Gran Turismo 5's realism extends even into the in-game weather. Polyphony Digital's Kazunori Yamauchi revealed at a TGS session that the game's weather system will dynamically alter factors like temperature, pressure and humidity in real time, working to simulate real weather systems. What they're calling the Dynamic Weather System will also change surface conditions to give a real chaotic weather feeling over the tracks.
Yamauchi says that the simulation doesn't actually let them set conditions on a track. All they can do is set tendencies, and it's up to the Dynamic Weather System to actually determine conditions. Their input is merely a sort of forecast for what the weather will really be.
Be sure to check out the new screenshots of the weather mode in the gallery below. Note that they don't do any justice to the live demonstration we saw. Effects like rain and snow hitting the windsheld were very impressive, and at some points startlingly realistic.