Here is my theory. I know that when bird hunting a dog can smell birds on cold/wet grass much better, to the extent that they will be convinced a bird is there when they likely flew hours earlier. This leads me to believe that the particles in the air that the animal smells will blow around more when it is dry and won't when it is wet. By this logic you could assume that when it is wet out there is less scent blowing, because the majority of it is adhered to a wet surface and isn't evaporating, making it more difficult for an animal to detect you because less of your scent particals are flying.
How's that for some science/BS?