ULTRAVIOLET SKY RADIANCE DISTRIBUTIONS OF TRANSLUCENT OVERCAST SKIES

The diffuse sky radiation component in the ultraviolet wavelengths is often at least 50% of the global irradiance under clear skies, and is the dominant component of ultraviolet global radiation under transluce nt overcast skies. The distribution of sky radiance was measured in a rural area and modeled for wavelength bands of ultraviolet-B (UVB, 280 -320 nm) and ultraviolet-A (UVA, 320-400 nm). Sky radiance measurement s were made during the summer of 1993 over a wide range of solar zenit h angles using radiance sensors mounted on a hand-operated hemispheric al rotation mount. UVB irradiance measurements were also made during e ach scan. Since the ratio of measured irradiance under overcast skies and that predicted for clear skies was not correlated with cloud base height, opaque cloud fraction, or solar zenith angle, it was concluded that the scattering from the clouds dominated the global irradiance, and this scattering was relatively unaffected by the scattering off op aque clouds in the translucent atmosphere. Analysis of the translucent overcast sky UVA and UVB radiance measurements using a semi-empirical distribution model showed that the spectral influences on multiple sc attering, single scattering, and horizon brightening components of the distributions agreed with basic atmospheric radiation theory. The bes t model used solar zenith, the sky zenith, and the scattering angle wi th resultant coefficient of determination values of 0.62 and 0.25 for the UVA and UVB respectively. The developed equations can be applied d irectly to the diffuse sky irradiance on the horizontal to provide rad iance distributions for the sky.