Huh, The only racing that isn't run in the rain here is Nascar, obviously because they are running on pavement ovals and ive never seen a oval race run in the rain. f3000s do run in the rain also.
And rain racing is awesome it adds a whole new element to racing were you have to drive a completely different line since the rubber on the track feels like ice and figuring out the limits without going off the track can get you a good finish. Ok I want rain now.