The steady state performance of radiators, exposed to a clear sky, is investigated. Radiators are subjected to a radiative environment consisting of infra-red and solar radiation. There is also a convective heat exchange between the radiators and the atmosphere. Four surface orientations (South, West, North, and East) are examined. For each surface orientation, six surface inclinations (15°, 3°, 45°, 6°, 75°, and 9° with respect to the horizontal) are considered. Both the daily heat loss and the instantaneous (hourly) heat rejection are considered for the fifteenth day of each month in a year. The climate used is that of Houston, Texas (Latitude 3°N and Longitude 95°W). The feasibility of using radiators to reject heat, for a given environment, depends on the time of the day and the magnitude of the Inlet fluid temperature. It is found that for fluid temperatures equal to or greater than 4°C, radiators oriented in all directions and at any surface inclination reject heat at all times of the year. The optimum surface orientation-inclination is determined by considering the daily total heat loss. In June and July, surfaces oriented to the south reject the greatest amount of heat, followed by radiators facing north, while east and west facing surfaces reject the least heat. For the period August-December-May, north oriented surfaces are superior, with an optimum surface inclination varying between 45° and 9°. In this same period of about ten months, east and west facing surfaces are the next efficient while south oriented surfaces reject the least heat and are inferior to horizontal surfaces.