This could actually be addendum to my Atmospheric Optics post. Sprites are special kinds of lighting that occur high above thunderstorm clouds (shelf clouds).
It is believed that sprites are formed only during really large storms - hence the rarity. Sprites often come in clusters. I would really like to see one day this amazing red thing! A friend of mine once caught it on the photo - blew my mind.
|Mind the blue jet.|
Space Weather - Space weather archive about sprites.
Wiki - Wiki article about sprites.
Eurosprite - Blog about following sprites.