A meteorological study on the local changes in radioactive fallout

(1) The reason for the remarkably high rate of deposition of radioactive fallout in the winter season (winter peak) along the Japan Sea side of the northern part of Honshu is studied, on the basis of observations at Akita (39043'N, 140006'E). The results of study show that when the wind is west to north-west and vertical instability of the air is high, the specific radioactivity in rain and snow water in Akita reaches five to eight times that under other weather conditions. The winter peak in radioactive fallout along the Japan Sea coast can be explained by abundant precipitation amount, the prevailing wind blowing from the west to north-west originated in the Siberian Continent, and vertical instability in the lower troposphere due to warming of the cold air of continental origin during its passage over the warm current of the Japan Sea. (2) The causes of the smaller amount of radioactive fallout at Osaka than that in other regions are studied. The ratio of 90Sr fallout between Tokyo and Osaka is 1.4 and that between Akita and Osaka is 2.7. Since rainfall amounts at Osaka and Tokyo are almost the same, the difference in 90Sr fallout between the two cities is due to other meteorological factors. It is made clear that in Osaka, owing to its local topography, 72% of the total rainfall is associated with winds from the N to ENE. In Osaka a wind with the west to north-west component carrying radioactive dusts seldom blows. This seems to be a main reason for smallness of the radioactive fallout amount there. (auth.)