The global atmospheric electric circuit maintains a continuous negative charge on the Earth's surface and a positive charge in the ionosphere, with a fair-weather 'return' current continuously discharging this separation. This charge separation is replenished primarily by active thunderstorms, which act as the generators of the global circuit, predominantly via cloud-to-ground lightning discharges that transfer substantial negative charge to the Earth's surface.
In the global electrical circuit model, thunderstorms worldwide act as batteries/generators driving current through the atmosphere: within thunderstorms, charge separation creates a negatively charged cloud...
Full option-by-option analysis available to enrolled students.