Abstract

With a calibrated Macbeth illuminometer measurements were made of <i>i</i><sub><i>z</i></sub> the brightness of the zenith sky and of <i>i</i><sub><i>g</i></sub> the energy flux across a vertical plane from the twilight horizon for the depression θ of the sun below the horizon from 0° to 13°. For clear sky conditions the <i>i</i><sub><i>z</i></sub>, θ and <i>i</i><sub><i>g</i></sub>, θ curves did not change within 30 percent with the season from October, 1937, to April, 1938, and were the same for evening and morning twilight. Calculation from the Rayleigh theory of molecular scattering and the observed <i>i</i><sub><i>z</i></sub> and <i>i</i><sub><i>g</i></sub> data showed that within ±30 percent the densities of the atmosphere from sea level to about 60 km were those of the density-height relation known to 20 km and extrapolated for a temperature of 218°K. It follows that the temperature of the twilight temperate zone atmosphere is 218°±15°K from 20 to 60 km. The influence of secondary scattering, determined from <i>i</i><sub><i>g</i></sub>, although small for small values of θ increased rapidly with θ to such an extent that the twilight zenith sky brightness measures gave no indication of the distribution of density above about 60 km.

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