Probability that it will be hit would be dependant on the distance and quantity of rain falling. In a light drizzle probably slight, in a heavier shower probably very high. Out to 50m probably pretty good chance it's going to be affected in all but the lightest shower.
Well, I have definately found that I have to aim a bit higher.
Like Tomcat said, it depends on the amount of rain and size of the drops.
I don't think there will be a real scientific calculatation to work this one out.
I think other than the fact that the air is denser in the rain (liek shooting in water) the pellet also weighs a bit more as it is wet. All of this will translate in reducing velocity down range and thus lowering the poi