Laboratory studies of charge transfer, when vapor-grown ice crystals bounce off a hailstone in simulated thunderstorm conditions, have shown that the sign and magnitude of the charge separated is a sensitive function of the cloud and cloud particle properties. In general, at temperatures colder than -20¿C, hail becomes negatively charged, but at warmer temperatures it becomes positively charged. The formation of the lower positive charge center in thunderstorms and the initiation of a lightning flash which precedes the commonly observed gush of rain on the ground may be explained in terms of this charging process.