Note: This requires at least GZDoom 1.1.04 or ZDoom 2.2.0. I haven't tested it on the SVN versions or older versions of either.
Spoiler:After seeing a few people's implementations of the whole rain/snow thing and wanting something a bit more robust for the large Cold as Hell maps I'm working on, I came up with this surprisingly easy-to-use way of doing nice looking weather effects. The main method of doing it (using two map spots to calculate a weather zone) isn't new, but I don't think I've seen another weather effect mod that uses the player's position to restrict the weather zone to a certain area around the player if desired. This is nice for larger maps, because it effectively cuts down the size of the weather zone to just what the player can see.
I've tested it on both ZDoom 2.2.0 and GZDoom 1.1.04. In GZDoom the rain and snow look nicer, but they work just fine under ZDoom.
Full documentation is included, along with a crappy example map that sucks. I've been replacing the snow in Cold as Hell with this, and it's hugely successful. It's basically at the point where if you can run the mod at all, you can run it with full weather effects with a very minimal FPS drop.
The DECORATE file includes definitions for a few default spawners, including snow, hard snow, rain, and hard rain. These can be expanded and configured to create just about any shit-falling-from-the-sky effect you could possibly want.
If you like this and want to use it (or any part of it) with your own WAD, go right ahead! Just give me a little credit in there somewhere. You're also free to use my awesome snow/rain sprites that I made in Photoshop in 2 minutes and looping rain sound that I think is the General MIDI "rain" effect recorded through some sound card or other.
I have to give credit to everyone in the ZDoom community who came up with similar rain/snow mods before me, especially solarsnowfall, whose work in the water resource mod got me thinking about how to do this.
So let me know what you think. Tell me if it works pretty well or if it makes your computer want to cough up a lung.
Protips for Anyone Using this: