번개의 원리 - 전하의 분리 이론들 <wikipedia>

Charge separation
The first process in the generation of lightning is charge separation.

The mechanism by which charge separation happens is still the subject of research,

  1. Polarization mechanism theory

Falling droplets of ice and rain become electrically polarized as they fall through the atmosphere's natural electric field; 
 

# 2. Electrostatic induction theory

 Opposite charges are driven apart by the above mechanism and energy is stored in the electric field between them. Cloud electrification appears to require strong updrafts which carry water droplets upward, supercooling them to -10 to -20 C. These collide with ice crystals to form a soft ice-water mixture called graupel. The collisions result in a slight positive charge being transferred to ice crystals, and a slight negative charge to the graupel. Updrafts drive lighter ice crystals upwards, causing the cloud top to accumulate increasing positive charge. The heavier negatively charged graupel falls towards the middle and lower portions of the cloud, building up an increasing negative charge. Charge separation and accumulation continue until the electrical potential becomes sufficient to initiate lightning discharges, which occurs when the gathering of positive and negative charges forms a sufficiently strong electric field.

There are several additional theories for the origin of charge separation.