The inactivation of ultraviolet ray on silver iodide as a sublimation nuclei was examined experimentally, using natural (solar) and artificial (mercury arc) light sources. The silver iodide particles produced at the temperature of 600-700•Ž were introduced into the diffusing boxes which were equipped by three different kinds of filter, i.e. cutting off the light of wave-length shorter than about ƒÉ-240 mƒÊ, ƒÉ-345 mƒÊ and ƒÉ-415 mƒÊ respectively. Then they were irradiated by ultraviolet ray of natural or artificial source. In order to examine the nucleating properties of silver iodide particles, the small part of them were sampled by a syringe at proper time intervals and then injected into the cold box at the temperature of -14•`-16•Ž. Then the number of ice crystals formed in the cold box was counted by visual observation. The logarithm of counted number was plotted against time (minutes) and the decay constant was determined graphically from the slope. On the other hand, the effective energy distribution of ultraviolet radiation which actually exerted on the silver iodide particles in each diffusing boxes was computed, using the properties of filter transmissions and the light source. Thus the decay constant for the light of unit energy in different ranges of wave-length was finally determined. According to these results, the efficiency of inactivation became better, as the wave-length of ultraviolet ray irradiated became shorter. But in the case of solar radiation the intensity of radiation shorter than ƒÉ-350 mƒÊ was so weak that the actual effect was almost due to the longer side of the radiations. The rate of decay was about 1/500 per hour when irradiated through the cello phane-filter by natural light on a clear day. 1. 緒 論 沃化銀粒子に よつて 氷晶をつ くる場 合,紫 外線 に照射された粒子は,そ の機能が著 しく減衰する ことは多 くの実験に よつて明 らかにされた(1)~ (4)。 Inn(2)は440 mμ より短波長の光を沃化銀粒 子に 照射す ると色が変化し,波 長が短 くなるにつ れて その影 響も大きくなることから,こ の波長 よ り短い光に よつて沃化銀が分解するのだとした。 また 氷晶形成の機能の減衰も この光吸収に起因す るものと考えられている。 我 々は この問題を更に詳 しくしらべ るため太陽 光 又は超高圧水銀灯を光源に し,分 光透過度のち がつたフィルターを別 々の拡散箱に取付けて,紫 外線照射による減衰の模様を 同時に しらべた。予 備的な実験は 昭和28年4月 から6月 まで気象 研究所中野分室において,本 実験は昭和29年2月 から6月 までの間 気象研究所において実施 した。2