A measure of the atmospheric transmission of incident solar radiation; if I0 is the flux density of the solar beam just outside the earth’s atmosphere, I the flux density measured at the earth’s surface with the sun at a zenith distance which implies an optical air mass m, and Imw the intensity which would be observed at the earth’s surface for a pure atmosphere containing 1 centimeter of precipitable water viewed through the given optical air mass, then turbidity factor 8 is given by 8 = (ln I0 – ln I)/(ln I0 – ln Imw).
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