This is a really cool effect that ou can use (provided that you didnt think of it already) to make maps where snow has just started to stick to the ground, very simple idea. You need cs_office.wad to play it as wel as cs_assault.wad, assault is included in the .zip but office.wad was too big, so if you don't have Counter Strike here is a link to the wad:
http://www.freewebs.com/coolfat3459/cs_office.zip