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PHYSICS OF LIGHTNING 30 2 Physics of Lightning E. Philip Krider University of Arizona INTRODUCTION Lightning is a transient, high-current electric discharge that occurs in the atmospheres of the Earth and other planets and that has a total path length on the order of kilometers. Most lightning is produced by thunderclouds, and well over half of all discharges occur within the cloud. Cloud-to-ground flashes (Figure 2.1), although not so frequent as intracloud flashes, are, of course, the primary lightning hazard to people or structures on the ground. The continental United States receives an estimated 40 million cloud-to-ground strikes each year, and lightning is among the nation's most damaging weather hazards (see Chapter 1, this volume). The peak power and total energy in lightning are very large. Thus far, it has not been possible to simulate in the laboratory either the geometrical development of a lightning channel or the full extent of lightning damage. Lightning is a leading cause of outages in electric power and telecommunications systems, and it also is a major source of interference in many types of radio communications. The possible effects of lightning on advanced aircraft, nuclear power plants, and sophisticated military systems are problems of increasing concern. Besides its many deleterious effects, lightning also has some unique benefits. The chemical effects of lightning may have played an important role in the prebiotic synthesis of amino acids, and today lightning is still an important source of fixed nitrogen, a natural fertilizer, and other nonequilibrium trace gases in the atmosphere (see Chapter 6, this volume). Also, lightning-caused fires have long dominated the dynamics of forest ecosystems throughout the world. The electromagnetic fields that are radiated by lightning can be used to study the physics of radio propagation and have been used for many years in geophysical prospecting. Also, lightning-caused "whistlers" are still being employed to study the characteristics of the ionosphere and magnetosphere. Lightning plays an important role in maintaining an electric charge on the earth and is therefore an important component of the global electric circuit (see Chapter 15, this volume). It is clear, therefore, that an understanding of the physics of lightning is important to further insight into our geophysical environment as well as for the development of optimum protection from the lightning's hazards. In recent years, new experimental techniques have enabled researchers to obtain a better understanding of the physics of lightning. Among these techniques have been applications of optical, acoustic, and electromagnetic sensors to measure the properties of various discharge processes on time scales ranging from tens of nanoseconds to several seconds. These measurements