As thunderstorms develop, interactions of charged particles produce an intense electrical field within a 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.
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 make the positive ground current flow up buildings, trees and other elevated objects in an effort to establish a flow of current, but air, a poor conductor of electricity, insulates the cloud and ground charges, preventing a flow of current until a huge electrical charge builds up.
Lightning flashes when the attraction between positive and negative charges become strong enough to overcome the air’s high resistance to electrical flow.