Lightning is a giant spark of electricity in the atmosphere between clouds, the air, or the ground. In the early stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground. When the opposite charges build up enough, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning. (The actual breakdown process is still poorly understood.)

The air breakdown creates ions and free electrons that travel down the conducting channel. This current flow temporarily equalizes the charged regions in the atmosphere until the opposite charges build up again.

Lightning from thunderstorms begins in a strong electric field between opposite charges within the storm cloud, and can stay completely within the cloud (intra-cloud lightning) when the charge regions are similar strength (balanced) or can reach the ground (cloud-to-ground lightning) when one of the regions is much stronger than the other (unbalanced).

Lightning is one of the oldest observed natural phenomena on earth. It can be seen in volcanic eruptions, extremely intense forest fires (pyrocumulonimbus clouds), surface nuclear detonations, heavy snowstorms, in large hurricanes, and obviously, thunderstorms.