On the Transmission Coefficient of Ultraviolet Solar Radiation at Misima

Abstract

In this paper we report the seasonal change of the transmission coefficient of ultraviolet solar radiation observed at Misima over the period from September 1932 to November 1933. The instrument we used is the so-called “vital ray meter” made by the Tokyo Electric Company, consisting principally of cadmium photo-cell and aluminium leaf electroscope. Since the light we dealt with is not homogeneous, but covers the wavelengths ranginging from 0.29μ, to 0.32μ, the transmission coefficient has a meaning only in a sense of some mean. We define it as _??_ in the formula: where I_secz is the intensity observed, z being the zenith-distance of the sun, and I₀ the intensity outside the earth's atmosphere. We determined I₀ from the data of simultaneous observations made by Prof. R. Sekiguti on the summit of Mt. Fuji in June 1933 with our instrument and the standard cadmium cell, which had been in use there. The method of calculation is the one which was also suggested by him and is as follows. We calculate I₀/I_secz by numerical integration for the value of ozone-depth, which is determined from the observation with the standard photo-cell, and then multiplying I₀/I_secz by the value of I_secz, observed with our instrument, I₀ is obtained. The transmission coefficient was found to be markedly dependent upon sec z, so, in order to investigate the seasonal variation, we constructed isopleth diagram with season as abscissa and sec z as ordinate. We see from this diagram that the transmission coefficient becomes maximum in November, December and January and minimum in June and July. This may be attributed to the corresponding variation of ozone and dust content in the upper atmosphere.