O rly? I've actually noticed that, but it is nothing to do with weather; The phase of the moon is based on system clock. So change the date to, say, the next full moon, and the moon in game will be full. (And evil conscience will say, "Full moon tonight. Wish it was blood-red, though." Not sure if good characters will get a different message.)
But that is different from whether it is raining or so forth in any place in the world, which would be impossible to accurately predict as early as 2001.