go to the gamecore-decal folder and theres a rain.dds already made.
then go to your entity-bank-generic-decals folder and make a new copy of your steam.fpe or steam2.fpe as a renamed rain.fpe.
There is a particle.x model located in the decals folder also
take this model into a modeling program and scale it up i scaled on a x,y,and z axis 6 x larger then the orginal. Save this model as particle2.x and put it in the decal folder. Then edit the rain.fpe to read this
;header
desc = rain
;ai
aiinit = appear1.fpi
aimain = decal.fpi
aidestroy = disappear1.fpi
;orientation
model = particles2.x
textured = particles_d.tga
transparency = 0
defaultheight = 0.5
;identity details
strength = 0
isimmobile = 1
ismarker = 4
;decals
decalmax = 1
decal0 = rain
then when you put in your rain just rotate your model upside down so the emitter travel down instead of up like steam would.
And you should have rain, next i want to edit the dds of rain to a snow image, the decals use 256 x 4 width and height making a 1024 x 1024 image size then just make a new folder using the above pointing to the snow decal instead of rain. There are several other decals you can play with that are already made like mushroom, splinter just need to make .fpe for them.
edit: actually i don't think you even have to rotate the model and it works.
Here is a simple one room demo of the rain for anyone who want's to see it, i only stacked the rain entities 2 high for demo.
http://dbgamezone.com/fpscreator/demos/rain.rar