Measurement of Skylight Polarization with Surface Reflectance Over Railroad Valley, Nevada.

Abstract

A multipurpose field radiometer (Optical Research Corporation Model MSR7000) was modified to measure the degree of polarization and radiance of skylight either on the ground or on a ship. The atmospheric observations were performed over Railroad Valley (38°32'N, 115°44'W), Nevada (USA) where the altitude is 1435m in wavelengths from 400nm to 1000nm. The polarization initially increased with increasing wavelength and reached a maximum of 70% at around 500nm. It then decreased with a further increase of wavelength. This trend might be explained by contamination of aerosols in the atmosphere if the surface reflectance is low. Furthermore, at fixed wavelengths polarization decreased generally with increasing sun elevation, whereas the surface reflectance factor increased. Based on a sun-photometer measurement, the atmospheric aerosols were stable during the measurements. In a simulation in which the surface is assumed to be Lambertian with a fixed reflectance, the skylight polarization decreased with increasing sun elevation. The observed magnitude of changing skylight polarization with solar elevation is smaller than that of simulated results. This might be mainly due to neglecting polarization generated by the surface and partly due to neglecting directional dependence of reflectance.