I just yanked the particle effects for rain out of the envior gallery. Compiling now.
The rain is actually a cool effect. It has drops but also 'sheets of mist'. But it looks like a fairly small radius so you have to place them every 512 units or so. Probably goes through walls and roofs. (actually, just saw the comment, it says 700-1500 units.
particle system name
env_rain_001
The gutter drip and splash effects are also in the enviro gallery.
I grabbed the water fall out of the prop library, but haven't found the particles for it yet. The soundscape is in the enviro gallery.
I just added the muddy alpin ground too. It helps darken the map quite a bit, but the blend into the rocks is still the blend into apline ground, so the moss is bright, that's gonna take a new vmf.
I also yanked the enviro light out of the enviro gallery, but my map seems brighter than sawmill, there must be something else going on to make it feel as dark as it is. I've only played it once though.
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I added about 5 particle effects and filled up a large area with them. Barely even bumps the particle rendering up to 1-1.5ms in +showbudget. I think valve placed them closer than 700 units in sawmill, mine are about 512 and sawmills rain is thicker.
But I was wrong about lighting, sawmill really isn't that dark. I think it's just the new enviro, the darker textures for ground and they used mostly dark tex on all wood. And the murky water.
Rain does go through wall so that'll be a little tricky to set up just right.
It also starts at the height of the particle entity, I believe it just falls until it hits solid terrain. Water surface does not block it either.