At about midnight of 18th September,1954, a typhoon (No.14,1954) ran away toward the sea after attacking Japan. In place of the typhoon a colder and less moist air flowed in from the north. Just at the time, a new activity of artificial origin was detected in rain water at Niigata and Hirosaki, both situated along the Japan Sea coast of the northern part of the Main Island. From 22nd to 24th of September, the activity increased rapidly, spreading over a wide area in Japan and finally an activity as strong as 0.3 × 10-6 curie/l was counted in rain water at Yamagata.
At the beginning of November 1954, another new activity almost comparable to that in September visited again. The estimated dates of explosion were respectively 13th-16th September and 28th-30th October. As air trajectories at 700 mb and 500 mb extended from Siberia to Japan, it might be supposed that the explosion tests would have taken place somewhere in USSR.
Presence of fission materials was confirmed. It is to be noted, S. KOYAMA at Niigata Univ. estimated the presence of Np-239 and U-237 in rain water precipitated on 24th September. In autumn 1954, the activity was stronger along the coast of the Japan Sea than that on the Pacific side.
Since the middle of March 1955, the radioactivities which might be attributed to Nevada experiments were detected several times both in rain water and the air at Tokyo and other places.
by Japan Meteorological Agency / Meteorological Research Institute