Lightning is an electric discharge caused by electric potential differences in the atmosphere.

Thunderclouds (cumulonimbus) are formed by strong updrafts. Water vapor rising into the sky condenses into water droplets and then freezes into ice crystals. Heavy ice crystals that start falling from the top of the thundercloud coalesce with other ice crystals and water droplets, becoming hail as they continue their descent. At that point, the steadily growing ice crystals and falling hail begin to collide with each other.

Some of the water molecules in these ice crystals and hail ionize into H+ and OH- ions. Compared to the large OH- ions, the small H+ ions are more mobile, and those that are higher in temperature are even more mobile.

When low-temperature ice crystals and slightly higher-temperature hail rub against each other, the H+ ions transfer to the ice crystals from the highly mobile hail.