186 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



information about natural precipitation processes. There is at 

 least some jDossibility that the precipitation can be increased in cer- 

 tain cases. 



7. The conditions under which it appears possible that seeding 

 might cause, or increase, precipitation are almost or exactly the 

 same as those required for the natural release of precipitation. 



8. Inasmuch as the most favorable conditions for the augmenta- 

 tion of precipitation by seeding are almost or exactly the same as 

 those requisite for natural precipitation, definitive results cannot 

 be expected from isolated seeding experiments. 



9. On the basis of present phj^sical knowledge and synoptic ex- 

 perience there appears to be no prospect that large-scale effects can 

 be produced by seeding. 



In view of the above conclusions it is considered that attempts at 

 the practical application of cloud-seeding techniques to increase nat- 

 ural rainfall are premature. The entire problem is still in the 

 research stage, and any funds available should be devoted to research 

 on the basic mechanisms involved. Useful research results cannot be 

 expected from personnel without extensive training and experience 

 in cloud physics and synoptic meteorology. 



It is the opinion of the writer that not enough attention has been 

 paid to the meteorological factors in most of the past cloud-seeding 

 experiments. It is believed that this omission is in large part respon- 

 sible for some of the diametrically opposed conclusions of certain 

 workers in this field. 



There are two general approaches to further research in this field : 

 full-scale experimentation with a properly designed experimental 

 plan and adequate meteorological data ; and detailed studies of cloud 

 physics, both in the laboratory and in the free atmosphere. If 

 possible, both approaches should be prosecuted simultaneously. The 

 second plan is less expensive than the first and promises more funda- 

 mental results. However, the full-scale experimental trials must be 

 made in the end since a complete knowledge of cloud physics would 

 still leave out some important meteorological factors. 



In some quarters it is argued that cloud seeding has been a failure 

 and that further experimentation is unwarranted in view of the high 

 cost. It must be admitted that the high hopes of weather control 

 proclaimed by the popular press have not been realized. Nevertheless, 

 the demonstration that several cubic miles of supercooled cloud can be 

 converted to ice crystals with a few ounces or pounds of suitable 

 material is extremely spectacular in the light of previous efforts to 

 control meteorological processes. This should be sufficient to stim- 

 ulate further investigation. It is quite possible that further funda- 

 mental discoveries lie ahead which will not be uncovered unless the 



