Abstract Field and laboratory observations strongly support the view that the development of electric fields in thunderstorms is caused by charge separation during rebounding collisions between ice crystals and graupels, followed by their gravitational separation. Although several plausible microphysical mechanisms have been proposed to explain the physics of charge separation, none have been found to be fully consistent with the experimental evidence, and a fresh approach is necessary. We propose a new mechanism for charge separation in ice‐ice collisions based on the fundamental molecular mechanism of charge transport in ice, involving the diffusion of H+ ions (excess protons or positive ionic defects) via a proton hopping relay along the hydrogen‐bond network of ice and the trapping and release of H+ ions from L‐orientational defects. The collision of two ice particles leads to the formation of a transient crystalline ice bridge at the contact point, which is spontaneously driven by the tendency for ice sintering, permitting rapid H+ diffusion between the two particles. Charge separation is achieved by the asymmetry in the concentrations of H+ ions or L‐orientational defects between the two ice surfaces. The proposed H+ transport mechanism successfully explains the direction and magnitude of charge transfer as well as its dependence on the relative growth rates of the two ice surfaces observed in laboratory studies. In addition, it offers a molecular‐level interpretation of the empirical rule that, during a collision, the faster‐growing ice surface is positively charged at the expense of negative charging of the slower‐growing or sublimating ice surface.
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