{b) Interval Between Dropping of Dry Ice and the Appearance of Rain. 

 — In each of the experiments careful observation was made of the time- 

 interval between dropping the dry ice and the first appearance of pre- 

 cipitation or rain from the base of the cloud. If this is plotted against 

 cloud thickness, there is a distinct tendency for the interval to increase 

 Avith increasing cloud depth. It appears that the time-interval is 

 in two parts — a gestation time of about ten minutes and an additional 

 time which is a function of cloud thickness and might therefore be 

 related to the rate of fall of precipitation within the cloud. 



(c) Rough observations have also been made of the intensity of 

 precipitation and when plotted against cloud thickness it is found, as 

 one might expect, that there is a distinct tendency for rain intensity 

 to increase with cloud thickness. 



Conclusions 



The number of occasions on which the top of cumulus clouds have 

 changed from water drops to ice particles and on which precipitation 

 has been observed to fall from the base of the cloud after being seeded 

 with dry ice while similar clouds in the vicinity remain unchanged is 

 convincing proof that rain can be induced to fall artificially. The chances 

 of success are found to improve with decreasing temperature of the cloud 

 top and are greatest at temperatures between —8° and — 16°c. It 

 appears that the interval between seeding and precipation from the base 

 of the cloud is a function of cloud thickness and that the intensity of 

 precipitation is also a function of this thickness. 



Discussion 



Mr. Mordy, of the Pineapple Research Institute of Hawaii, gave an 

 account of the experiments being conducted by the Institute, and 

 described the techniques that had been developed. Mr. Mordy urged 

 caution in the interpretation of these results which were in the process 

 of evaluation. 



Mr. Andrew Thomson, of Canada, said that in Canada many similar 

 experiments had been carried out, particularly with regard to the pro- 

 duction of snowfall. He said that in clouds at temperatures below- 

 freezing rainfall always resulted, while clouds at temperatures above 

 freezing gave no rain. The maximum amount of rainfall measured was 

 in the vicinity of two-tenths of an inch. 



Some experiments had also been made in Canada using silver iodide 

 instead of dry ice, but it was found that the silver iodide was not nearly 

 so effective in the free air as it had been found to be in experimental 

 cloud chambers. Numerous cases had occurred also of dry ice causing 

 the formation of clouds in clear air and the production of holes in strato- 

 cumulus clouds. 



Mr. Simpson, U.S. Weather Bureau, Hawaii, described similar experi- 

 ments in the United States and stated that although results had been 

 rather negative up to the present, the experiments w^ere stOl being actively 

 continued. 



In answer to a question by Dr. R. G. Simmers, Mr. Mordy stated that 

 in Hawaii most of the experiments had been on geographically persistent 

 clouds. 



130 



