Measurements have been made of the spectral energy distribution of daylight (sunlight plus skylight) and skylight in the near-ultraviolet and the visible regions of the spectrum as a function of solar altitude for various atmospheric conditions as measured on planes of different orientations.
H. R. CONDIT and F. GRUM, "Spectral Energy Distribution of Daylight," J. Opt. Soc. Am. 54, 937-940 (1964)