as Jai says, watch the wet coping, a lot of folk lift the front wheel over the coping a touch anyway.
Concrete: I taught myself to pop out and roll in on some small concrete ramps in the rain, on my own, first time out on a new school bike, but just try it. I guess what will happen is the grip gives up less predictably.
Wood: damp, no totally dry is sometimes okay, but I've found you often get a kind of "slimey" coating on damp ramps, esp in the shaded part if it's not in full sun to dry out.
