Preliminary discharge processes in lightning flashes to ground

Abstract By using different techniques of recording the electrical effects of lightning discharges occurring at varying distances, it has been possible to sort out the major components of pre‐return stroke field changes. A comparison between these field changes has shown that the first return stroke of a lightning flash to ground is usually preceded by three successive and distinct discharge processes. The first, or breakdown stage, is identified from the reversal of its field change with distance as a discharge between the main negative charge centre and the positive charge centre near the base of the cloud at heights of 3.6 and 1.4 km respectively, the discharge probably just as often starting from the positive as from the negative centre. This is followed by the second stage during which the highly conducting channel then existing in the cloud is being negatively charged. The charging continues until the field in front of the channel has become sufficiently strong to propagate a downward‐moving stepped leader breakdown streamer. Stepped leaders have been observed to precede more than 75 per cent of all flashes to ground, though the radiation fields of most stepped leaders are so small that they can only be detected by using very high amplification. The probable mechanisms involved in the preliminary stages are discussed.