The charge on the base of the storm cloud induces a shadow of opposite charge on the surface of the earth beneath it.
As the storm cloud builds, the difference in potential between the cloud base charge and the ground charge increases, with the cloud base charge trying to pull the ground charge off the surface of the earth.
As a thunderstorm builds, various mechanisms produce a stratified charge in the storm cloud, with a particular charge at the base of the storm cloud. Remember playing with magnets as a kid? Like charges repel and opposite charges attract.
As the charged storm cloud travels across the earth, it drags the ground charge along beneath it. When the ground charge reaches a structure, the cloud base charge pulls it up onto the structure. If, before it moves away, the charge on the base of the storm cloud manages to concentrate sufficient ground charge potential on the structure to overcome the dielectric of the intervening air, an arc (lightning strike) occurs.
