Can ball lightning occur on clear nights? I think I've seen actual ball lightning before (that or it was a REALLY weird atmospheric phenomena). I think I've seen them on three separate occassions back when I lived out in the country. They were all the same shape... they looked like colored balls encased in a lighter color. I saw red, green, and purple lights that slowly shimmied their way downward, sort of how you'd see a tadpole or sperm move, and then they just vanished.
Those flashes of lights I saw out on the horizon also occurred on clear nights.