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LIGHTNING PHENOMENOLOGY 28 Figure 1.4 The ratio of intracloud to cloud-to-ground lightning as a function of latitude. From Prentice and Mackerras (1977). Lightning Location Networks The study of lightning phenomenology has made a major advance in the last decade with the introduction of new magnetic direction-finding techniques (Krider et al., 1980) that provide the means to monitor ground strikes over areas exceeding 106 km2. Extensive networks of lightning direction finders have been established for forest-fire detection in the western United States, Canada, and Alaska. Figure 1.6 shows the coverage as of the summer of 1984, and it can be predicted that within the next few years the entire United States will be covered. One expanding lightning detection network covers the East Coast and is approaching the Mississippi River to provide coverage of the eastern part of the United States (Orville et al., 1983). This network is operated by the State University of New York at Albany in a multidrop communication network that links all the direction finders to one computer. Data are now retrieved on the time, location, number of strokes in the flash, polarity of the charge lowered to ground, and amplitude of the peak magnetic radiation field that can be related to the maximum current in the first stroke. These data, in turn, can be analyzed and related to the meteorological patterns producing the observed phenomena. To report all initial results would far exceed the space available in this brief paper; nevertheless, it is interesting to note a few observations. The highest ground flash rate recorded by the East Coast Network occurred on June 13, 1984, when 50,836 flashes were detected in a 24-hour period over an area of approximately 250,000 km2. The highest hourly summary was 7800 flashes with the highest 5-minute rate exceeding 10,000 ground flashes per hour. These results are remarkable when it is realized that these flash rates were from storms in only three statesâPennsylvania, New Jersey, and part of New Yorkâand at the time were producing approximately 3 percent of the entire global lightning activity. Other results indicate that lightning is recorded in every week of the year along the East Coast and that the polarity of the lightning ground strikes shows a change from negative to positive in the fall and a shift back to negative in the spring. A discussion of positive lightning and its characteristics is presented by Rust (Chapter 3, this volume). Figure 1.5 Lightning flash density estimates on an annual basis. Adapted from Maier and Piotrowicz (1983) and MacGorman et al. (1984).