How do you add rain to a map?

[That's the plan]

The title says it all.

 

[DrSquishy]

func_precipitation is bad, use info_particle_systems set to the rain particles such as env_rain_001. This particular one spawns particles in a 1024*1024HU box (with the particle system at the center) so just raise it above your map and place one every 1024HU