Lightning attachment process involving connection of the downward negative leader to the lateral surface of the upward connecting leader

High‐speed video camera records, with a temporal resolution of 20 µs and a spatial resolution of 2.4 m per pixel, of a downward negative lightning flash that terminated on a 440 m high building are examined. The attachment process in this flash exhibited an unexpected behavior in that the downward leader tip connected to the lateral surface of the ~400 m upward connecting leader (UCL) below its tip. It appears that the effect of the downward leader on the UCL is significant, while the effect of the UCL on the downward leader is negligible, except for the final 80 µs preceding the beginning of the first return stroke. The ratio of speeds of the downward leader and the UCL tends to decrease with time, ranging from 1.8 to 0.12, although the lower 80–100 m or so of the UCL were too faint to allow speed measurements. Key Points Downward leader can make contact with upward leader below its tip Ratio of speeds of downward and upward leaders tends to decrease with time Mutual influence of downward and upward leaders is discussed