Daytime radiative freezing

Terrestrial radiative cooling utilizes materials that allow solar radiation to pass through or reflect away while efficiently radiating heat across the atmospheric transmission window. Here we show that liquid water, when its thickness is in a proper range, is highly transparent to the solar spectrum and emits strongly in the infrared. By incorporating a solar reflector underneath, a thin water layer of proper thickness exhibits a solar reflectivity of 96.6% and a thermal emissivity of 0.94. During outdoor testing, the water-based radiative cooler achieved sub-ambient temperatures of 6 °C at noon and 11 °C around 18:00 when direct sunlight was absent. Notably, the water temperature remained below freezing for most of the day, despite ambient temperatures above 0 °C.