PART V — SEVERE STORMS 



theoreticians and fifteen senior ex- 

 perimentalists (assuming, of course, 

 that these researchers are equipped 

 with the necessary skills), one might 



expect significant progress in our 

 detailed understanding of lightning 

 in the next ten to fifteen years. There 

 is certainly no assurance of success 



in any lightning research. It is clear, 

 however, that a successful effort to 

 understand lightning must be a long- 

 term effort. 



Reduction of Lightning Damage by Cloud Seeding 



Lightning is an important cause of 

 forest fires throughout the world and 

 especially in North America. In an 

 average year, about 10,000 forest fires 

 are ignited by lightning; in a severe 

 season, the number may rise to 

 15,000. The problem is particularly 

 acute in the western states, where 

 lightning ignites over 70 percent of 

 the forest fires. Here, hundreds of 

 fires may be ignited in a single day, 

 many of them in remote and inac- 

 cessible regions. These peaks in oc- 

 currence, along with existing heavy 

 fire loads, tax fire-suppression agen- 

 cies beyond reasonable limits of man- 

 power and equipment. Fire-suppres- 

 sion costs can be very high; direct 

 costs may approach $100 million per 

 year while losses of commercial tim- 

 ber, watersheds, and other forest 

 resources may be several times this 

 amount. In addition to loss of human 

 lives, lightning fires constitute a 

 growing threat to homes, businesses, 

 and recreational areas. 



Potential Modification Techniques 



What steps could be taken in 

 weather modification to alleviate the 

 lightning-fire problem? The most ob- 

 vious is to reduce the number of 

 cloud-to-ground discharges, particu- 

 larly during periods of high fire 

 danger. Those characteristics of dis- 

 charges most likely to cause forest- 

 fire ignition might be selectively mod- 

 ified to decrease their fire-starting 

 potential. Also, the amount of rain 

 preceding or accompanying lightning 

 could be increased in order to wet 

 forest fuels and thus decrease the 

 potential for fire ignition and spread. 



A Seeding Experiment — The large 

 losses in natural resources each year 



caused by lightning-ignited forest 

 fires has prompted the Forest Service 

 of the U.S. Department of Agricul- 

 ture to perform a series of experi- 

 ments in the northern Rocky Moun- 

 tains which are aimed at reducing 

 fire-starting lightning strokes by 

 massively seeding "dry" thunder- 

 storms over the national forests. 

 Following is a summary of results of 

 the studies of lightning-fire ignition 

 and lightning modification. 



The first systematic program of 

 lightning modification was conducted 

 in western Montana in the summers 

 of 1060 and 1961. This two-year 

 pilot experiment was designed to test 

 the effect of seeding on lightning 

 frequency and to evaluate lightning- 

 counting and cloud-seeding methods 

 in mountainous areas. Some 38 per- 

 cent fewer ground discharges were 

 recorded on seed days than on days 

 when clouds were not seeded. Intra- 

 cloud and total lightning were less 

 by 8 and 21 percent, respectively, 

 on seed days during the two-year 

 period. Analysis of these data by a 

 statistical test showed that, if seeding 

 had no effect, differences of this 

 magnitude would occur about one 

 in four. Also, the experiment con- 

 firmed the need to develop a contin- 

 uous lightning-recording system that 

 could resolve the small-scale details 

 of individual lightning discharges. 

 Subsequently, a continuous lightning- 

 recording system and improved cloud- 

 seeding generators were developed. 



Building a Data Base 



A new lightning-modification ex- 

 periment was begun in 1965, with 

 the first phase to last for three sum- 



mer seasons. The objectives were 

 to gain additional information on 

 the frequency and characteristics of 

 lightning from mountain thunder- 

 storms and to determine if there is 

 a significant difference in the occur- 

 rence and character of lightning from 

 seeded and unseeded storms. It was 

 not designed to confirm or reject a 

 single mechanism by which lightning 

 is modified by seeding. Rather, a 

 primary objective was to build a 

 body of observations of lightning 

 from both seeded and unseeded 

 storms and to use these data to build 

 appropriate hypotheses and models 

 for testing in future experiments. 

 Appropriate statistical tests were in- 

 cluded in the design of the experi- 

 ments as a basis for evaluating dif- 

 ferences attributable to treatment. 



Analysis of data on the basis of 

 the life cycle of individual thunder- 

 storms occurring in 1965-67 (14 no 

 seed, 12 seeded storms) gave the 

 following results at the given level of 

 significance for two-tailed tests: 



1. Sixty-six percent fewer cloud- 

 to-ground discharges, 50 per- 

 cent fewer intracloud dis- 

 charges, and 54 percent less 

 total storm lightning occurred 

 during seeded storms than dur- 

 ing the unseeded storms. 



2. The maximum cloud-to-ground 

 flash rate was less for seeded 

 storms. Over a 5-minute inter- 

 val, the maximum rate averaged 

 8.8 for unseeded storms and 

 5.0 for seeded storms; for 15- 

 minute intervals, the maximum 

 rate for unseeded storms aver- 

 aged 17.7 as against 9.1 for 

 seeded storms. 



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