Time-intervals between negative lightning strokes and the creation of new ground terminations

Abstract

On average, negative cloud-to-ground (CG) lightning flashes produce 3 to 5 return strokes, and a new ground termination is produced when any stroke after the first strikes the ground in a different place. In order to understand better the physical factors that affect the formation of new ground terminations, high-speed digital video cameras with time-resolutions and exposure times ranging from 125μs (8000 frames per second) to 2ms (500 frames per second) were used to record images of cloud-to-ground lightning in southern and southeastern Brazil and southern Arizona (USA), between February 2003 and September 2007. Some relevant information regarding the formation of new channels was obtained from the analysis of the previous interstroke time intervals and the number of previous strokes following the same path to ground. Although most of the subsequent strokes tend to follow the previously formed channel, this tendency is not observed in the second stroke (that is, the first subsequent stroke). 52% of the new channels occur in the second stroke. Contrary to what it was generally assumed in some past studies (Kitagawa et al., 1962; Malan, 1956; Rakov and Uman, 1990; Rakov et al., 1994; Winn et al., 1973), the formation of a new channel stroke is not clearly dependent on the interstroke interval that precedes it. In general, most of the new channels occur after a single usage of the channel and in these cases the previous interstroke time interval is not an important parameter. However, when the channel is used more than once, a new channel occurs mostly after a long interstroke interval.