Atmospheric electrical discharges in the presence of water and ice particles

Abstract

A variety of electrical discharges were photographically examined in a preliminary survey of water-involving discharge phenomena that occurred in simulations of the thunderstorm environment. The investigation was limited to situations showing no clear evidence of water drop disruption. Discharges between oppositely charged falling drops of water exhibit a luminosity distribution that suggests electron emission from the negative drop. Measurements indicate that the current density in this discharge may be 20 amp/cm2 or more as is the case with metallic cathode glow discharges. It is conceivable that a comparable emission occurs in clouds whenever a positive streamer comes within 40 μ of a particle's surface, and that this emission quenches weak streamers. Photographs show weak streamers that terminate on either ice or water surfaces, and stronger streamers and sparks that glide over the surface of a water drop. These latter observations demonstrate that in the arclike discharge produced by an intense spark, a water surface does not supply charge as copiously as a metallic one. Positive streamers are attracted to water and ice particles, which facilitates charge transfer across the air-water interface whether it occurs by ion capture or by electron emission. Since long sparks are often preceded by streamers, the paths of sparks in an atmosphere containing water or ice particles should be modified by charges deposited on the particles by the streamers. Paths were observed in several situations involving 50- to 70-cm-long impulsive sparks and target volumes nonuniformly populated with either fog droplets or with falling cloudburst densities of 2-mm-diameter drops. Effects on the paths were slight, and they could be detected only statistically; but in all situations there was, if anything, a slight tendency for the sparks to avoid wet regions of the target volume. Just the opposite tendency was found in a situation involving 2-cm-long sparks and a much denser distribution of drops. The assorted observations are related by citation to various published discussions of the lightning path, the rain gush, the transfer of charge from precipitation elements to the lightning channel, and the radio noise from clouds.