A Microscopic Mechanism for the Separation of Charge in Thunderstorms

We propose a plausible microscopic mechanism for charge transfer during rebounding collisions between ice crystals and graupel within thunderstorms. The key concept is that charge is transferred through the diffusional transport of H + ions across the ice‐ice contact junction during collisions. The transferred charges are stabilized by the trapping of H + ions at L ‐orientational defects in ice particles. The distinct formation pathways of ice crystals and graupel within a cloud lead to asymmetries in their respective H + ion and L ‐defect concentrations. Graupel, formed by the accretion of supercooled droplets containing dissolved atmospheric CO 2 , has a higher H + concentration than ice crystals, which grow exclusively from vapor. Additionally, warming of graupel due to riming increases its L ‐defect concentration relative to that of crystals, according to microscopic analyses of ice growth processes. These differences in H + and L ‐defect concentrations between ice crystals and graupel drive charges in opposite directions, with the net charge transfer determined by the interplay of these two factors. A theoretical model based on this mechanism predicts that positive charge on graupel increases with rising liquid water content (LWC), while negative charge becomes stronger in low LWC regions at colder temperatures. These trends align with common features observed in most laboratory studies and with the formation of two polarities of storm dipoles under different LWC regimes. We show that the magnitude of charge separation during crystal‐graupel collisions, calculated based on this model, broadly agrees with the values found in laboratory studies.