Good idea, I would love to see rain like in Assassins Creed Black Flag for example, where the rain will also rimple/animate the normal maps (see: ?v=U99B31GbrzQ, looks a lot better from the side by the way, see: ://youtu.be/s0HQlTogOWg?t=15).
Aside from that, I think the raindrops itself are a post process effect, although it could be a particle effect, I’m not certain. It’s interesting that they light up when thunder strikes (see: ://youtu.be/EKjddHoqlEg?t=30 at around 0:30).
I think that game did the rain quite well.
Something like would be cool for like 80-120 bucks, but it can’t have too much of a hit on performance of course.