Statistics and characteristics of cloud‐to‐ground lightning with multiple ground contacts

Abstract

During the summer of 1997, 386 cloud‐to‐ground (CG) lightning flashes were recorded on videotape near Tucson, Arizona. Within this sample, 136 flashes (35%) struck the ground in two or more places that were separated by tens of meters or more. Within this subset of 136, which we term “multiple channel flashes” (MCFs), 88 exhibited two or more separate and distinct channels between the cloud base and the ground, a type that we term a “new channel flash” (NCF); 37 forked or produced channels that struck the ground in two or more places and shared a common channel at higher altitudes, a type that we term an “altered channel flash” (ACF); and 11 exhibited both types of behavior. The ratio of NCFs to ACFs in Arizona was 99/48 or 2.1, a value that is consistent with measurements in Florida after allowing for differences in the cloud base heights. Altogether, the 386 CG flashes produced 558 different strike points; therefore, the average number of strike points per CG flash was 1.45. 67% of the new strike points were produced by the second stroke in the flash, and 72% of the changes in channel geometry occurred after there had been just one stroke in the previous channel. Ten of the 48 ACFs (21%) developed along a visible branch of the previous channel, and seven of the 136 MCFs (5%) contained strokes that reilluminated the first channel after a second channel had formed. The presence of a long, continuing luminosity during the initial three or more consecutive video frames appeared to lower the probability of forming either a new or altered channel on the second stroke (from 47% to 33%), but the statistics are limited, and our ability to identify individual strokes is biased by poor time resolution.