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Surface emissivity in the thermal infrared region is an important parameter for determining the surface radiation budget in climate, weather, and hydrological models. This paper focuses on estimating the spatial and temporal variations of the surface emissivities using thermal infrared remotely sensed data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard NASA's Terra satellite. We developed a regression approach to use the ASTER and MODIS data for estimating the broadband emissivity (BBE; 8-13.5 mum). The regressions were calibrated using a library of spectral emissivity data for terrestrial materials. We applied these regressions to ASTER and MODIS data to obtain emissivity maps for several arid regions of the Earth. In the 8-9-mum band for sparsely or nonvegetated desert areas, emissivity values between 0.66 and 0.96 have been observed, which are due to the low emissivity of quartz-rich sands at these wavelengths. As a result, the range of BBE is between 0.86 and 0.96. The seasonal variation over a two-year period and the dependence on land cover/soil type were also investigated.