Atmospheric extinction of infra‐red radiation

Abstract The effects of scattering on the transmission of 2·6 to 12 micron radiation are analysed as a continuation of an earlier paper. Monochromatic extinction coefficients are determined on the basis of the original haze models and of the exact values of the complex index of refraction for water. The individual extinction cross‐sections are computed with a high degree of accuracy by means of a modified van de Hulst expression. The results are in good agreement with existing experimental determinations of window transmission except in the 8 to 12 μ region, where they support Roach and Goody's (1958) conclusion on the importance of wing absorption by molecular lines. The computed continuous spectrum of haze extinction presented graphically and in tables, further reveals interesting details which will be useful in determining the relative importance of absorption and scattering in atmospheric infra‐red radiation. A simple cloud model is also considered, and it is shown that even tenuous clouds may be quite opaque to infra‐red radiation over a wide band. In the window regions, clouds and dense hazes scatter a considerable portion of the incident radiation, thereby contributing to the infra‐red sky background usually attributed to black‐body emission.