As the experiment of cloud seeding with silver iodide has been made by the Chubu Electric Power Company at the top of Mt. Fuji from November 1955, in concert with this the observation of the number of ice crystal nuclei by the cold box (its schematic diagram is shown in Fig. 2) was made at Nakano Branch of Meteorological Research Institute (Tokyo) in order to detect the increase of the number of ice crystal nuclei in the air near the ground at Tokyo due to the cloud seeding and to make a datum to determine whether the cloud seeding might be effective or not to the rainfall observed at Tokyo.
It is necessary to know the number of ice crystal nuclei in the air near the ground and its variation under various weather conditions on unseeded days for the detection of the increase of its number, therefore the observation has been continued over about a month (in actual 20 days) from February to March 1956. (Observations were made for two 5 minutes at each observation time of 11 h, 14 h and 16 h 30 m every day. But on the seeded days the number of observation has been especially increased).
However, since the number of ice crystal nuclei in the atmosphere is few and the detection by the method of small sampling as usually used can not be used for this observation, the continuously sucking air method was adopted. (Distributions of temperature and liquid water content in the cold box for the actual observation are shown in Figs. 3, 4, and 6, respectively).
The results obtained from these observations are as follows: For the number of ice crystal nuclei in the air near the ground at Tokyo (with the exception of seeded days),
(i) Its number increased with wind velocity as shown in Fig. 7 and
(ii) Their maximum and mean values were 0.8 and about 0.25, respectively, per litre of air as shown in Fig. 10 and
(iii) Its number did not depend upon wind direction and weather conditions throughout this observation. (that is, it could not be found for its number to decrease on rainy days). (cf. Figs. 8, 9 and 7)
For the effect of the cloud seeding with silver iodide at the top of Mt. Fuji. (throughout this observation only 4 seeding experiments were made on Mar. 2, 3, 9 and 16)
(iv) On Mar. 2 and 16, on which silver iodide smoke might be considered to direct towards the district of Tokyo from its path estimated from the direction of cloud motion and the weather map, the increase of the number of ice crystal nuclei was obviously found as shown in Figs. 11-(i), and 14-(i), (cf. Figs. 11-(iii), 14-(ii) and and -(iii). At about 12 h on Mar. 2, 1956, the estimated trajectory of the smoke has directed to ESE as shown in Fig. 11-(ii), but the wind direction has changed from NNW to WSW since about 14 h 30 m when the front would be supposed to pass through the Kanto district, and this will be also confirmed from Figs. 11-(i), -(iii) and -(iv).) and on other days, that is, Mar. 3 and 9 no increase could be found because of the deviation of the path of the smoke from the district of Tokyo as shown in Figs. 12-(ii), -(iii), 13-(ii), and -(iii),
(v) Their maximum values amounted to about 3 or 4 times as large as that of unseeded usual days as shown in Figs. 11-(i) and 14-(i).
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