The water surface temperature of Lake Okeechobee under clear sky conditions was investigated in this study through the use of Automatic Picture Transmission (APT) thermal infrared data from the National Oceanic and Atmospheric Administration (NOAA) satellite Advanced Very High Resolution Radiometers (AVHRR). Cloud-free data were acquired during the months of May and December 1989. An image processing approach and regression model were used to convert the grey-level digital numbers of the APT imagery into temperature values. A third-order polynomial equation was successfully used to convert the digital numbers into temperature values with a high coefficient of determination (r2=0·98). The satellite-derived temperatures for Lake Okeechobee were correlated with ground-truth temperature measurements (r2= 0·74). The results of this study indicate that APT data are capable of being converted into lake surface temperatures with acceptable accuracy. Results of paired Mest and variogram analyses indicate that the temperature uniformity of the lake varied with the season, space, and time of day