Electrification produced by the growth of soft hail in thunderclouds

Detailed studies of the electrification associated with the growth of an artificial pellet of soft hail have been made by Reynolds, Brook & Gourley (1957), Latham & Mason (1961a) and Magono & Takahashi (1963a, b). The apparent contradictions between the results of these separate studies are shown to be merely the consequence of the different conditions under which they were performed and it is demonstrated that the temperature gradient theory affords a qualitative explanation for all the results obtained. Strong negative charging of the growing pellet is produced by two processes, both manifestations of the theory: (1) Collision between the pellet and supercooled droplets which freeze onto its surface and shatter to eject positively charged ice splinters. (2) Momentary contact between ice crystals and a rimed pellet of irregular surface structure; the charge transfer resulting from this process depends markedly upon the violence of the impact.

Calculations show that appreciable negative charging of the growing pellet will occur in a thundercloud over ranges of droplet diameter 0 < d < 120 μ, pellet radius 0.25 < r < 2.5 mm and environmental temperature −35° < T < −4°C. The combined rate of charging inside a thunderstorm by means of these two processes is estimated to be at least 3C km−3 min−1, which can easily account for the electrification of a moderate thunderstorm; this riming process may also be able to generate sufficient charge to explain the electrification of extremely severe thunderstorms.