> The paint's solar reflectance is so effective, it even worked in the middle of winter. During an outdoor test with an ambient temperature of 43 degrees Fahrenheit, the paint still managed to lower the sample temperature by 18 degrees Fahrenheit.
That statement is confusing to me, by ambient temperature do they mean surface temperature? I thought it was impossible to cool something below ambient without adding energy into the mix?
Yes I think they simply misured the surface temp of a nearby panel not painted with this white paint.
Depends on what you use as "ambient". Those panels and paints emit radiation in frequencies where atmosphere is transparent, so they use "coldness of outer space" as ambient.
Actually, it's not. There are materials that are both generally highly reflective and also have blackbody emissions with strong peaks in frequencies that the atmosphere is transparent to. Such materials can radiate enough energy into space to passively self-cool below nearby ambient temperature, even during the day.