Radiative cooling
A heat-transfer process that lets surfaces lose thermal energy by emitting infrared radiation, sometimes cooling buildings, materials, or landscapes without active refrigeration.
What it is
Radiative cooling is the loss of heat through thermal radiation. Every warm surface emits infrared energy. If more energy leaves a surface by radiation than arrives from sunlight, nearby objects, and the atmosphere, the surface can cool. This is part of Earth's ordinary energy balance and also a design tool for buildings and materials.
The atmospheric window
Some thermal infrared wavelengths pass through the atmosphere more easily than others. NASA describes this transparency as a window through which heat from Earth's surface can escape toward space. Passive radiative cooling materials try to emit strongly through that window while avoiding heat gain from sunlight.
Nighttime and daytime cooling
Radiative cooling is easiest to notice at night, when a clear sky can let exposed surfaces become cooler than the surrounding air. Daytime radiative cooling is harder because sunlight adds heat. Modern passive daytime materials pair high solar reflectance with high infrared emittance so they can reject sunlight and radiate heat at the same time.
Materials and surfaces
Useful radiative cooling surfaces are engineered around spectral behavior: how they reflect, absorb, and emit at different wavelengths. Examples include white or reflective roof coatings, multilayer films, porous polymers, ceramic particles, paints, textiles, and experimental recycled composites. Durability, dirt, humidity, cost, and installation matter as much as laboratory performance.
Buildings and cities
Cool roofs are the most familiar urban example. They reflect more sunlight than dark roofs and often have high thermal emittance, which helps them shed absorbed heat. On a city scale, reflective and radiative surfaces can reduce roof temperatures, cooling demand, and some urban heat island stress, though the benefit depends on climate, roof area, building use, and season.
Limits and tradeoffs
Radiative cooling is not magic air conditioning. Clouds, humidity, dust, nearby warm surfaces, and low sky exposure reduce the cooling effect. In cold climates, very reflective or emissive roofs can increase winter heating needs. A material that performs well on a test stand may also face aging, cleaning, glare, fire, or code issues in real buildings.
Why it matters
Cooling demand is rising as cities grow and heat waves intensify. Radiative cooling matters because it attacks part of the problem without compressors or refrigerants: surfaces can be designed to reject sunlight and emit heat more effectively, lowering some cooling loads while complementing insulation, shade, ventilation, and efficient mechanical systems.