Lightning is an effect of electrification within a thunderstorm. As the thunderstorm develops, interactions of charged particles produce an intense electrical field within the cloud. A large positive charge is usually concentrated in the frozen upper layers of the cloud, and a large negative charge, along with a smaller positive area, is found in the lower portions.
The earth is normally negatively charged with respect to the atmosphere. But as the thunderstorm passes over the ground, the negative charge in the base of the cloud induces a positive charge on the ground below and for several miles around the storm. The ground charge follows the storm like an electrical shadow, growing stronger as the negative cloud charge increases. The attraction between positive and negative charges makes the positive ground current flow up buildings, trees, and other elevated objects in an effort to establish a flow of current. But air, which is a poor conductor of electricity, insulates the cloud and ground charges, preventing a flow of current until huge electrical charges are built up.
Lightning occurs when the difference between the positive and negative charges--the electrical potential--becomes great enough to overcome the resistance of the insulating air and to force a conductive path for current to flow between the two charges. Electricial potential in these cases can be as much as 100 million volts. Lightning strokes proceed from cloud to cloud, cloud to ground, or, where high structures are involved, from ground to cloud.