A set of equations for full spectrum and 8‐ to 14‐μm and 10.5‐ to 12.5‐μm thermal radiation from cloudless skies

TLDR

The study develops new equations (15–17) for atmospheric effective emittance across the 10.5–12.5 µm, 8–14 µm, and full thermal spectrum based on a physical model. A comprehensive experiment at Phoenix, Arizona measured full‑spectrum thermal radiation, the 8–14 µm and 10.5–12.5 µm subrogions, surface air temperature, and vapor pressure. Analyses show that variations in water‑vapor thermal emittance arise from fluctuating concentrations of water dimers, and equation (17) markedly outperforms prior models.

Abstract

A comprehensive experiment was conducted at Phoenix, Arizona, involving the monitoring of full spectrum thermal radiation and those fractions of that flux that are contained within the 8‐ to 14‐μm and 10.5‐ to 12.5‐μm subrogions. Also monitored were surface air temperature ( T 0 ) and vapor pressure ( e 0 ). Analyses of the data established the source of water vapor associated thermal emittance (ϵ) variations of the cloudless sky as being due to the variable atmospheric concentration of water dimers—pairs of water molecules linked together by weak hydrogen bonds. New equations ((15)–(17)) based on this physical model were thus developed for the effective emittance of the atmosphere in both the 10.5‐ to 12.5‐µm and 8‐ to 14‐μm wavebands, as well as for the full thermal spectrum. Equation (17) was then shown to be a significant improvement over previous equations that have attempted to model this phenomenon.