The process by which the large electric fields found within thunderclouds are generated; the process by which particles bearing opposite electrical charge are given those charges and transported to different regions of the active cloud. Accounting for the rapid and extensive separation of electric charge within thunderstorms is still one of the central problems in the study of thunderstorm electricity. Many theories have been proposed to explain charge separation, including the breaking-drop theory, the ion-capture theory, a theory involving the Workman–Reynolds effect, and a mechanism involving the bounce of ice crystals from growing graupel. None is entirely satisfactory in being able to account fully for the observed charge separation required to maintain a very active thunderstorm producing one discharge per second or so. Much evidence points toward particle-size difference and hence falling- speed difference as a necessary factor in the transportation of the oppositely charged particles in opposite directions in the updrafts of convective clouds, in regions where ice crystals are produced in the presence of graupel in regions between updraft and downdraft.