The first step in an investigation of the use of satellite imagery data in automated short-range forecasts of clouds and precipitation is to examine the variation in ground brightness under clear-sky conditions. Bedford, Massachusetts, Albany, New York, Pittsburgh, Pennsylvania, and Dulles International Airport, Virginia, were selected as test sites for the period March to December 1977. Data were corrected for error and noise. Navigation was fine-tuned to an accuracy of 1 to 2 pixels. Average, standard deviation, maximum, minimum and range of brightness were computed over a range of area sizes around each station in order to establish a stable sample size. The statistics stabilized by a 5 x 5 pixel area. Although the number of cases was small, necessitating combining stations, the correlation between brightness and sun angle was high. The slope of the regression curve between these variables was steeper in spring and fall than in summer. Additional data are needed, but it is expected that they will refine rather than revise significantly the present results. (Author).