Atmospheric transmission in the 1 to 14μ region

The transmission of the atmosphere for radiation of wave-lengths between 1 and 14 μ has been determined at sea-level, and its dependence on meteorological conditions investigated. Measurements have been made over paths of 2264 and 4478 yd., and the correlation with visibility and humidity studied in detail at four chosen wave-lengths, 2.18, 3.61, 10.01 and 11.48 μ,. Spectral transmission curves for typical conditions have been recorded for the complete range 1 to 14 μ and, in addition to the numerous absorption bands due to water vapour arid carbon dioxide, some bands caused by the rarer constituents, in particular N2O and HDO, have been observed. Throughout the wave-length range investigated, the transmission varies with the visibility, the effect being less marked at the longer wave-lengths. For example, when, under typical conditions, the visual transmission falls from 75 to 50% per sea mile, the corresponding change at 2.18 μ is from 85 to 73%, and at 10.01 μ from 87 to 83%. At the latter wave-length there is a strong dependence on the quantity of water vapour in the path. Assuming that the observed variations of infra-red transmission with visibility are due to the scattering of radiation by salt nuclei the characteristics of a suitable size distribution have been calculated. As the humidity increases the nuclei absorb moisture and increase in size. The distribution is in agreement with the limited observations on size and concentration that have been reported. For a visual transmission of 40% per sea mile the radius of the most frequently occurring droplet is calculated to be 0.4 μ.