Corona, breakdown and humidity in the rod-plane gap

A survey is given of our knowledge of the effects of humidity on corona and breakdown when a positive impulse is applied to a rod-plane gap. It is shown that the minimum in the humidity correction factor Kh which exists, when the impulse wavefront is increased in the range 1 < Tcr < 1000 ¿s, is now substantiated by a number of experimental observations. The potential gradient for streamer propagation is discussed; its variation with humidity is similar to that of the breakdown condition with the lightning impulse (Tcr ~ 1 ¿S). The changes that occur for longer wavefronts are discussed in terms of the known facts relating to the increased probability of leader formation and propagation, as humidity increases.