If you do find a cheap and cheerful technology that does work long-distance, do come back and tell us!
Oh, rain sensing, here's one an astronomer made, might be suitable? simple rain sensor - DIY Astronomer - Stargazers Lounge - though that will take a while to dry out after the rain stops, should that matter. I vaguely recall someone here made a piezo-electric sensor for detecting raindrop impacts, maybe have a search for that.