Abstract
Calculations have been made of the electrical evolution of a thundercloud under the action of various precipitative charging mechanisms. the cloud is cylindrical, with a defined width and depth. the finite dimensions of the cloud permit the distributions of axial field and charge to be calculated. This was not possible with the infinite parallelโplate cloud model employed by earlier workers, which in any event has been shown to provide greatly exaggerated rates of field growth.
The calculations take into account various criticisms of other precipitative treatments. These features include the important effects of the dependence of the charge transfer, under an inductive mechanism, on the charge carried by the larger of the two colliding particles and the angle defining their point of separation. Levitation, ionic dissipation and multiple collisions of cloud particles with precipitation are also discussed and shown to be unimportant in situations of interest.
It is found that the waterโwater inductive charging mechanism is quite inadequate to explain thundercloud electrification. the available experimental evidence is insufficient to permit a conclusion to be drawn concerning the importance of the corresponding waterโice mechanism.
Two established mechanisms which appear to be capable of producing breakdown fields in the available time โ although only marginally โ both involve collisions between hail pellets and ice crystals. These are the inductive mechanism and the thermoelectric mechanism. Although these two processes are about equally powerful, the details of the predicted fieldโgrowth rates, the field structure and the charge distributions under these two mechanisms are quite different. Analysis of the available field evidence is consistent with the inductive theory in some cases and the thermoelectric theory in others. However, a combination of these two processes appears to be consistent with all the evidence, and to provide a tenable theory of thundercloud electrification.