Definition: | ratio of the vertical optical thickness of a turbid atmosphere to the vertical optical thickness of the pure and dry atmosphere (Rayleigh atmosphere), related to the whole solar spectrum
where δR is the optical thickness with respect to Rayleigh scattering at the air molecules, δA, δZ, δW are the optical thicknesses with respect to Mie scattering and absorption at the aerosol particles, to ozone absorption, and to water vapour absorption respectively
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