“Ever since Benjamin Franklin’s time lightning has been understood to be a large electrical discharge similar to that seen when a conductive object (like a metal doorknob) is touched after a static electric charge is picked up (by feet scuffing across carpet, for example). But whereas the spark from static electricity measures a centimeter or less in length, a lightning channel can span five kilometers or more. (Also, cloud-to-ground lightning involves electrical currents on the order of tens of thousands of amps. In contrast, a circuit breaker for a common household circuit is usually rated at 20 amps.) Because of its extreme scale, lightning is a complex physical phenomenon.”
More here from Scientific American.