It would be nice if we could come up with some minor differentiation between ball lightning and sheet lightning.
According to Wikipedia, sheet lightning is intracloud lightning which "can be seen when lightning is close to the horizon."
"Ball lightning is described as a floating, illuminated ball that occurs during thunderstorms. They can be fast moving, slow moving or nearly stationary. Some make hissing or crackling noises or no noise at all. Some have been known to pass through windows and even dissipate with a bang."
The ball lightning spell works as follows:
"You create one 5-foot-diameter ball of concentrated electricity that rolls in whichever direction you point, dealing electricity damage to creatures it strikes. It moves 30 feet per round. As part of this movement, it can ascend or jump up to 30 feet to strike a subject. If it enters a space with a creature, it stops moving for the round and deals 1d6 points of electricity damage per caster level (maximum 15d6), though a successful Reflex save negates that damage.
The ball moves as long as you actively direct it (a move action for you), otherwise it merely stays at rest, damaging any creature in its space. It cannot push aside unwilling creatures or batter down large obstacles. The lightning winks out if it exceeds the spell's range."
Just a thought...since sheet lightning requires a cloud, perhaps it can only take that form within a natural or magical cloud, and gains concealment?