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CHARGING MECHANISMS IN CLOUDS AND THUNDERSTORMS 120 Drop charging occurs in the rain stage from drift charging at cloud edges and selective ion capture within the cloud. The latter mechanism enhances the electric field by gravitational separation of negatively charged precipitation drops and positively charged cloud droplets. In addition, breakup charging of colliding drops may result in significant charges on raindrops and drizzle drops. Since fields are weak in the clouds we have considered, induction charging is ineffective. However, under the special circumstance of deep (warm) convection, as discussed in the evaluation of charging mechanisms, induction may lead to higher fields through a positive feedback. As the cloud top rises above the freezing level the newly formed cloud droplets, as well as drizzle drops carried in the updraft, remain in the liquid state. Soon some of the larger drops freeze until, at about the â 15Â°C level, the cloud top takes on a fuzzy outline indicating a substantial number of ice particles. Such a glaciated cloud usually undergoes a growth spurt from the release of latent heat. If the air above is not too warm, convection may continue up to the base of the stratosphere, resulting in an intense thunderstorm. Typically on a day that has isolated thunderstorms, the first cumulus clouds in the early afternoon reach only the cloud stage. Somewhat later cloud tops are higher and reach the rain stage. It is often not until middle afternoon that cloud tops are high enough to glaciate. This is the onset of the hail stage, since what follows is the beginning of hail- like precipitation as droplets collide and freeze onto larger ice particles. HAIL STAGE The glaciated portion of the cloud contains ice crystals in a saturated vapor environment maintained by the presence of more numerous cloud droplets. Since the saturation vapor pressure for ice is less than water, the ice crystals grow rapidly by vapor diffusion. As the ice crystals grow larger than about 100 Âµm, they begin to collect cloud droplets. This riming process continues within the upper portion of the cloud until the particles are transformed into soft hail (also termed "snow pellets" or "graupel"). These ice particles are not nearly so dense or large as typical hailstones. When the size and liquid water concentration are large enough, the accretion of water occurs too rapidly for immediate freezing. Water will then infiltrate the rime structure and increase the particle mass, fall speed, and growth rate. In a strong updraft the water in soft hail may refreeze higher in the cloud, resulting in ice pellets (i.e., small hailstones). Further growth by riming may be followed by a descent to a region where wet growth can again occur. A cycle of wet and dry growth may be repeated several times to produce the multiple layering found in larger hailstones (see also Chapter 7, this volume). The above description for the growth of soft hail and hailstones indicates the complexity of particle interactions in the hail stage. Although the growth of ice precipitation is governed by the collection of cloud droplets, the charging of ice precipitation appears to be linked to collision with smaller ice particles (Gaskell and Illingworth, 1980; Latham, 1981; Jayaratne et al., 1983). Therefore, we will consider the separation of charge for collisions of precipitation, such as soft hail and hailstones, with frozen drops and ice particles. First, the induction charging discussed for the rain stage will be extended to drops and ice particles rebounding from ice precipitation with dry or wet surfaces. Then we discuss thermoelectric charging and interface charging. Induction Charging The concept of induction described for the rain stage can be applied in the hail stage after considering a few alterations. Of primary importance is the charge relaxation time for ice that is a factor of 1000 slower than for liquid water. Theoretical estimates by Gaskell (1981) for charge transfer between a 100-Âµm ice sphere and a much larger one, including the effect of a surface conductivity, yield a relaxation time constant of Ï = 100 Âµsec, which is much longer than an estimated contact time of less than 1 Âµsec. Since the amount of charge transferred during contact is proportional to 1 â 3-t/Ï, an ice sphere charges at less than one hundredth the rate of a comparable water drop. This would increase the induction-charging time from a few minutes for drizzle drops to several hours for ice pellets. However, induction charging of wet hail would proceed to the maximum amount in Eq. (9.8) about as rapidly as in the water-drop interactions. Another aspect of induction charging is the effective contact angle found from the average over the range of bouncing interactions. In the rain stage the average is about 70Â° for collisions between large and small drizzle drops. Experimental measurements of charge separation for larger precipitation particles (both water drops and ice spheres) colliding with cloud droplets indicate that the average contact angle is greater than 85 (e.g., see Jennings, 1975; Gaskill, 1981). In contrast, ice particles almost always separate after colliding with hail, resulting in an average contact angle of 45Â°. When we consider both the effects of contact angle and charge relaxation, we can compare the strength of induction charging for various particle interactions in the hail stage. For collisions between dry hail and ice