This is just the basics of getting precipitation into your maps. You should experiment for a more desired result, by adding things like fog, sounds, etc. It gives your maps a better atmosphere and helps you set the mood for the game.
Hammer - Add Rain/Snow to your maps! [HD Vid. Tutorial]
A tutorial on how to add rain, snow, snowfall, or ash to your map. Works for any source game!
Posted by infl1ct1on on - Basic Mapping/Technical