The Electrification of Precipitation and Thunderstorms

Abstract

The fundamental physical processes responsible for the observed electrification of ordinary precipitating clouds, as well as thunderstorms, is considered in quantitative terms and new facts are emphasized. Cosmic rays and radioactivity, sometimes supplemented by corona and photoionization, produce ion pairs in the atmosphere. These ions are transferred to cloud droplets or ice crystals by diffusion or by electric fields and thereby establish a droplet charge distribution. This distribution is profoundly influenced by the observed differences between the positive and negative light ion conductivities in the atmosphere. Electrification by induction frequently supplements these processes. Rain formed by the association of these cloud elements may be highly electrified and a mixture of positive and negative drops is usually produced. Experimentally wellestablished processes are considered that can selectively charge such rain or selectively discharge it. It is shown that either process can establish a free space charge distribution that is transferred toward the ground by gravity, and may be large enough to develop electric fields exceeding the dielectric strength of air. Lightning then intervenes. The conditions necessary to establish gross free charge distributions by regeneration are specified and shown to be met under frequently occurring meteorological situations. Many of the observed characteristics of thunderstorms are traceable to the nonuniform semiconducting nature of the lower atmosphere and its nonohmic behavior. The analysis reflects the results of earlier work.