Humidity Effects on the Positive Leader Steps in Laboratory Long Spark Discharges

Abstract The stepwise development of positive lightning leaders is still not well understood. A recent laboratory study indicated, at high absolute humidity, positive leaders can do steps due to the merging of a separate luminous structure and the primary leader channel, similar to the steps of negative leaders. The humidity may play a key role in the formation of positive leader steps, however, the humidity effect on the positive leader steps has never been explored. In this paper, we examine numerous positive long spark discharges at different humidity levels with the synchronized discharge current and high‐speed camera frames recording the evolution of leader channel. The positive leader propagation manners at different humidity levels are compared both morphologically and electrically. The effect of humidity on steps is further analyzed statistically. We found that the positive leader steps characterized by steep‐rise current pulse and abrupt channel elongation, which may be led by separate luminous structures, only appear under the condition that high absolute humidity is above a certain threshold. As the ambient humidity increases, these positive leader steps occur more frequently.