Characteristics of skylight at the zenith during twilight as indicators of atmospheric turbidity 2: Intensity and color ratio

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Abstract
This is the second of two papers based on an extensive series of measurements of the intensity and polarization of light from the zenith sky during periods of twilight made at an altitude of 3400 m on the island of Hawaii. Part 1 dealt with the skylight polarization; part 2 is on the measured intensity and quantities derived from the intensity. The principal results are that (1) the polarization and intensity of light from the zenith during twilight are sensitive indicators of the existence of turbid layers in the stratosphere and upper troposphere, and (2) at least at Mauna Loa primary scattering of the sunlight incident on the upper atmosphere during twilight is strongly dominant over secondary or multiple scattering at wavelengths beyond ~0.60 μm, whereas this is much less true at shorter wavelengths. It is suggested that the development and general use of a simple twilight polarimeter would greatly facilitate determinations of turbidity in the upper layers of the atmosphere.