Atmospheric transmittance determination from sky radiance in real time in the tropics

A method is presented for determining in real time the infrared atmospheric transmittance along the line of sight of a tracking radiometer from measurements of background sky radiance. The case at hand involves observations from a high-altitude site in the tropics at wavelength bands from 8 to 21 μm. Hawaiian radiosonde atmospheric profiles were analyzed using the lowtran 5 spectroscopic code. Model atmospheres were found unsuitable for characterizing the variable meteorological conditions. The infrared transmittance and sky radiance are shown to have a very close correlation that allows determination of transmittance within a few percent from radiance data in spite of very large fluctuations in tropospheric humidity and variations in effective atmospheric temperature.