Clear sky radiance distributions in ultraviolet wavelength bands

Abstract

In the ultraviolet, the diffuse sky radiation component is often at least 50% of the global irradiance for middle latitudes. Thus, knowledge of the sky radiance distribution is important for modeling ultraviolet irradiance in vegetative or urban canopies. In this study, the distribution of clear sky radiance was measured and modeled for wavelength bands of ultraviolet-B (280–320 nm) and ultraviolet-A (320–400 nm). Sky radiance measurements were made in a rural area over a wide range of solar zenith angles using radiance sensors mounted on a hand-operated hemispherical rotation mount. The measured sky radiance distribution in the UVA waveband differed greatly from that in the UVB waveband. The sky UVB radiance varied less across the sky hemisphere than the sky UVA radiance, in accordance with theory. A distinct region of minimum sky radiance was commonly found in the UVA waveband, but not the UVB waveband.