A concise review is presented of the more important features of the meteorological and electrical behaviour of thunderstorms from which the main conditions to be fulfilled by any satisfactory theory of charge generation, are enumerated. The various theories are then examined, as far as possible from a quantitative point of view, in the light of these requirements.
None is found to be entirely satisfactory.
It is concluded that the mechanism which most accords with the observed facts is the charge generation and separation which, according to the results of laboratory experiments, will be associated with the growth of graupel pellets in the supercooled region of the thundercloud.