Charge generation in Thunderstorms by collision of ice crystals with graupel, falling through a vertical electric field

When an ice-crystal impinges on the lower surface of a particle of hail or soft hail part of the charge induced by the electric field is transmitted to the ice-crystal. The relaxation time for the exchange of charge varies from a few milliseconds to about 10^–6 seconds and is sufficiently short to enable this gain of charge. In a positive field the soft hail will therefore get a negative and the ice-crystal a positive charge. In an electric field E the final charge of a sphere with a radius R is e₁ = –2.12 ER². At temperatures between –18° and –26° the number of ice crystals is sufficient to allow a time constant of about 5 minutes for the charging process. The charge per droplet and per g of water derived from this process are in good agreement with measurements of Ross Gunn. The growth of precipitate and the formation of strong electric fields can be brought into correspondence with the timing and development of thunderstorms.