The Passive Radiative Cooling Ceiling, Recomputed: Planck-Spectrum Integrals for Selective and Blackbody Emitters at Three Water-Vapour Loadings

A surface facing a clear sky radiates through the 8-13 micrometre atmospheric window and can cool below ambient. From numerical Planck-spectrum integrals with a stated band-averaged atmospheric transmittance model (zenith window transmittance 0.85, 0.65, 0.40 at precipitable water vapour 5, 15, 30 mm), the net night-time cooling power at 300 K is 110.8, 72.7 and 36.4 W/m2, identical for an ideal selective emitter and an ideal blackbody. Maximum drops with no non-radiative transfer are 68.7, 37.8 and 17.0 K (selective) against 20.0, 12.7 and 6.1 K (blackbody). Comparison with published experiments shows exposed-surface powers are already near the dry-sky ceiling, and remaining headroom lies in suppressing non-radiative and solar parasitic gains.