Lightning is a giant electric spark in the sky. Most lightning that people see takes place between a cloud and the ground. But lightning also occurs within a cloud, between a cloud and the air, and between two clouds. Lightning in the atmosphere may damage aircraft traveling through it, but it does not cause harm on the ground. However, lightning that strikes the earth can kill people and cause fire. Lightning strikes the earth about 100 times each second. Lightning heats the air explosively, creating the sound that we hear as thunder.
A lightning flash from a cloud to the ground consists of one or more strokes. A stroke appears as a single brightening of a channel (path) between the cloud and the ground. Typical cloud-to-ground flashes are up to 10 miles (16 kilometers) long. Flashes in clouds may travel horizontally through lines of clouds for more than 100 miles (160 kilometers).
Until the mid-1700
