How can I add rain to the map?
I am guessing this is a troll since you are "DRY"tek asking how to make a map "WET".
You can load some weather presets or start with these settings and adjust for your pleasure.
Pay attention to the DENSITY setting 0-No rain, 100-Monsoon
Select a rain texture you like and adjust the speed.
Snow is just a different texture, slow the speed, add Randomize amount, and add bouyancy so the flakes float around.
Remember to test loading presets on a garbage map cause they change all sorts of things and will wreck all your work. But you can see the settings and make mental notes.
Dont forget to go to MISC ENVIRONMENT and add some extra SPECULAR and GLOSS to make stuff look all plasticy and wet