Number of Significant Experiments 



Thirty-eight complete experiments were made up to 25th August, 

 1948. In the majority of these rain was observed to fall from the 

 selected cloud after it had been seeded with dry ice. The most con- 

 vincing proof that this was brought about by the dry ice is the great 

 number of occasions on which the top of the cloud was observed to change 

 from water drops to ice particles and on which precipitation fell from 

 the cloud shortly after seeding, while similar clouds in the vicinity showed 

 no such change. 



A full description of the experiments and of the main results is being 

 published shortly in papers by Squires and Smith in the Australian 

 Journal of Scientific Research. 



Deductions From the Observations 



In making deductions from the experiments, eighteen of them have 

 been rejected either because the prevailing weather conditions made 

 observation difficult or because there was some doubt whether the rain 

 which fell was in fact caused by the seeding process. In the remaining 

 twenty the observations were adequate and the conditions were suffi- 

 ciently stable and clear-cut for the results to be known with certainty. 

 The main deductions which can be drawn from these limited data are 

 as follows : — 



(a) The Effect of Cloud Temperature. — It is found that the chances 

 of successfully producing rain increase as the temperature of the top 

 of the seeded cloud decreases below zero. Of the twenty tests being 

 considered, precipitation was produced in fifteen cases and quite 

 definitely not produced in five, as detailed in the table. 



Four of the five failures occurred between and — 8° c. All ten 

 experiments between — 8° and — 16° c. were successful, and there was 

 a single unexplained failure at — 20°c. 



Although temperature has been chosen as a convenient parameter 

 for estimating the probability of success it is obvious that the growth 

 of ice nuclei in the cloud is influenced by a number of other factors, 

 including the degree of supersaturation, the total water content, and 

 the time for which the nuclei are exposed. It is not suggested that 

 temperature is the sole controlling factor. Any of the above either 

 individually or collectively might influence success or failure. 



129 



5 — Pac. Congress 



