Measurement of the Photometric Properties of the Upper Atmosphere

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Abstract
This paper describes the procedure in and the results of measurements made of the photometric properties of the atmosphere. The data were required for the solutten of a number of visibility and similar problems; the need for data is outlined, and the relevant photometric properties of the atmosphere are described. The experiments fall into two groups: measurements made in clean air, and those made in industrial atmospheres. In both atmospheres the polar distribution of light scattered by an element of the atmosphere was measured by polar nephelometers built for the purpose. In the clean air measurements, a nephelometer was carried on an aeroplane and a series of flights was made, several up to 30,000 ft., over different parts of the country and at different times of the year. A series was also obtained at sea level on a ship. On one series of experiments several other measurements were made in addition, including the brightness of a searchlight beam measured from the air, and the air-ground transmission. Both these formed checks to the nephelometry. In the industrial atmospheres a nephelometer was carried on an observation balloon. In these experiments the air-ground transmission was also measured carefully; and it was possible to deduce from the results the absorption as distinct from the scattering coefficient. Three series of measurements were made over two industrial towns to heights of 3,000 ft. The polar distributions of scattered light obtained varied greatly; there appeared to be little correlation with altitude or with scattering coefficient in most cases, though one type of curve was usual on and near the ground. Some measurements were made in extremely clear air, when distributions not unlike the Rayleigh distribution were obtained. There is a hope that the size of the scattering particles might, in some cases, be deduced from the shapes of the curves. In industrial atmospheres the absorption coefficient seems to be independent of the scatter, and there is a slight indication that it is small away from the vicinity of sources of pollution. Its amount is very variable. The meteorological conditions for each flight are given in an appendix, and another appendix outlines other methods which have been tried for obtaining some of the data.

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