Nanophotonic Radiative Sky Cooling

Sky & Sun

Solar reflectance R_solar 96%

Atmospheric-window emissivity ε_win 90%

Ambient air temperature 30°C

Convection coefficient h 8 W/m²K

Live Readouts

Surface temp

30.0°C

Sub-ambient ΔT

0.0°C

Net radiative cooling power

0 W/m²

How it works

Earth's atmosphere is almost transparent to infrared light between 8-13 µm — the "atmospheric window" — letting that band radiate straight through to the ~3 K of outer space instead of warming the air. A nanophotonic coating (alternating SiO₂/HfO₂ layers or polymer-embedded microspheres, each tuned to sub-wavelength thickness) is engineered to do two things at once: reflect almost all sunlight (0.3-2.5 µm) so it never heats up, while emitting strongly only inside that 8-13 µm window.

P_solar = (1 − R_solar) · S            [absorbed sunlight, S ≈ 900 W/m² by day]
P_window = ε_win · σ · (T_s⁴ − T_sky⁴)  [radiates through the window to cold sky]
P_other  = ε_other · σ · (T_s⁴ − T_amb⁴) [remaining IR, reabsorbed by the atmosphere]
P_conv   = h · (T_amb − T_s)            [convective/conductive coupling to the air]

dT_s/dt  = ( P_conv − P_solar − P_window − P_other ) / C_eff
Steady state:  P_solar + P_window + P_other = P_conv

R_solar — fraction of sunlight the coating bounces away; every percent left over is absorbed heat.
ε_win — how strongly the coating emits specifically inside the 8-13 µm window that sees space; higher means more heat escapes past the atmosphere.
T_sky is modelled ≈30 K below ambient — the effective radiative temperature of the window's view of space on a clear day — while the non-window band radiates against the atmosphere near T_amb (ε_other fixed at 0.15, a well-designed coating).
Because P_window can exceed the absorbed solar and convective gain even while the sun is up, the surface settles below ambient air temperature with zero energy input — this is real, deployed technology (Stanford's 2014 demonstration and since-commercialised roof/paint coatings).

Sunlight (reflected / absorbed) IR through atmospheric window → space IR reabsorbed by atmosphere

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