This study analyzes the two-dimensional speed profiles of 107 stepped leaders and 93 dart leaders recorded by high-speed cameras in Utah (USA), together with data from lightning location system (NLDN). The results show that the stepped leader speed right before the ground contact (final speed) has a very strong correlation (R = 0.85) with the peak current of the initiated return stroke. It also shows that the correlation between the average speed and the peak current of the return stroke is strong (R = 0.73). The same analysis for dart leaders did not show any significant correlation with the peak current of the prospective return stroke (R = 0.41 to average speed and R = 0.29 to final speed). This paper discusses why stepped leaders exhibit a significant correlation, while dart leaders do not. In addition, why final speed is better correlated with peak current than average speed.

Recoil leaders develop in lightning flash decayed channels. The propagation of a recoil leader depends on its intensity and on the conductivity of the decayed channel. When the recoil leader is strong enough to propagate over the entire channel, a subsequent return stroke happens. When the recoil leader is not intense enough, only a partial reconstruction of the channel occurs, that is, only part of the decayed channel is reionized. The present work aims to analyze the herein named secondary recoil leader that originates near a previously formed recoil leader. When these secondary recoil leaders develop and connect to previous recoil leaders, they provide enough energy for the recoil leader to reionize the whole decayed channel of the lightning flash. High-speed videos analysis of upward lightning flashes shows that secondary recoil leaders play an important role on the formation and progression of dart leaders/subsequent return strokes.

The present work shows high-speed videos of two upward flashes that started with positive upward leaders and, instead of being followed by negative subsequent return strokes, they were followed by positive subsequent return strokes. In both cases, after the positive leaders developed, recoil leaders appeared in their decayed branches as would be usual in negative upward lightning flashes. However, in these flashes the negative end of a recoil leader connected to a positive leader of an intracloud flash nearby. The connection initiated a downward positive leader that re-ionized the decayed channel of the upward flash all the way to the tower giving origin to a positive subsequent return stroke. This work shows that recoil leaders do play an important role in the occurrence of bipolar upward flashes and their interaction with intracloud flashes can provide explanations for all types of bipolar upward flashes initiated by upward positive leaders.