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 I_m,w 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 θ is given by θ=(ln I₀-ln I )/(ln I₀-ln I_m,w).