The proof that lightning is a form of electricity came when

When Benjamin Franklin flew a kite amid a thunderstorm and gathers a charge in a Leyden jar when the kite is struck by lightning.  

Much the same as a battery, these clouds have a + end and a - end. The + or positive, charges in the cloud are at the top. The - or negative, charges are at the base. At the point when the charge at the base gets solid enough, the cloud lets out energy.