Return stroke speed models

Abstract

The return stroke speed is one of the main parameters in the modelling of return strokes. The available experimental observations show that the return stroke speed decreases with height, both in the first and the subsequent return strokes. The experimental data also seem to indicate that there is no relationship between the return stroke current and the return stroke speed. This observation is somewhat against the theoretical intuition where a larger electric field at the return stroke front associated with a larger return stroke current is expected to expedite the neutralisation process giving rise to a larger return stroke speed. What parameters of the lightning channel that control the return stroke speed is one of the most important questions in lightning research. A current pulse propagating along a perfect conductor located above the ground will move at a speed equal to the speed of light in air. Since the speed of propagation of the return stroke front is significantly less than the speed of light one may hypothesise that the process of electrical breakdown that converts the partially ionised leader channel to a highly conducting return stroke channel decides the speed at which the return stroke is propagating. In this chapter, we will consider some of the models that have attempted to take this point into consideration and predict the return stroke speed as a function of other current parameters.