A dramatic scene of a lightning strike illuminating a dark, cloudy night sky. The dark clouds obscure most of the sky, but there is a break in the clouds where the lightning flashes. The bolt of lightning is jagged and bright white, and it illuminates the clouds from behind. The light from the lightning also reflects off the bottom of the clouds, creating a brief flash of light on the horizon. Lightning is a powerful electrostatic discharge that occurs in thunderstorms. It is caused by the buildup of electrical charges within a cloud, or between a cloud and the ground. When the electrical potential difference becomes great enough, the air breaks down and allows a rapid discharge of electricity, which we see as a flash of light.