Lightning is a giant spark. Inside a thunderstorm, collisions among ice particles charge the cloud, typically with a negative region toward the base and positive higher up. The ground beneath can become positively charged in response. Air is an insulator until the electric field grows strong enough to tear electrons free. A faint, stepped leader of ionized air then feels downward from the cloud. When it meets an upward streamer from a tree, tower, or the earth, a brilliant return stroke floods the channel. That is the flash the eye sees, and it can heat the air to temperatures hotter than the surface of the Sun.

The rapid expansion of that superheated air is thunder. Light reaches you almost at once. Sound lags, so counting seconds between flash and boom estimates distance. Most strikes are inside clouds or between clouds. The ones that hit ground start fires, split wood, and kill people and livestock because the current follows the easiest path, including wet skin and metal.

Lightning also fixes a little nitrogen from the air into compounds plants can eventually use, and it is part of the global electric circuit that fair-weather current slowly closes. A storm is not only rain. It is a machine for separating charge until the sky has no choice but to reconnect in a line of light.
