Simply question, do snail show when it's raining because they like it or/and need hydration and food or are they showing up just to escape somewhere, because they're scared of drowning?
Most types of snails survive droughts (that would kill otherwise through Dehydratation) by producing much mucos around themselves. This mucos will dry out if the sun comes up, so they hide under leaves or in the ground. If it's raining, those rings of mucos hydralise or "melt" and the snail begins to crawl. Like most amphibians snail take their oxygen by their skin furthermore they have lungs. By raninning they start moving for searching food, cause they dry out in sun.