Abstract

The excellent data of zenith polarization and color ratio (CR) during twilight obtained by Coulson at the Mauna Loa Observatory, Hawaii are subjected to a reinterpretation, especially with regard to the frequent deviations from the clear average. It is shown that a moderate lift of the earth’s shadow by distant clouds (or by low level haze) will reduce the red/green CR, while greater lift shifts the CR peak to a smaller solar depression. The effect on zenith polarization at wavelengths 0.6 μm is seen to correspond to a slight reduction of overall polarization for a moderate lift, and for a large lift to a shift of the polarization minimum (depolarization by stratospheric aerosol) from 4° solar depression angle to 2°. A generally high level of polarization, but with a barely noticeable 4° minimum, had also been observed earlier in Germany by Steinhorst when the stratospheric aerosol load was very small. The difference is explained by a higher and stronger aerosol layer in the tropics combined with a higher and cleaner troposphere.

© 1981 Optical Society of America

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OSA Recommended Articles
Volcanic twilights over Hawaii, April 1982

J. L. Gras
Appl. Opt. 25(9) 1377-1378 (1986)

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