Atmospheric Attenuation In The Infrared Windows

None of the so-called atmospheric windows in the infrared are completely transparent, even in an atmosphere free of haze, fog, dust, or other particulate matter. Absorption by the atmospheric gases occurs because of many, very weak absorption lines in the windows and the extreme wings of very strong lines whose centers are located outside the windows. Most of the absorption lines between the visible and approximately 18 pm result from simultaneous changes in the vibrational and rotational energy levels of the absorbing molecules. At longer wavelengths most of the lines are pure rotational. A few pure rotational H2O lines appear at wavelengths as short as approximately 8 μm. (Ref. 1)