Processes causing electrification of ice crystals in thunderclouds

Abstract

Jayaratne and Saunders (1991) have performed a useful service by reporting their laboratory experiments demonstrating, contrary to the findings of Odencrantz and Buecher (1967), that ice crystal charging does not take place in an unmixed cloud of ice crystals. Jayaratne and Saunders' further cold box experiments showing that the ice crystals become electrified when a fan in the cold box is turned on, support their view that an ice-ice charging process takes place on the surfaces of the moving ice-covered fan-blades. Their conclusion, that this phenomenon might be capable of producing each second the several coulombs of charged cloud particles necessary to maintain the electrification of a thundercloud, appears justified. However, because their paper is lacking in experimental details and because it is unknown whether the conditions in their cold chamber accurately duplicate those in a thunderstorm, this point is not possible to resolve. There is good reason to doubt J and S's final conclusion, that “… in thunderstorms, ice crystal charges are similarly acquired by collision processes RATHER THAN BY ANY OTHER MECHANISM”. It is puzzling to understand why the authors have chosen to ignore ion attachment, an important ice crystal charging process that does not involve collisions between ice crystals. Beginning with Gish and Wait (1950), numerous investigators have demonstrated that ion currents flow to the tops of thunderclouds as predicted by Wilson (1920). Marshall et al. (1989) and Byrne et al. (1989) have found that ice particles at cloud top carry appreciable charges that appear to be derived from the Wilson current. In an average storm there can be little doubt that this mechanism too could be producing charged ice crystals at the required rate of about a coulomb per second (Vonnegut, 1990).