The initial leader of a lightning discharge; an intermittently advancing column of high ionization and charge that establishes the channel for a first return stroke. The peculiar characteristic of this type of leader is its stepwise growth at intervals of about 50– 100 μs. The velocity of growth during the brief intervals of advance, each only about 1 μs in duration, is quite high (about 5 × 107 m s−1), but the long stationary phases reduce its effective speed to only about 5 × 105 m s−1. To help explain its mode of advance, the concept of a pilot streamer was originally suggested, but has been supplanted by analogy to recent work on long laboratory sparks.
A stepped leader (possibly also referred to as a step leader) is a path of ionized air which extends downward from a thundercloud during the initial stages of atmospheric breakdown during a lightning strike. Often, there are multiple, branching stepped leaders. As the step leaders form, these branches of ionized air form in a stepwise fashion, followed by a short period of inactivity, until the final step leader reaches the ground, a tall object on the ground, or a positive streamer extending upward from a ground object.