When the electric field becomes strong enough, an electrical discharge occurs within clouds or between clouds and the ground. During the strike, successive portions of air become a conductive discharge channel as the electrons and positive ions of air molecules are pulled away from each other and forced to flow in opposite directions.

The electrical discharge rapidly superheats the discharge channel, causing the air to expand rapidly and produce a shock wave heard. The rolling and gradually dissipating rumble is caused by the time delay of sound coming from different portions of a long stroke.