By generating electric fields, charge separation creates the appropriate conditions for lightning. But what produces charge separation?
Efficient charge separation depends on strong updrafts, ice crystals, and supercooled water (water that is liquid at temperatures below its normal freezing point). These requirements make the electrification process extremely complex and controversial. One explanation of the process that has gained observational and theoretical support involves porous ice structures called graupel, also known as soft hail or snow pellets.
Strong updrafts carry a mixture of supercooled cloud water drops and aerosol particles upward until the air is cold enough for the formation of ice crystals seeded by the aerosol particles. ("Aerosol" refers to either solid or liquid particles suspended in air. Dust, haze, and smoke are examples of aerosol particles.)
Large ice crystals may fall rapidly, attaining terminal speeds 20 meters per second greater than the speeds of nearby smaller particles. The crystals collect the smaller, slower, supercooled water drops in their path. The drops freeze on the surface of the falling ice crystals, building up what will become the porous mass of graupel particles.
When graupel particles fall through supercooled water and ice crystals, they acquire one charge, and the water-ice mix acquires the opposite charge. The difference in fall speeds of the graupel and the smaller, slower water drops and crystals leads to charge separation, and where the mix of particles is favorable, sufficient charge separation for lightning develops.