[Dynamix]: To answer both questions at once, weather will affect gameplay by not only reducing visual ranges, add some real-life hazards in the game (for instance, lightning will NOT be just a pretty background graphic. Expect your shorts to smell like ozone if you're not careful).
We've also got fog effects. Fog in T2 is altitude based, creating some spectacular scenes, like when you're on a mountain peak looking down on a fog-shrouded valley. Visibility while in fog will make it pretty tough to see what's what.