[Imp]

On the weather it does not realy mater what type it is as game cant model gunflashes starshells etc tend to regard anything under 30 hexes as inclement weather or night dusk dawn & like DD adjust settings though rally had not occured to me. With TI units adjusting becomes a bit of a problem usually just restrict search slightly.
The premise for the above & assume DDs is the same if visibility is set to say 10 hexes thats the max distance you can see for whatever reason. But poor weather or night means you can only see well to less than that vehicles will be visible at max range infantry perhaps half.
While it would be nice to have weather probably beyond the engine you would need wind to. Seeing firing into a rainstorm upwind would be a lot harder than downwind.