Summary of results from the spectral and angular sky radiation measurement program

Abstract Extensive measurements of the thermal IR radiance of the sky were obtained at 6 U.S. locations: Tucson, AZ; San Antonio, TX; Gaithersburg, MD; St. Louis, MO; West Palm Beach, FL; and Boulder City, NV. Fifty thousand observations were obtained at half hour intervals during 1979 and 1980. Each observation consists of measurements in 7 spectral bands, wavelengths (in microns) of 8.1–13.7, 8.3–9.1, 9.4–9.9, 10.0–11.4, 14.0–15.8, 16.6–21.6 and 6–17 at zenith angles of 0, 20, 40, 60 and 80°. The data have been expressed and presented as apparent sky emissivities. It is shown that the measured spectral and angular sky emissivities can be reliably estimated from a knowledge of the total (global) sky emissivity, using an empirical “sky emissivity” equation. The results are of particular relevance to the performance of radiative cooling systems designed to make use of spectral and/or angular selectivity.