PART VI — PRECIPITATION AND REGIONAL WEATHER PHENOMENA 



merit technique result in a pre- 

 dictable cloud response? 



2. Which of the various possible 

 cloud responses would be useful 

 to society, in what ways, and 

 under what conditions? 



3. Given that a useful cloud re- 

 sponse can be predicted from 

 a particular treatment of some 

 specific set of initial cloud con- 

 ditions, are our abilities and 

 tools for diagnosing the occur- 

 rence of these conditions suf- 

 ficient to permit exploitation of 

 such treatment? In what time- 

 space scale? In what economic 

 framework? 



4. What is the proper division of 

 resources between : 



(a) basic research, where the 

 sought-for end product en- 

 hances knowledge about 

 clouds and their physical 

 response to seeding; 



(b) pilot projects, where the 

 chief objective is assess- 

 ment of the economics of 

 a particular cloud-modifi- 

 cation scheme; and 



(c) field operations, where the 



principal aim is to maxi- 

 mize the field of a changed 

 weather element? 



Projected Scientific Activity — Be- 

 cause of the complexity of the at- 

 mosphere and our limited knowledge 

 about modifying it, it is likely that 

 the skill in recognizing seeding op- 

 portunities can be developed only 

 from the results of a number of care- 

 fully designed experimental projects 

 aimed at testing seeding hypotheses 

 in various types of weather situations 

 in different parts of the country. 

 Project WHITETOP and the Bureau 

 of Reclamation Upper Colorado Pilot 

 Project are examples of what these 

 projects might look like, each of 

 which will require from three to ten 

 years. Until such studies are carried 

 out, scientists will probably be unable 

 to specify how much precipitation can 

 be changed, under what conditions, 

 and how often these conditions oc- 

 cur. Technology is already at hand 

 and scientific principles of experi- 

 ment design are known. We must, 

 however, be prepared to accept dis- 

 advantages as well as advantages to 

 the underlying population. 



Economic and Social Implications — 

 The interactions of cloud seeding with 

 society are clearly enormous, but they 

 are hard to detail because we lack 



firm information as to how much and 

 how often precipitation can be modi- 

 lied, and also because most studies 

 have emphasized the scientific as- 

 pects with little regard for the eco- 

 nomic, social, and political issues. 



Since there are few places in the 

 United States where the economy is 

 tied to a single economic enterprise, 

 almost any change in precipitation is 

 likely to disadvantage some while 

 working to the advantage of others. 

 We sorely need studies to learn the 

 full scope of public cost and public 

 benetit of changes in weather. We 

 can start by using the natural vari- 

 ability of weather and determine just 

 how a departure of weather from 

 long-term normality works its way 

 through the economy of a region. 

 Such studies — involving the collec- 

 tive effort of sociologists, economists, 

 and meteorologists — should be en- 

 couraged. 



Even with such knowledge, one 

 comes ultimately to the thorny is- 

 sues of how we decide when and 

 where to practice weather modifica- 

 tion, and how the disadvantaged are 

 to be compensated. Will insurance 

 companies, for example, "pay off" in 

 a region of cloud seeding if evidence 

 develops that increasing rainfall also 

 increases hail? 



The Status of Precipitation Management 



Research and operational weather- 

 modification programs since the late 

 1940's have served to identify proce- 

 dures that appear related to precipi- 

 tation increases. At the same time, 

 these results have indicated areas 

 where real understanding and com- 

 petence are insufficient. 



A number of cloud-seeding tech- 

 niques have been developed. Ground- 

 based seeding with silver iodide 

 (Agl), whose crystal structure re- 

 sembles that of ice (see Figure VI-3), 



is the most common technique, espe- 

 cially for winter storms in moun- 

 tainous terrain. The seeding ma- 

 terial is carried aloft by vertical 

 motion resulting from the instability 

 of the air or from the lift due to the 

 mountain barrier. One remaining 

 fundamental problem involves diffu- 

 sion of the seeding material. Proper 

 seeding procedures require (a) that 

 the proper number of nuclei reach the 

 effective level in the cloud, and (b) 

 that the effect of the seeding will be 

 felt in the desired location on the 



ground. The diffusion process is a 

 rather complex function of vertical 

 temperature distribution and the 

 three-dimensional wind field. 



Airborne seeding with silver iodide 

 or crushed dry ice is frequently em- 

 ployed with summer convective 

 storms. The primary limitation of 

 aerial operations is whether or not 

 the aircraft can fly in weather condi- 

 tions where seeding will be effective. 



Various experimental designs and 

 statistical evaluation procedures have 



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