Mechanism of charge transfer between colliding ice particles in thunderstorms

We derive a theoretical mechanism for the charge transfer between colliding ice and soft hail particles, based on current ideas of surface melting. According to this theory, each of the ice surfaces is covered with a film of quasi-liquid water, whose thickness can be estimated as a function of temperature and local curvature. We show that during contact of two particles, fluid will flow from the thicker to the thinner layer, and we estimate the volume transferred in atmospherically relevant situations. We postulate a charge structure within the surface-melted layer based on observations on similar systems and show that this leads to estimated charge transfers consistent in approximate magnitude and sign with trends observed in the laboratory.