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CHARGING MECHANISMS IN CLOUDS AND THUNDERSTORMS 114 9 Charging Mechanisms in Clouds and Thunderstorms Kenneth V. K. Beard University of Illinois at Urbana-Champaign Harry T. Ochs Illinois State Water Survey INTRODUCTION Since the time of Benjamin Franklin, a major difficulty in identifying the causes of cloud electricity has been our inability to obtain adequate measurements within clouds. This observational problem is now being remedied by modern electronics and instrumented aircraft. More quantitative theories of charging have become available since the 1940s along with our improved understanding of the atmosphere. In addition, better laboratory simulations of cloud physics in recent decades have led to improved measurements of microscale charge separation. With all these advances we should not be surprised to find that the number of possible charging mechanisms has proliferated. Thus, a modern task has been to sort through the possible mechanisms in trying to identify their relative contribution to cloud electrification (Mason, 1972; Latham, 1981; Takahashi, 1982). A major purpose of this chapter is to describe the mechanisms that charge cloud and precipitation particles. We also evaluate their relative role in cloud-scale electrification and assess our state of knowledge. A broader evaluation of cloud electrification is found in Chapter 10 of this volume. There are two major categories of charging mechanisms: the microscale separators, which ultimately lead to charged cloud and precipitation particles, and the cloud-scale separators, which can result in field intensification and lightning. The first category includes the creation of ion pairs in the air and charge separation on individual cloud and precipitation particles. These mechanisms are coupled with other microscale separators to produce net charges on cloud and precipitation particles, for example the attachment of ions by diffusion to cloud drops and the charging that results from particle collisions. Once the cloud and precipitation particles become appreciably charged, a larger scale separator such as differential sedimentation is needed to create electrification on the cloud scale. Convection can also act as a cloud-scale separator by redistributing ions and particles. Much of the emphasis in this chapter will be on describing the microscale separators that produce the charged cloud and precipitation particles. In discussing the charging mechanisms we consider the electrification of convective clouds. These clouds can produce spectacular displays of lightning and are the most important cloud link in the global electric circuit. We start the discussion of charge separation in the simple environment of a small cumulus cloud. This nonprecipitating cloud stage is followed by sections on the rain stage and hail stage. An abbreviated discussion of the charging mechanisms associated with these three stages is found in the evaluation section of this chapter.