GENERAL METEOROLOGICAL OPTICS 
tions in visual range; the polarization of sky light; the 
depletion of direct sunlight; and the luminance of the 
sky in daytime. Only some aspects of sky luminance 
will be briefly mentioned here, as the other topics are 
treated elsewhere in this Compendium.” 
The luminance of the sky represents a considerable 
portion of that direct sunlight which has been depleted 
in its passage through the atmosphere. A practical 
aspect of this luminance is the resultant illumination 
without which the earth’s surface would be in darkness 
except where reached by direct or reflected sunlight. 
The most obvious characteristic of the sky’s luminance 
is 1ts blue color which is caused by the preference of the 
seattermg process for the shorter wave lengths of the 
incident radiation. Far from being a pure blue, the color 
is composed of other wave lengths to an extent that 
varies with the state of atmospheric turbidity, because 
with increase in size of the scattering particles the longer 
wave lengths increasingly participate in the scattering 
process. In the sky light, the ultraviolet component, 
whose intensity may exceed that of the direct sunlight, 
has biological (erythematous and bactericidal) and tech- 
nological (dye-fading, photographic) effects. 
The luminance of the sky is significant from a meteo- 
rological viewpoint because it enters as a factor in the 
appraisal of the atmospheric radiation balance and 
serves as a criterion of the turbidity of the air. In this 
respect, even mere estimations of the variations in the 
sky blue have been shown to be of practical value." 
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