ATMOSPHERIC ELECTRICITY

Owing to the somewhat explosive development of the science of atmospheric electricity during the past decade this article covers a broad field of activity. The article begins with a description and discussion of the work that has been performed to understand the electrical properties of the basic materials involved in generating processes in the atmosphere, namely ice, water, and sand and similar particulate matter. A comprehensive treatment of the thermoelectric effect in ice is given, and the electrification of phase changes in ice is also discussed. Field measurements of the electrification of snowstorms and blizzards are described, together with simulation experiments performed in the laboratory, and the results and interpretations compared. A following section deals with the electrification of water and water drops during freezing, fragmentation, disintegration, evaporation, and splashing and bubbling, the laboratory experiments relating to a given phenomenon being critically compared. Another section considers the electrification of sand and dust, with particular reference to sandstorms and dust devils, and also measurements of the electrification of volcanic plumes and their interpretation. The study of the electrification of convective clouds is almost certainly the most important aspect of the science of atmospheric electricity, and it is studied in detail. First, the observations of the electrification of convective clouds are described in order to lay a foundation for discussion; secondly, laboratory investigations of the electrical processes occurring within clouds are presented and compared critically. The observations and laboratory experiments so described enable theoretical models of the growth of cloud electrification to be projected; the models and their associated equations are examined thoroughly. Another aspect of cloud electrification studies is that of electrical discharges involving lightning. The electrical and optical measurements of the lightning flash are discussed and include comparisons of intra-cloud and cloud-to-ground strokes and theories of the discharge mechanisms. The phenomenon of ball lightning is also discussed. A short section is devoted to an extreme phenomenon of cloud electrification, the tornado. Studies of the electromagnetic radiation emitted from colliding water drops are described in a section that is primarily devoted to presenting an account of recent research into electromagnetic radiation associated with the lightning discharge; spectra are shown that are thought to be associated with different components of a lightning flash. The last three sections are concerned with ions and aerosols in the atmosphere (involving ion-aerosol interactions, the electrode effect, and radio-activity), the electrification of the upper atmosphere and space, and a consideration of the global electrical circuit and its related electrical 'balance sheet'.