PART V — SEVERE STORMS 



Current Status of Hail Prevention 



Hail losses in the United States, 

 including damage to property and 

 agricultural crops, have been esti- 

 mated at $200 million to $300 million 

 annually. While damage from hail- 

 storms can occur in nearly every 

 state, major hail losses are concen- 

 trated in a belt extending from west- 

 ern Texas through the High Plains 

 into Alberta, Canada. 



Most property owners respond to 

 the hail risk by buying insurance, 

 since damages by hail are typically 

 covered in a homeowner's compre- 

 hensive policy. However, insurance 

 coverage is less satisfactory for agri- 

 cultural crops, because of the high 

 premiums required in regions of high 

 hail hazard. Crop hail insurance pre- 

 miums in the Great Plains can range 

 up to 22 percent for a standard policy. 



During a period of crop surpluses, 

 it may be debatable whether crop 

 losses from hail justify any substan- 

 tial research effort. However, from 

 the point of view of the effects of 

 hailstorms on society, and consider- 

 ing the trauma of a hailstorm loss 

 and the fact that destruction of prop- 

 erty by hail is a net economic loss, 

 investigation of artificial hail preven- 

 tion deserves attention. 



In regions of high hail hazard, it 

 appears likely that an ability to re- 

 duce hail damage by as little as 5 or 

 10 percent would provide a net eco- 

 nomic benefit. It is anticipated that 

 hail reduction of 50 to 75 percent 

 should be possible, with a resulting 

 higher net economic benefit. 



Data Base: Large-Scale Field 

 Experiments 



Attempts to prevent hail by cloud 

 seeding were initiated shortly after 

 the early experiments of Schaefer and 

 Langmuir in the late 1940's. The 

 projects were based mostly on the 

 concept of reducing hailstone size 



through increases in the number of 

 hailstone embryos. Silver iodide was 

 the most common seeding agent and 

 was frequently released from net- 

 works of generators on the ground. 

 The early projects in this country 

 suffered from numerous handicaps, 

 including a lack of knowledge of 

 cloud processes and of resources for 

 any significant evaluation studies. 



The early hail-suppression projects 

 in the United States were conducted 

 for commercial sponsors and em- 

 ployed little or no statistical design. 

 Some randomized experiments using 

 ground-based generators were car- 

 ried out in Argentina, Switzerland, 

 and Germany. They yielded evidence 

 that silver iodide could affect hail- 

 storms, but that the effect could be 

 unfavorable as well as favorable. 



Throughout the 1960's, under- 

 standing of hail-formation processes 

 was advanced through a number of 

 extensive observational programs of 

 hailstorms in the United States and 

 abroad. The work carried out in 

 the Soviet Union during this period 

 is especially noteworthy, but observa- 

 tional programs carried out in north- 

 east Colorado also deserve mention. 



Improved understanding of hail 

 growth processes led to more sophis- 

 ticated systems for treatment. Seed- 

 ing was increasingly carried out from 

 aircraft and represented attempts to 

 influence specific parts of a hail- 

 bearing cloud rather than attempts 

 to increase ice-nucleus concentrations 

 throughout large volumes. This lo- 

 calization of the seeding treatment 

 reached its apex in the development 

 in the Soviet Union of a system to 

 introduce seeding agents into special 

 regions within a cloud by means of 

 artillery shells. 



There is increasing evidence that 

 the seeding treatment used through- 

 out the 1960's has been effective in 



eliminating hail from certain storms 

 and reducing hail damage in other 

 instances. Review of the evidence 

 from a number of hail-prevention 

 projects leads to the conclusion that 

 the projects were successful in some 

 instances. More recent results indi- 

 cate substantial success in hail pre- 

 vention in the United States, East 

 Africa, France, and the Soviet Union. 

 Indeed, a leading Soviet scientist is 

 quoted as saying that "the problem 

 of hail control is successfully solved." 



Mathematical Modeling 



During the past five years, sub- 

 stantial advances have occurred in 

 mathematical models of cumulus 

 clouds. An ability to create realistic 

 mathematical models of hailstorms 

 would provide the basis for a better 

 understanding of hail-formation proc- 

 esses and mechanisms for hail pre- 

 vention. 



Initial cloud-modeling attempts 

 utilized relatively simple one-dimen- 

 sional steady-state models. These 

 simple models were helpful as fore- 

 runners of more complex models 

 which now simulate realistically the 

 life history of a large rain shower. 



In addition to modeling the dy- 

 namics and life history of the large 

 cumulonimbus clouds, greater atten- 

 tion has been given to the mathemati- 

 cal simulation of individual hailstone 

 growth. Early efforts at development 

 of a mathematical formulation of 

 hailstone growth are being continued. 

 More recent work has given greater 

 insight into the hailstone growth 

 process, and shows that the primary 

 region of hailstone growth appears to 

 be in the higher and colder parts 

 the hail-bearing clouds. (See Figure 

 V-13) This information, derived 

 from the mathematical analysis, is 

 consistent with field observations. It 

 is of particular importance since it 



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