A theoretical model of thundercloud charge separation is presented. The model assume that moisture in the updraft moving as a stream with uniform velocity condenses into particles of graupel in reaching cooler regions of the atmosphere. Falling graupels acquire a negative charge by shedding the inductive positive charge which is carried away by the updraft. Charged graupels moves downward subjected to gravity, force due to the collective electric field and air resistance. Non-linear equations governing the dynamics of the system demonstrate existence of a stable equilibrium electric field of the order of magnitude needed for dielectric breakdown of air. Model also shows that thundercloud electric field could undergo pulsations at infrasonic frequencies and synchronously emit acoustic waves at the same frequency. Preferential positive inductive charging of graupel is explained as a consequence of the proton conductivity of ice.