The existence of regions of net charge in a thunderstorm. During transient collisions of ice crystals with riming graupel pellets, charge is transferred. The separating particles then carry equal and opposite charges; the larger (often negative) particles fall while the smaller ones (often positively charged ice crystals) are carried up in the updraft to produce a vertical electric field that eventually produces lightning. The charge transfer process is not completely understood, but possible processes include charges on the surface layers of the particles, charges on dislocations in the ice lattice, temperature differences along surface features that may be broken off during collisions, and contact potential differences between the surfaces of the interacting particles. See breaking-drop theory, ion-capture theory.