The density of the upper atmosphere and the brightness of the twilight sky

The intensity of the zenith skylight was measured for solar zenith angle of 90° to 106° for wave-lengths of 0.75μ, 0.59μ, 0.52μ, and 0.44μ. The polarization was measured for the 0.44μ region. The difference between the measured polarization and the computed polarization of the primary scattered light was used to determine the fraction of the total zenith skylight that was due to primary scattering. The equation relating the atmospheric molecular density to the intensity of the primary scattered zenith skylight contains an extinction term. It was shown that the extinction term can be evaluated most accurately for the blue light. The computed atmospheric molecular densities agree with those given by the N.A.C.A. tentative standard atmosphere.