Hello!
In T2k 2.2 there are some effects of rain, snow, and fog described in the section dealing with "tactical visibility" (p.223).
MERC: 2000 has a little more: The optional rule "weather" gives some more info: If one uses the noise level table from MERC, several kinds of precipitation (
odd word, had to look for it in the dictionary. All kinds of "water" that falls from the skies. Well, you know what I mean, hopefully!) have an effect, either reducing the ranges or make it more difficult to hear sounds, that ar more quiet.
Fatigue may be increased in "hot" and "very hot" weather, "deep snow" reduces movements. All kinds of cold weather require special clothing.
After all, in my eyes the original rules are not that helpfull. When I work with the effects of weather, I borrow them from "Midgard", a German RPG, that we still play. But I'm sure, there will be rules in most RPGs. Maybe these can easily be adopted.