Radiative cooling works by allowing objects to emit infrared radiation between 8 and 13 micrometers through the atmosphere into outer space. This process occurs because the vacuum of space radiates at approximately 3 Kelvin, which is far colder than surrounding air or nearby objects.
Communities in India used shallow ceramic trays filled with water under the night sky before modern refrigeration existed. Thick layers of hay insulated these trays against ground warmth so that water could lose heat upward faster than convection replaced it.
Lord Kelvin underestimated the age of Earth because his model ignored substantial heat released by radioisotope decay since that phenomenon was unknown in the 19th century. Convection within the mantle also remained unaccounted for in his initial calculations regarding planetary thermal evolution.
Daytime radiative coolers developed in 2014 can achieve sub-ambient temperatures of 5 degrees Celsius under direct sunshine. Field tests recorded cooling powers reaching 96 watts per square meter during peak daylight hours using multi-layer thermal photonic structures.
The James Webb Space Telescope uses radiative cooling to reach operation temperatures near 50 Kelvin while its large reflective sunshield blocks radiation from the Sun, Earth, and Moon simultaneously. The telescope structure remains permanently in shadow behind this shield and cools by radiation alone without mechanical systems.