Charge separation due to riming in an electric field

Ice pellets were grown by accretion of supercooled drops, 20–100 μm diameter, at temperatures between −5 and −15°C in a small wind tunnel at an air speed of about 10 ms−1. An electric field of about 50–150 kV m−1 could be applied at the surface of the pellet. The pellet, and induction rings upstream and downstream of the pellet, were connected to electrometers and analysing circuits of sufficient sensitivity and speed to measure, correlate and display individual charging events. Charged particles entering the apparatus could be excluded from analysis by the upstream induction ring. The downstream ring was used to identify charged particles originating at the pellet. Significant charge separation occurred only in the presence of an electric field, and was consistent with charging by electrical induction. Charges between 2 and 30 fC (1 fC = 10−15 Coulomb = 3.10−6 e.s.u.), depending on drop size, were observed at a rate of about one to ten events per 1,000 accreted drops. Evidence suggesting that these charging events are the result of some drops colliding with the pellet at grazing incidence is described. This process seems adequate to account for the generation of charge in thunderclouds.