Passive Daytime Radiative Cooling: How a Coating Can Sit Below Air Temperature Under the Sun

Radiative cooling coatings are often described as "cooler than the air, even in sunlight." That sounds like a violation of common sense, so it is worth being precise about the physics — and about where the tropics complicate it.

The atmospheric window

The Earth's atmosphere is largely transparent to infrared radiation in the 8–13 μm wavelength band, often called the atmospheric window. A surface that emits strongly in that band sends energy straight through the atmosphere to the cold of space (~3 K), with little absorbed on the way. That is a genuine heat sink that exists day and night.

Two properties a coating needs

A daytime radiative cooling (PDRC) coating must do two things at once:

1. Reflect almost all solar radiation (0.3–2.5 μm) so the sun does not heat it. 2. Emit strongly in the 8–13 μm window so it radiates to space.

Get both, and the coating can have a net heat loss even at noon. The surface temperature drops below the surrounding air — passive, no electricity. This is not reflection of sunlight; it is active radiation to a colder body through a transparent slice of the atmosphere.

Why the tropics are the hard case

The same humidity that limits ordinary reflective roofs also limits radiative cooling. Water vapor absorbs and re-emits infrared across much of the spectrum — and although the 8–13 μm window is the clearest part, tropical atmosphere still closes part of it. Cloud cover is the bigger issue: a cloud at 30 °C radiates strongly downward and effectively caps the radiative sink. Under thick monsoon cloud, the space-cooling path is largely blocked and the coating performs like a good reflective roof, not a sub-ambient one.

What to expect honestly

On a clear tropical night, radiative cooling coatings can run 5–10 °C below air temperature. On a clear tropical day, 2–5 °C below ambient is a realistic field figure for a well-made selective emitter; some laboratory reports are higher, but they are not field conditions. Under cloud, expect little or no sub-ambient effect.

The technology is real and grounded in well-established physics. The mistake is to assume the dramatic lab numbers survive a humid, cloudy Southeast Asian summer unchanged. They do not — and the next articles look at what that means for actual roofs.