When Lightning Strikes

As cloud formation continues, the two opposite charges increase in strength. Since unlike charges attract, there is a powerful tendency for the charges to join and neutralize each other. Each charge exerts a strong electrical potential, or pressure, in an effort to bridge the air gap from cloud to ground. Air, a poor electrical conductor, resists the passage of the charges. At some critical point, however, the resistance of the air is overcome. A small discharge, called a pilot streamer, moves toward the Earth carrying negative charge. A stronger current, called a stepped leader, follows and ionizes the air in its path (see Electricity). The stepped leader moves in a series of jagged spurts, each about 150 feet (45 meters) long. When the pilot streamer touches the Earth, a high-current return streamer leaps from the ground toward the cloud. It travels along the path of ionized air created by the stepped leader. This is the part of the stroke that produces the brilliant flash we see.