You've fried a connection wire to your alternator and consequently cooked the alternator. Unless for some reason your battery's given out, or it's terminals have corroded or been tampered with, then the rain sounds like a key indicator. First, there's electrical goop they put around the connections, but it could drip with travel and time, and leave you a little vulnerable to rain and condensation right where you don't need it, which in general sends a short - right to the bib connection and cooks it like a piece of glass in a bonfire. After which your connection is completely useless, the battery does not get charged, and usually the alternator fries, even though it will keep spinning. Just no charge. It doesn't take much goop, nor much time to fry. Usually casing around the wire at that point will cook, and when you go to put another alternator on, you have an exposed piece of wire just waiting for moisture to jump a charge between it and the alternator and you get the problem all over again, mysteriously each time it rains. What you need to get is the wire replaced, which might be a pain, or, the casing replaced, which can be done simply by getting this red gasket goop for a few bucks, painting the wire carefully back up to the point to where the ring/bib connector is, and making sure you have new electrical goop at the new connection. That electrical goop is also only a few bucks, and looks like KY jelly. I wouldn't bet too much it isn't, but it's not that expensive to get the right stuff. A rubberized sleeve should then pop over the top of that connection to give the assembly a bit of help. That sleeve should be on the connection wire. If it's cooked, get a new one when you drop off your alternator for a core discount on a new/used one. If you get a used rebuilt one, get one with a warrantee, usually fifteen to 30 bucks more on a better model alternator. For diagnosis in the future, some auto part shops have a cart they pull right out their front door, hook up to your alternator and battery and tell you what's working or not, even how much of a percentage of an alternator is working, and usually this is free. They won't do the labor to replace the alternator, but they might recommend someone. Belts are the worst aspect of replacing alternators, since they need the correct tension, and each car has their favored system to work this
