Radiant temperature of the zenith at altitude

¹ Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania

Using a thermistor radiometer, measurements have been made of the radiant temperature of the zenith sky from ground level to 40,000 ft. The lowest temperatures measured were near –135 C, with an air temperature near –50 C at 36,000 ft. There was a high variability in sky temperature at altitude not observed at ground level. Although the low sky temperature as compared with air temperature points to the large thermal load imposed by radiant heat loss on man exposed to a high-altitude thermal environment, computation indicates this would amount to only about 77 Kcal/m²/hr for the completely clothed pilot.