We present here rigorous analytical formulas derived from asymptotic formulas of the radiation transfer theory for determination of optical thickness and single scattering albedo on the basis of measurements of reflected or transmitted solar radiance or irradiance in stratus clouds. The detailed analysis is carried out of the error of the methodology proposed. The formulas derived are applicable to the interpretation of spectral measurements of solar radiation under continuous cloudiness carried out at the SP–22 Arctic station in summer 1979. The values of optical thickness and single scattering albedo revealed a pronounced spectral dependence.