BRUSH func_precipitation 
A brush entity that creates rain and snow inside its volume.
KeyValues
Density (0-100%) <integer> = 5 
This is the amount of particles that fall down from top side of brush. However distance from first particle to second depends on a brush volume size!
Color (R G B) <color255> = 100 100 100 
Color added to sprites which are rendered transparently (probably rain and snow particles)
Precipitation Type <choices> = Rain (0) 
No description provided.
Possible Values:
| Name | Value | 
| Rain | 0 | 
| Snow | 1 | 
| Ash | 2 | 
| Snowfall | 3 | 
| Particle Rain | 4 | 
| Particle Ash | 5 | 
| Particle Rainstorm | 6 | 
| Particle Snow | 7 | 
Inner particle distance <float> = -1 
Distance between the inner and outer particle systems. If not set, this will be automatically selected from the precipitation type keyvalue.
Inner near system <particlesystem> 
Name of the inner near particle system. If not set, this will be automatically selected from the precipitation type keyvalue.
Inner far system <particlesystem> 
Name of the inner far particle system. If not set, this will be automatically selected from the precipitation type keyvalue.
Outer system <particlesystem> 
Name of the outer particle system. If not set, this will be automatically selected from the precipitation type keyvalue.
Inherited from BaseEffectBrush
Name <target_source> 
The name that other entities refer to this entity by.
Global Entity Name <string> 
Name by which this entity is linked to another entity in a different map. When the player transitions to a new map, entities in the new map with globalnames matching entities in the previous map will have the previous map's state copied over their state.
Parent <target_destination> 
The name of this entity's parent in the movement hierarchy. Entities with parents move with their parent. Set an attachment point via 'parentname,attachment'.
Inputs
Alpha <integer> 
Changes the density of the rain, and may add additional particle effects like fog or leaves. Accepts inputs from -1 to 255.