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Thunderstorm electrification is associated with the development of the ice phase in cumulus clouds. Laboratory experiments of graupel charging during ice particle collisions reveal a charge sign dependence on temperature and cloud conditions which is consistent with observations of the electric field sign and magnitude in thunderstorms. Several experiments and theories are reviewed and it is concluded that detailed information is needed on the surface conditions of graupel pellets in order to identify the charge transfer processes. Furthermore, information concerning droplet size distributions in the active electrification regions of storms is required.