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POSITIVE CLOUD-TO-GROUND LIGHTNING 45 rent in the channel ceased and the remaining activity was intracloud. PRACTICAL IMPLICATIONS The destructive nature and practical importance of + CG flashes to the electrical power industry are at least partially documented. The multiline grounding and wire-strand fusing in high-voltage transmission lines cannot be explained by normal negative flashes to ground in Japan. Because of the usual occurrence of continuing current, + CG flashes may ignite a disproportionate number of fires, especially in grasslands and forests. Of the 75 + CG flashes reported in the Rocky Mountain study, all had field changes indicative of continuing current. The apparent pattern is for + CG flashes to strike outside the rainfall, further enhancing the likelihood of their starting a fire. One third of all storm days in a 3 year period had + CG flashes within a 30-km radius of the U.S. Forest Service observing site at Missoula, Montana. Thus on any given day the fire-starting probability from + CG flashes in mountain thunderstorms appears significant. The results to date of the observations in severe storms suggest that + CG flashes may be correlated with storm severity and tornado occurrence. If this hypothesis were shown to be true, it would enhance existing severestorm detection and warning capabilities. Additional observations to test this hypothesis are in progress. An area of increasing importance and study is the effect of lightning on new-generation aircraft (see Chapter 5, this volume). The relevance of + CG flashes to this problem is currently unknown; however, the reports of strikes to aircraft in the winter storms in Japan suggest that + CG flashes were involved. There are several aspects of + CG flashes needing additional clarification to ascertain whether these flashes pose an unusual threat to aircraft. They include the presence of fast return-stroke rise times and large peak currents; the frequent occurrence of continuing current; and the apparent tendency, especially in squall lines, for + CG flashes to propagate through large horizontal and vertical extents and to be in low radar reflectivity regions, which can appear "innocent" on radar. Because + CG flashes are a small percentage of the total flashes that most storms produce, appear to have large spatial extent, and do not seem to have a unique electric-field change, it is difficult to verify + CG flashes. However, because of their potentially devastating nature and their possible link to storm severity, + CG flashes have become an important research topic in several parts of the world. AREAS NEEDING ADDITIONAL RESEARCH It is worth noting those general areas of research that are needed to increase our understanding of + CG flashes and our ability to determine and cope with their effects on important technologies. These research areas are listed below but not in order of priority. 1. Measure electric-field changes and wave forms for a large number of confirmed + CG flashes to establish their typical characteristics with greater certainty. 2. Determine what storm types and environmental conditions are conducive to + CG flashes for storms throughout the year. 3. Relate the production of + CG flashes to storm evolution and structure, including also flash initiation and propagation characteristics. 4. Determine typical and extreme peak currents. 5. Evaluate the capabilities of automatic strike-locating systems in identifying + CG flashes, especially their detection efficiency and false identification rate. 6. Determine the significance of + CG flashes to aviation, especially to new-generation aircraft (typified by composite structures and computer-controlled flight). 7. Determine the importance of the threat posed by + CG flashes to power distribution systems, including whether they are the cause of the unexpected large number of faults on power lines in various parts of the United States. References Berger, K. (1967). Novel observations of lightning discharges: Results of research on Mount San Salvatore, J. Franklin Inst. 283 , 478-525 . Brook, M., M. Nikano, P. Krehbiel, and T. Takeuti (1982). The electrical structure of the Hokuriku winter thunderstorms, J. Geophys. Res. 87 , 1207-1215 . Brook, M., P. Krehbiel, D. MacLaughlan, T. Takeuti, and M. Nakano (1983). Positive ground stroke observations in Japanese and Florida storms, in Proceedings in Atmospheric Electricity , L. H. Ruhnke and J. Latham, eds., A. Deepak Publishing, Hampton, Va., pp. 365-369 . Fuquay, D. M. (1982). Positive cloud-to-ground lightning in summer thunderstorms, J. Geophys. Res. 87 , 7131-7140 . Nakahori, K., T. Egawa, and H. Mitani (1982). Characteristics of winter lightning currents in Hokuriku district, manuscript 82WM205-3, IEEE Power Engineering Society 1982 Winter Meeting (Jan. 31-Feb. 5, 1982, New York). Orville, R. E., R. W. Henderson, and L. F. Bosart (1983). An East Coast lightning detection network, Bull. Am. Meteorol. Soc. 64 , 1029-1037 . Rust, W. D., W. L. Taylor, D. R. MacGorman, and R. T. Arnold (1981a). Research on electrical properties of severe thunderstorms in the Great Plains, Bull. Am. Meteorol. Soc. 62 , 1286-1293 . Rust, W. D., D. R. MacGorman, and R. T. Arnold (1981b). Positive cloud-to-ground lightning flashes in severe storms, Geophys. Res. Lett. 8 , 791-794 .
