Earth's surface acts as an electrical conductor. The air between acts as a slightly leaky electrical insulator. Between the ionosphere and the ground there's an average difference of 250,000 volts. What we do notice is the 100-fold more powerful currents that flash at up to half the speed of light between the ground and a thunder cloud in lightning strokes. Differences of hundreds of millions of volts between ground and cloud drive these awesome discharges.
Thunderstorms are the electrical generators that keep the global electric current flowing.
Even though Franklin didn't know the magnitude of these currents and voltages, he did know he was playing with powerful electrical forces when he sent a kite toward a thunderhead and drew sparks from an attached key in 1752. Several years earlier, Franklin established his fame as an electrical scientist when he designed the crucial experiment to prove thunder clouds are electrified. In this, an observer would stand in a shelter with an iron rod projecting through the roof. The observer could then draw sparks from the rod if a passing thunder cloud were electrically charged. Connection between frequency of lightning and global temperature could be used to track any global-warming trend.