Humans have harnessed this basic natural process for thousands of years. Desert peoples in Iran, North Africa, and India manufactured ice by leaving pools of water exposed to clear desert skies overnight, when radiative cooling happens naturally; other cultures constructed “cool roofs” capped with reflective materials that scattered sunlight and lowered interior temperatures. “People have taken advantage of this effect, either knowingly or unknowingly, for a very long time,” says Aaswath Raman, a materials scientist at UCLA and cofounder of the radiative­cooling startup SkyCool Systems.