Angular dependence of the emissivity of bare soils in the thermal infrared

Emissivity is one of the main factors to take into account when studying processes that take place in the Earth surface by using radiance measurements in the thermal infrared, such as surface energy balance, land surface temperature (LST) retrieval, classification of different types of surface, etc. For this reason it is necessary to study the factors that can influence the emissivity. The present work evaluates one of these factors: the variation of the emissivity with the zenithal observation angle over bare soils, specifically the variation of the relative emissivity calculated from measurements of radiances, almost simultaneous, at nadir (0^o) and at a certain angle (¿). The measurements of radiance were taken with the aid of a straightforward goniometric system that allows the measurement from nadir observation to 70^o (at 10^o increments) for a fixed azimuthal angle. The results show a significant decrease of emissivity with observation angle, which is especially accentuated in the case of sandy soils with high quartz content.