info_particle_system
Name: env_rain_001
Start Active?: Yes
Flag as weather: Yes
EDIT: It has a radius of 512 and a height of 1024, so be sure that each system is not any closer than 700 units to each other.
The way I've handled it in the past is by creating a 1024x1024x1024 cylinder, placing an info_particle_system with the proper info at the top dead-center, and then shift-dragging it to where I need. In the overhead x/y view, you can see these cylinders and where they overlap.
Also, it'll go through buildings (unless you're using that collision rain, but it's very intensive on systems), so be sure to place it mainly outside.