Lightning is an electrical discharge in the atmosphere, like a giant spark. There is still debate about the cause of the separation of charges needed to create the discharge. Atmospheric scientists believe that strong updrafts in the clouds sweep droplets of water upward, cooling them far below the freezing point. When the droplets collide with ice crystals the droplets become a soft mixture of water and ice. As a result of these collisions the ice crystals become slightly positively charged and the water/ice mixture becomes negatively charged. The updrafts push the ice crystals up higher, creating a positively-charged cloud top. The heavier water/ice mixture falls, making the lower part of the clouds negatively charged.

The ground under the cloud is charged by induction. The build-up of negative charges on the underside of a thundercloud attracts the positive charges in the ground. The negative charges are repelled further into the ground, leaving a positively charged surface.