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OVERVIEW AND RECOMMENDATIONS 8 This report will not present an encyclopedic review of all the electrical phenomena that occur in the atmosphere but will simply give some examples that illustrate a few of the basic processes and some of the important interrelationships. Notably absent, but important to an overall understanding of atmospheric electricity, are discussions dealing with electromagnetic phenomena such as sferics, Schumann resonances, and whistlers. These and many other subjects, however, have been reviewed in the two-volume Handbook of Atmospherics, edited by Volland (1982), and recent proceedings of international conferences on atmospheric electricity (Dolezalek and Reiter, 1977; Orville, 1985). Human and biological effects of atmospheric electricity are also important research areas that are not considered in this study. The Planetary Boundary Layer The planetary boundary layer (PBL) is the lowest few kilometers of the atmosphere where interactions with the surface, man, and the biosphere are the most pronounced. Galactic cosmic rays are the main source of ionization in the PBL; however, near land surfaces, ionization is also produced by decays of natural radioactive gases emanating from the soil surface and by radiations emitted directly from the surface. Ionization from radioactive sources depends on soil type and surface structure and on the meteorological dispersal rate; this ionization normally decreases rapidly with altitude, and at about 1 km its contribution to the total ionization is less than that from cosmic rays. Other sources of ions in the PBL include lightning; electrification due to waterfalls, ocean surf, and man-made sprays; a variety of combustion processes; point discharge or corona currents that are produced whenever the ambient electric field exceeds breakdown; and frictional processes associated with blowing dust, snow, or volcanic ejecta. In the troposphere, atmospheric trace gases are numerous and variable, and the ion chemistry is complicated by clustering processes and the relatively long lifetime of the terminal ions. As a further complication, clouds and other aerosols play an important role as sinks for small ions and thereby alter the ion distribution (see Chapter 11). Over continental areas, the loss of ions by attachment to aerosols can be larger than the loss by recombination. Some atmospheric aerosols are hygroscopic, and the particle size increases with relative humidity. At large humidities, fog and cloud droplets form and produce a large decrease in the electrical conductivity of the atmosphere. Since a decrease in conductivity can be a precursor of fog, it might be possible to improve forecasts of the onset of fog by electrical measurements. Turbulent transport and convection within the PBL are important processes that govern the momentum, heat, and moisture exchanges between the atmosphere, geosphere, and biosphere. These processes influence the mean wind profile, the vertical distribution of temperature, water vapor, trace gases, aerosols, and the ion distribution throughout the troposphere. Turbulent mixing and convection can prevent the buildup of radioactive emanations near the ground and can also disperse aerosols to a greater altitude in the troposphere. Electrical processes in the PBL are complex, highly variable, and span a tremendous range of space and time scales. The electrical variables respond to many of the lower atmospheric processes but usually have little influence on the phenomena to which they respond. Within the PBL, local turbulent fluctuations of space-charge density impose a time-varying electric field that is comparable in magnitude with or even greater than the electric field maintained by global thunderstorm activity. Since the PBL is the region of the atmosphere with the greatest resistance, it is this layer as well as the generators that control the currents in the global circuit. Electrical processes in the PBL are discussed in greater detail in Chapter 11.