I
don't think this was used in the game but you can
have rain in your levels, i found a texture called
rain in the sfx texture set and decided to see
what it could do.
What
we need to first is have an area for the rain to
fall in, if you create it the same size as the
room then you won't see the edge of the texture
and you wouldn't see the rain effect.
load
up the sfx texture set and find the texture called
rain and then create a brush which covers the
whole of your sky opening and make it touch the
floor.

Save
and then compile and go sing in the rain.
If you have allot of this in your level then it
could cause slowdown the same way as fog and
mirrors do but the end results are pretty neat.