Riming Electrification as a Charge Generation Mechanism in Thunderstorms

Riming electrification was studied in cold room experiments simulating thunderstorm conditions. When both ice crystals and supercooled droplets coexist in the experimental chamber, high electric charge occurs on the riming probes. Both the sign and magnitude of riming electrification are highly dependent on the temperature and cloud water content. Electric charge of the order of 10−4 esu (33 fC) is separated on riming with each ice crystal collision under conditions typical of the in-cloud environment of continental thunderstorms. This amount of charge is sufficient to produce the high electrification required for lightning discharge within reasonably short periods of time. The effect of the electrical field on charge separation during the riming and effect of the freezing potential of drops of rime also were examined. It appears that these effects play only a secondary role for charge separation in thunderstorms. Three different physical mechanisms of charge separation during riming are proposed and discussed to explain the riming charge under different conditions of temperature and cloud water content. Possible differences in the character of thunderstorms at various geographical locations are briefly considered.