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In this paper a method for cloud cover assessment at night-time (when only thermal infrared data are available) is presented. It is based on the analysis of long wave radiation transfer processes in partially cloudy areas, which led to the formulation of a simplified model of the surface-cloud-atmosphere system. The model was implemented in an operational and iterative algorithm to solve the radiative equations. The algorithm was validated using ground data collected at four meteorological stations in Argentina during November 1997 and May-June 1998, which were compared to cloudiness derived from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer thermal data. Differences between observed and calculated cloudiness were within ±1 okta in 78% of the cases studied, giving a bias of +0.5 oktas and a standard deviation of ±1.0 oktas in cloudiness estimation. [ABSTRACT FROM AUTHOR]