Fair weather atmospheric electricity: its origin and applications

Abstract—An entirely electrostatic view of the earth upper atmosphere system, as understood for example, by Lord Kelvin, underwent a paradigm shift around 1900 with definitive observations of sustained ion production in atmospheric air. Cosmic ray ionisation of atmospheric air, together with a potential difference between the upper atmosphere and the earth’s surface, yield vertical atmospheric current flow in regions of fair weather. The current’s origin was ultimately explained through the global circuit model of the Scottish atmospheric physicist C.T.R. Wilson, Nobel prize recipient for the cloud chamber. In Wilson’s conceptual model, charge exchanged by disturbed weather electrifies the ionosphere, with a return current flowing in fair weather regions. Sources of internal (thunderstorms) and external (energetic particles and cosmic rays) earth system variability can therefore influence the global circuit and the current flowing. One situation, of particular interest because it occurs commonly globally, is when the vertical current passes through an extensive horizontal cloud layer. As the electrical conductivity is reduced in cloudy air compared with clear air, charge accumulates at the upper and lower boundaries of the layer clouds in the vertical current flow if the cloud-air conductivity transition is sharp. Changes in the current flow which arise from a variety of sources (e.g. climate system variability, space weather or even the radon gas release associated with some earthquakes) can therefore potentially be communicated into clouds. New insights into the global relevance of fair weather atmospheric electricity are now emerging through novel instrumentation and measurements, arising from the close relationships between energetic particles, ions, aerosol and cloud droplets.