The observed temperature at which the charge transferred to riming graupel particles during collisions with ice crystals in thunderstorms reverses sign. The reversal temperature is affected by the cloud liquid water content such that graupel charges positively (negatively) at high (low) liquid water contents and temperatures above (below) the reversal temperature. The process can account for the positive charging of ice crystals carried to the top of the clouds, for the negative charge region typically at temperatures between −10° and −20°C, and for the lower positive charge center.