Abstract

Two physical processes causing the attenuation of brightness contrast of objects by the atmosphere have been isolated. One of these is related to scattering by the particles in the atmosphere and the other is related to convection currents of small local air masses of a variety of optical densities which cross a line of sight during the observation of an object through the atmosphere. These small air masses momentarily deflect the line of sight from the object to its background, thereby changing the apparent contrast of the object viewed. This attenuation process is designated as “optical haze” to distinguish it from reduction in contrast caused by scattering processes. Some authors have referred to this as “shimmer.” In the case of a black object viewed against the horizon as a background, the intensity of optical haze has been found to vary with time throughout the day. This temporal variation of optical haze was found to have at least one minimum during the day, usually after sunrise and before sunset. During the afternoon minimum, the optical haze was found to practically vanish at which time scattering alone was responsible for the atmospheric attenuation of brightness contrast and hence the two attenuation processes could be isolated on days in which the scattering was reasonably constant. On such days, studies of the relative importance of the two physical attenuation processes mentioned have indicated that as much as fifty percent of the atmospheric attenuation of brightness contrast can be attributed to optical haze. This suggests a need for modifying the theoretical treatment of the subject of atmospheric attenuation (to make allowances for optical haze which is ordinarily not considered as a factor).

© 1950 Optical Society of America

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