Rain in Doom 2! (effect)

Thanks for looking at this thread =D

Okay, so here comes a somewhat stupid question.

I have seen Wad's with rain in some of the maps for decoration/ "setting the mood".

How can i put effects like this in my maps? rain and lightning
but mostly rain!

Assuming you're mapping for one of the ZDoom family - grab a spawner from this page and follow the instructions that are embedded in the wad as a DOCUMENT lump.

Thank You Very Much