The non-obvious bit is that our atmosphere doesn't block all infrared; there are gaps in the spectrum that allow nearly all of the heat to pass into space and never return.
The slick thing that happens with this coating/paint is that it preferentially emits heat in the infrared bands that pass through our atmosphere to never return.
Colored and paintable bilayer coatings with high solar-infrared reflectance for efficient cooling (science.org)
(looking for unnaturally high heat emissivity from the atmospheres of distant planets in the atmospheric infrared spectrum windows might be a good indication of advanced intelligent life on the planet.)
Xiulin Ruan, a professor of mechanical engineering at Purdue University, didn’t set out to make it into the Guinness World Records when he began trying to make a new type of paint. He had a loftier goal: to cool down buildings without torching the Earth.
In 2020, Dr. Ruan and his team unveiled their creation: a type of white paint that can act as a reflector, bouncing 95 percent of the sun’s rays away from the Earth’s surface, up through the atmosphere and into deep space. A few months later, they announced an even more potent formulation that increased sunlight reflection to 98 percent.
The paint’s properties are almost superheroic. It can make surfaces as much as eight degrees Fahrenheit cooler than ambient air temperatures at midday, and up to 19 degrees cooler at night, reducing temperatures inside buildings and decreasing air-conditioning needs by as much as 40 percent. It is cool to the touch, even under a blazing sun, Dr. Ruan said. Unlike air-conditioners, the paint doesn’t need any energy to work, and it doesn’t warm the outside air.
In 2021, Guinness declared it the whitest paint ever, and it’s since collected several awards. While the paint was originally envisioned for rooftops, manufacturers of clothes, shoes, cars, trucks and even spacecraft have come clamoring. Last year, Dr. Ruan and his team announced that they’d come up with a more lightweight version that could reflect heat from vehicles.
“We weren’t really trying to develop the world’s whitest paint,” Dr. Ruan said in an interview. “We wanted to help with climate change, and now it’s more of a crisis, and getting worse. We wanted to see if it was possible to help save energy while cooling down the Earth.”
