The role of water particles in the atmospheric transmission of infra‐red radiation

Abstract The attenuation of infra‐red radiation by scattering on atmospheric haze particles, in the form of water droplets, is discussed in some detail. The spectrum is divided into two regions, the near infra‐red to λ 2·25 μ, where the refractive index of water may be assumed constant and real, and the region 2·25 to 12μ, where the index is complex and varies with the wavelength. This paper is concerned with the first region. It is shown that certain haze models, involving a continuous drop‐size distribution and previously used in explaining the attenuation of visible wavelengths and the aureole around the sun, adequately explain the window transmission measured by Gebbie and co‐authors (1951) along a horizontal path. The observed optical thickness of the whole atmosphere with respect to near infra‐red sunlight is also explained by means of these models. The infra‐red attenuation spectrum appears as a logical extension of the visible one and represents the continuous background upon which gas line and wing absorptions must be superposed. The magnitude of the latter can thus be assessed with more confidence.