Two quick ideas,

• Fog during rain would be even more atmospheric. I know Gleba uses some kind of fog, could maybe rip from that.
• Would be cool if there were a few intensities chosen at random when raining. Like light, heavy, thunderstorm, etc.
  -- This could effect things like the rate at which fires are put out (or even the ability to do so) and the amount of fog.

1. Yes, I think it is possible, and it should not hurt FPS significantly
2. I like the idea. It could also be implemented as options, for example with configurable probabilities for different rain intensities

Fog would be really cool and give some new gameplay