Lightning is a massive electrostatic discharge caused by unbalanced electric charge in the atmosphere, either inside clouds, cloud to cloud or cloud to ground, accompanied by the loud sound of thunder.

A typical cloud to ground lightning strike can be over 5 km (3 mi) long. A typical thunderstorm may have three or more strikes per minute at its peak. Lightning is usually produced by cumulonimbus clouds up to 15 km high (10 mi) high, based 5–6 km (3-4 mi) above the ground. Lightning is caused by the circulation of warm moisture-filled air through electric fields. Ice or water particles then accumulate charge as in a Van de Graaf generator. Lightning may occur during snow storms (thundersnow), volcanic eruptions, dust storms, forest fires or tornadoes. Hurricanes typically generate some lightning, mainly in the rainbands as much as 160 km (100 mi) from the center.

When the local electric field exceeds the dielectric strength of damp air (about 3 million Volts/m), electrical discharge results, often followed by more discharges along the same path. Mechanisms that cause lightning are still a matter of scientific investigation. Fear of lightning is called astraphobia. The study or science of lightning is called fulminology