Lightning happens because of the build-up and discharge of electrical energy in a thunderstorm cloud. Inside the cloud, turbulent winds carry water droplets and ice particles. When these particles collide, they cause a separation of electric charges: the bottom of the cloud becomes negatively charged, and the top becomes positively charged. This charge separation creates a strong electric field between the cloud and the ground or within the cloud itself. When the electric field becomes strong enough to overcome the insulating properties of the air, electricity rapidly discharges as a flash of lightning. This discharge equalizes the charge difference and produces an intense flash of light and a sudden expansion of heated air that causes thunder.
More specifically, the negative charge at the cloud base attracts a positive charge on the ground. As a step leader of negative charge descends from the cloud, positive streamers from the Earth's surface rise to meet it. Once they connect, a powerful current flows, creating the visible lightning bolt. Thunder is the sound created by the rapid heating and expansion of air caused by the lightning strike.
In short, lightning is a natural electrical discharge caused by the charge separation within clouds and between clouds and the Earth during storms, with thunder being the explosive sound from the heated air.
