The earth's negative electrical surface charge is the accumulated effect of thousands of thunderstorms. The average thundercloud generates a current of about one amp during its active period. With an estimated 1000 to 2000 thunderstorms continually active around the globe emitting close to 5000 lightning strikes every minute, there is an electric current of 1000 to 2000 amps continually being transferred to the surface of the earth. Since the earth's surface is highly conductive, this charge is evenly and rapidly dispersed across the land surface.

Even the atmosphere conducts electricity (to a lesser degree) because of ions in the air.  Thus, electrons continually flow from the earth’s surface into the atmosphere where an equal and opposite (positive) charge is maintained in the ionosphere. The whole circuit is referred to as the global electrical circuit. (see graphic) It is estimated that this current would disappear in less than an hour if all thunderstorm activity ceased. The global electrical circuit is responsible for the weather.  It also maintains an unlimited supply of electrons everywhere on the surface of the earth.

see also: The Earth’s Electrical Surface Potential

Above: Diagram shows fair weather circuit, also called “global electrical circuit,” showing normal potential between the ground and atmosphere. Credit: NASA/MSFC (Dooling)