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 wave radiations compared with the latter, which is responsible for the blue color of the sky. Direct sunlight, moreover, varies in the form of its distribution curve with the time of the day and year, and with latitude on the earth’s surface. Different again, is the distribution which accompanies an overcast sky. (', 37 39; ')

It is necessary therefore to adopt as a standard an average curve, representing the conditions most frequently encountered.



6. Spectral Energy Distributions of Natural and Artificial Sunlight

(The solid line represents the distribution of average noon sunlight, while the broken line is that of Priest's precision artificial sunlight.)

Such an average, for noon sunlight at Washington, D. C., is given in and the solid line in. It is the mean of forty determinations, half of which were made at the summer solstice (June 21) and the other half at the winter solstice (December 21), both high and low atmospheric transmissions being included. The authority is of the. Average noon sunlight, thus defined, corresponds roughly to a black body temperature of 5000°K. the distribution not being strictly Planckian.