Truth is, that this kind of effect cannot be replicated in FPSC the way you probably know it from AAA games like HL2, Stalker, Skyrim etc. because FPSC only does them as 'huds'.
You'll never see it raining through a window or see the raindrops hit a surface.
The only way to make it appear halfway realistic, is to build the level without any 'cover' and to enter/leave buildings via 'teleport' scripts so the player is always either inside or outside.
In case you find my grammar and spelling weird ---> native German speaker ^^