As an electrical storm builds, various mechanisms create a stratified charge within the storm cloud, with an electrical charge at the base of the cloud. Since we are mostly concerned with cloud-to-ground lightning, we are concerned primarily with the charge on the base on the storm, as that charge induces a “shadow” of opposite charge on the surface of the earth beneath it.
As the storm charge builds, so does the cloud base charge. Since like charges repel, and opposite charges attract, the cloud base charge induces an opposite charge on the surface of the earth beneath it – it pushes away the same charge and pulls in the opposite charge. The cloud base charge attracts, or pulls, on the ground charge, trying to pull it off the surface of the earth. It is this tendency for the storm base charge and the ground charge to equalize through the intervening air which causes cloud-to-ground lightning.
As the storm cloud travels over the earth’s surface, it drags this ground charge along beneath it. When the ground charge reaches your facility, the storm cloud charge pulls it up on, and begins concentrating ground potential on your facility. If, before the storm cloud travels away, it manages to concentrate enough ground potential on your facility so that the difference in potential between the storm cloud base charge and the charge on your facility exceeds the dielectric strength, or resistance, of the intervening air, the air breaks down electrically, and a potential equalizing arc occurs; a lightning strike.
Since we are concerned with lightning strikes to objects and structures on the surface of the earth, and some 95% of all ground strikes are negative cloud-to ground lightning, for the purpose of this discussion we will describe negative cloud-to-ground lightning.