The thermal radiance of clear skies

Measurements of the longwave radiance of the sky were made during the summer of 1979 at Tucson, Arizona; Gaithersburg, Maryland; and St. Louis, Missouri. The global longwave radiation (wavelengths greater than 3 μm) was monitored with a pyrgeometer and the distribution of this radiation in several spectral bands at five different zenith angles was monitored with a spectral radiometer. This paper presents results for the global sky radiation during clear sky conditions. The spectral radiometer was used to calibrate the pyrgeometer and to detect the presence of clouds. The results can most appropriately be summarized in terms of the correlation between the global sky emissivity ϵsky and surface dewpoint temperature Tdp(°C). The global sky emissivity is defined as the ratio of sky radiance to σTa4, where Ta is the absolute air temperature near the ground, and σ is the Stefan-Boltzmann constant. Based on 2945 night-time measurements in all three cities we find ϵsky=0.741 +0.0062Tdp with a standard error of estimate of 0.031. A similar relationship with almost identical coefficients holds during daylight hours.

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