Where does lightning have the potential to travel farther - vertically or horizontally?
The answer is fairly obvious once you ponder it for a few moments. First, consider how far reaching the atmosphere is vertically, then consider how much greater are the distances of the Earth's surface horizontally. Horizontal lightning arcs therefore have the greatest potential distance to travel.
The base of a cumulonimbus [thunderhead] can be several miles across, and it can be tall enough to occupy middle as well as low altitudes; though formed at an altitude of about 3,000 to 4,000 meters (10,000 to 12,000 feet), its peak can reach up to 23,000 meters (75,000 feet) in extreme cases (http://en.wikipedia.org/wiki/Cumulonimbus).
A bolt from the blue is a form of lightning that strikes out of an apparently cloudless sky. In fact, it actually originates from the anvil cloud atop a thunderstorm which may be as far as thirty miles [~50,000 meters] away (http://en.wikipedia.org/wiki/Bolt_from_the_blue).
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