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CHARGING MECHANISMS IN CLOUDS AND THUNDERSTORMS 129 items of secondary importance shown in parentheses). Charging appears to be well described by diffusion, drift, and selective ion capture for the nonprecipitating cloud stage (mechanisms 1-3). The situation in the rain stage is complicated by the addition of breakup and induction (mechanisms 4 and 5). We suspect that drift, selective ion capture, breakup, and induction are responsible for charges and fields in shallow clouds. However, it is difficult to find an explanation for the stronger electrification in convective clouds over a few kilometers deep. The basis of lightning from clouds with tops warmer than freezing remains a mystery. A major problem in the rain stage is that our knowledge of the suspected mechanisms is still rather rudimentary. There is clearly a need for additional research on charging by ions, breakup, induction, and convection to understand the electrification of warm clouds. In the hail stage we add thermoelectric and interface charging (mechanisms 7 and 8). Recent laboratory studies of charge transfer involving ice particles rebounding from simulated hailstones in the process of riming have shown that interface charging is the dominant mechanism. The roles of temperature, liquid-water content, and solutes are most likely important in altering the rime structure and thereby the contact potential and contact area. More research is required to understand these effects and the details of charge transfer. The electrification process becomes more complex as a cloud develops. The cloud stage involves mechanisms 1-3, whereas the rain stage includes 1-6. All the mechanisms listed in Table 9.1 may occur in the hail stage. We might ask, as many have before us, which separation mechanisms are essential to cloud electrification. The answers, if we had them, would depend on which aspect of cloud electrification we consider. For example, the essential mechanism for lightning depends on whether we are looking at the field development in the rain or hail stages or whether we are concerned with the charge centers associated with cloud-to-ground, in-cloud, or cloud-to-cloud lightning. Yet another aspect of lightning is the mechanism that initiates the stroke. Clearly the idea of an "essential" mechanism is an oversimplification. A more useful approach is to examine the interdependencies. We should be asking how the various charge-separation mechanisms are related. Some answers should be forthcoming as we incorporate the knowledge gained from recent laboratory studies of individual mechanisms into models of cloud electrification and compare the findings to field observations. With continued progress in laboratory, field, and modeling research we should achieve, in the next decade, a much improved perspective of the charging mechanisms in clouds and thunderstorms. 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