Abstract

By use of the Rayleigh scattering theory for pure air, primary scattering and known upper air densities from rockets, the brightness of the zenith twilight sky was calculated and values were obtained two to four times greater than those observed at Sacramento Peak, New Mexico ( J. Opt. Soc. Am. 42, 353 ( 1952). The attenuation of the atmosphere there was observed to be twenty percent above that of the Rayleigh theory, and the ozone thickness was measured to be about 2.2 mm. When the absorption of the Chappuis band of ozone in the visible spectrum was added to the Rayleigh theory, the calculated sky brightness came into agreement with observation for solar depression angles below the horizon from about 0° to 6°. For the sun below 7° the calculated zenith sky brightness fell rapidly below the observed brightness, showing that primary scattering in the atmosphere above about 60 km does not contribute appreciably to the brightness, as has long been known ( J. Opt. Soc. Am. 28, 227, 1938).

Calculation showed that during the day the clear sky is blue according to Rayleigh, and that ozone has little effect on the color of the daylight sky. But near sunset and throughout twilight ozone affects the sky color profoundly. For example, in the absence of ozone the zenith sky would be a grayish green-blue at sunset becoming yellowish in twilight, but with ozone the zenith sky is blue at sunset and throughout twilight (as is observed), the blue at sunset being due about 13 to Rayleigh and 23 to ozone, and during twilight wholly to ozone.

© 1953 Optical Society of America

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