Clear sky radiance distributions in ultraviolet wavelength bands

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. Nonlinear regression modeling of the sky radiance in the two wavebands showed that the isotropic sky was a poor descriptor of the UV radiance distribution. An anisotropic model of the clear sky UV radiance distributions was developed that used the scattering angle between the sun and the location in the sky and the sky zenith angle as predictor variables. The model coefficients of determination (r(2)) values were 0.93 and 0.69 for the UVA and UVB waveband models respectively. The model equations can be applied directly to the sky diffuse irradiance on the horizontal to provide radiance distributions for the sky. In addition, these distributions can be used to estimate correction factors for the shadowband method of diffuse irradiance estimation. Since the surface albedo of the measurement location was low and not explicitly used in the model equations, the distributions modeled may not apply to skies over snow cover or other high albedo surfaces.