Measurements of spectral radiance of the horizon sky

Typical spectral radiance data are presented for a rectangular portion of the horizon sky under various weather conditions and at different solar positions. Representative curves were selected from a total of seventy-nine spectral radiance measurements which covered meteorological ranges (at 665 mmu) from 10 km to 106 km and solar altitudes from -6 degrees to 74 degrees . The north horizon at noon with a 30-km or greater meteorological range typically produced a peak spectral radiance of 5 microW cm(-2)sr(-1)mmu(-1). The maximum spectral radiance value obtained in the study was 38 microW cm(-2)sr(-1)mmu (-1) for the south horizon in winter. The reciprocal dispersion varied from 1.8 mmu/mm at 400 mmu to approximately 8 mmu/mm at 1000 mmu. A mixture of fog and haze produced a spectral radiance curve with two maxima.