REVIEW OF LIGHTNING PROPERTIES FROM ELECTRIC-FIELD AND TV OBSERVATIONS

From analysis of simultaneous electric field and TV records of 76 nega tive cloud-to-ground lightning flashes in Florida, various lightning p roperties have been determined and several new facets of lightning beh avior inferred. Only 17 % of the flashes were single-stroke flashes, l ess than half the commonly claimed percentage (e.g., Anderson and Erik sson, 1980). The initial electric field peak (and, by inference, curre nt peak) for the only strokes in single-stroke flashes was smaller tha n for first strokes in multiple-stroke flashes. Half of all flashes, s ingle and multiple stroke, struck ground at more than one point, with the spatial separation between the channel terminations being up to ma ny kilometers. One third of multiple-stroke flashes had at least one s ubsequent stroke whose distance-normalized initial electric field peak exceeded that of the first stroke in the flash. Thus such flashes are not unusual, contrary to the implication of most lightning protection and lightning test standards. Subsequent strokes of the order of 2 th rough 4 were more likely to create a new channel termination on ground than strokes of the order of 5 and higher. Further, leaders of lower- order subsequent strokes following previously formed and not-too-aged (100 ms or less) channels were more likely to show stepping, as oppose d to continuous propagation (i.e., to be dart-stepped leaders rather t han dart leaders), than were leaders of higher-order strokes. Finally, lower-order subsequent return strokes exhibited a larger initial elec tric field peak than did higher-order strokes. The second leader of th e flash (the first subsequent leader) encounters the least favorable p ropagation conditions of all subsequent strokes: more than half of the second leaders either deflected from the previously formed path to gr ound or propagated in a stepped, as opposed to a continuous, fashion a long the lowest part of that path. It is important to note that inters troke intervals preceding second strokes are similar to or shorter tha n those preceding higher-order strokes. These observations indicate th at channel conditions for the propagation of a subsequent leader are d etermined not just by the immediately preceding channel heating and co oling processes but rather by the entire channel history. In particula r, the status of the channel apparently depends on the number of strok es that have participated in its cumulative conditioning. The overwhel ming majority of long continuing currents, those with a duration longe r than 40 ms, were initiated by subsequent strokes of multiple-stroke flashes as opposed to either the first stroke in a multiple-stroke fla sh or the only stroke in a single-stroke flash. Strokes that initiate such long continuing currents were (1) relatively small (in terms of b oth return-stroke field peak and, as determined from an independent st udy in New Mexico, stroke charge), (2) followed relatively short inter stroke intervals, and (3) showed a tendency to be preceded by a relati vely large stroke. Millisecond-scale K and M electric field changes ap peared different in terms of both microsecond-scale pulse content and interevent time intervals. Often no microsecond-scale K and M field pu lses were detected. When they were present, such pulses were highly va riable and sometimes irregular in waveshape, as opposed to the alleged characteristic K-pulse waveform described by Arnold and Pierce (1964) , which has been extensively used in atmospheric radio-noise studies. There is a remarkable similarity between many lightning characteristic s in Florida and in New Mexico.