If air, an insulator, is subjected to a very strong electric field, the air molecules become ionised and become conductingWhen charge builds up on a cloud, the electric field removes electrons from the air. The air suddenly becomes conducting and the cloud discharges and the air conducts the charge to the ground.Example If a lightning strike transfers 35,000C through a potential difference of 50,000V the amount of energy released is given by Energy=QV
=35,000×50,000
=1750MJ
=35,000×50,000
=1750MJ