EFFECTIVE CLEAR SKY TEMPERATURES IN THE 8-TO 14-MICRON BAND
ARMY ELECTRONICS COMMAND FORT MONMOUTH NJ
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Apparent sky temperatures were measured on clear days and nights during a period in July and August of 1964 at Flagstaff, Arizona. A portable radiometer, with a two-degree field of view, sensing in the wavelength band 8 to 14 microns, was used to read sky temperatures at 5-degree intervals of zenith distances angles from the zenith to the horizon. These values were converted to radiances and summed over the whole sky, to arrive at effective wholesky temperatures. The apparent sky temperature at any zenith distance is not dependent on azimuth, but does vary with the time of day. The elevation scan of temperatures at a given time is duplicated by that of an equally clear sky at a different time if the zenith temperatures are the same. The effective whole-sky temperature is consistently the same value as the apparent sky temperature at a zenith distance of 54 degrees. Mathematical equations were developed empirically to express the dependence of radiance or apparent sky temperature on zenith distance. Graphs of the spectral radiance of the clear zenith sky, obtained from a report by Ohio State University Research Foundation, were integrated over the band 8 to 14 microns to arrive at apparent zenith temperatures for locations at altitudes from sea level to 14,000 feet. These ranged from 21C to 82C.