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

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. Plain Language Summary: Lightning is driven by the propagation of positively and negatively charged, thermally‐ionized plasma channels, known as positive and negative leaders. Leaders propagate in a continuous or stepwise manner, characterized by intense reilluminations and abrupt elongations. The physics underlying the positive leaders is still mysterious. Previously, it was commonly accepted that the physics underlying the positive and negative leader steps differed. Recently, researchers found that, at least at high absolute humidity, positive leaders can form steps due to the merging of a separate luminous structure and the primary leader channel, similar to the steps of negative leaders. It seems that humidity plays an important role in the formation of positive leader steps, but the humidity effect on the positive leader steps has never been explored. In this paper, we reported the effect of humidity on the steep‐rise type steps. 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. Key Points: Humidity plays a key role in the formation of positive leader stepsThese steps only occur under high humidity above a certain thresholdAs the ambient humidity increases, these positive leader steps occur more frequently [ABSTRACT FROM AUTHOR]