissivity specifically within the 8–13 µm atmospheric window.
Fraction of incoming sunlight (0.3–2.5 µm) reflected back to the sky. High reflectance stops heat before it enters the material.
Efficiency with which the surface emits thermal infrared. Tuned to the atmospheric window, that emission escapes to space.
The surface must not absorb infrared coming down from the atmosphere — otherwise net cooling collapses.
Cooling day and night — the harder half is the sun.
Radiative cooling has worked at night for as long as the sky has existed. The scientific breakthrough of the last decade is doing it under direct sunlight.
With no sun to fight, any surface with reasonable infrared emissivity will radiate to the sky and cool. This is why cars, grass and rooftops can drop well below the surrounding air temperature under a clear sky — and why frost forms on clear nights above 0 °C.
A passive daytime radiative cooler must reflect over 95% of sunlight while simultaneously emitting strongly in the 8–13 µm window. Recent Stanford and Columbia work has shown surfaces that cool below ambient at solar noon, with no power input at all.
Same sun. Three very different surfaces.
Under identical midday sunlight (~1,000 W/m²) and a 30 °C ambient air temperature, the choice of roof material dramatically changes surface temperature, absorbed sunlight and net cooling.
Illustrative values based on published measurements of dark asphalt, white cool roofs, and passive daytime radiative coolers (Raman et al., 2014; Zhai et al., 2017; Mandal et al., 2018). Real-world performance depends on humidity, cloud cover and surface condition.
Where radiative cooling is already going to work.
From individual buildings to entire agricultural systems, passive radiative cooling is quietly being tested and deployed across the world. Each application shares the same starting point: reduce the heat, keep the power off.
Passive cool roofs and façade coatings can reduce indoor air-conditioning demand by 10–40% in hot climates.
MEER's school-roof program in Freetown, Sierra Leone, is measuring indoor temperature drops in real classrooms.
Cooler wards without extra grid load — critical in regions where refrigerated cold-chains and patient safety hinge on ambient temperature.
Radiative-cooled greenhouses and shade structures can moderate crop stress during heatwaves without irrigation penalties.
Cool coatings and covers reduce evaporation losses and thermal degradation of stored water in arid regions.
Substations, data-center roofs and industrial equipment benefit from lower surface temperatures and reduced cooling load.
Paints, films and fabrics engineered as retrofit surfaces — turning any existing roof, wall or tent into a radiative cooler.
Metasurfaces, polymer nano-composites and glass-based photonic coatings are extending PDRC to more climates and surfaces.
How MEER puts radiative cooling into practice.
MEER combines highly reflective surfaces with radiative-cooling materials on real rooftops, then measures what actually changes — surface temperature, indoor comfort, health outcomes and cooling energy use.
Primary scientific sources.
Every claim on this page is grounded in peer-reviewed research. These are the papers, groups and public resources we recommend for readers who want to go deeper.
Radiative cooling doesn't live in isolation.
It sits inside a wider set of physical principles that together explain how Earth manages heat — and how we can help.