A method is described whereby the opacity of the atmosphere is determined by the extinction of a beam of light. By the use of a chopped light beam, photocell, and a. c. amplifier the apparatus may be made independent of any amount of stray light and hence can function during both the daylight and night hours. Continuous records have been made of atmospheric opacity over a period of several weeks and the results compared with the extinction to be expected as the result of Rayleigh scattering by atmospheric condensation nuclei. The results show a maximum of scattering and hence greatest opacity in the hours near sunrise when the relative humidity is highest and the nuclei largest. During other hours of the day, the opacity is a function of the density of condensation nuclei, their size (as determined by the relative humidity), and the amount of non-nucleic matter (largely dust) in the atmosphere.

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