what causes lightning

Lightning is a giant electrical discharge caused by the buildup of opposite electrical charges within a thunderstorm that eventually overcomes the insulating properties of the air.

Charge Separation. Turbulent air currents in a thunderstorm cause collisions between ice crystals, hail, and water droplets, which separates electrical charges within the cloud.
Electric Field Formation. The top of the cloud typically becomes positively charged, while the base becomes negatively charged, creating a strong electric field.
Electrical Discharge. The atmosphere normally acts as an insulator, but when the charge difference becomes great enough, it breaks down, allowing electricity to discharge.
Types of Lightning Strikes. Lightning can occur between regions within a cloud, between different clouds, or between a cloud and the ground, where it connects with upward-moving streamers.