things you need:
Unreraled
notepad


#1: open the map YYY_04f
search for the actor zoneinfo4,
open it's properties under zoneFx.
http://www.lucasforums.com/picture.php?albumid=540&pictureid=7126)

#2: copy everything into notepad

http://www.lucasforums.com/picture.php?albumid=540&pictureid=7127)

#3: open you map, and add a zoneinfo (under info, zone info, make sure you uncheck "placeable classes only!!!)
#4 open the zone info
and paste in what was in your notepad.
http://www.lucasforums.com/picture.php?albumid=540&pictureid=7128)

#5: go to Build, Build All

#6: got to the particle systems box and find "Rain_Heavy"
right click and add Heavy rain here. ( you need to add a few all over the map about 4 or 5 should do)
http://www.lucasforums.com/picture.php?albumid=540&pictureid=7129)


RESULT


http://www.lucasforums.com/picture.php?albumid=540&pictureid=7130)