Lightning kills about 150 to 200 people annually in the United States. It is caused by a build-up of electrical charge in the clouds which discharges to the ground. Voltages of 20 to 200 000 000 V have been reported. The current is unidirectional and achieves levels of 30 to 270 kA ( B§rQsíeiQ..i1991). One flash is composed of a series of 10 to 15 strokes lasting for milliseconds. A direct hit is probably not survivable. Most victims are injured by sideflash from the high voltages developed when a nearby tree or other object is struck. Because of the extremely short exposure to the flash there is too little time for the skin to be penetrated by the current. The energy arcs over the outside of the person, producing the so-called 'flashover effect' ( CoxJ 992). As a result deep burns are infrequent.
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