QUOTE("Pete Hines")
As you travel around you’ll see the weather change from one type to another. It’ll be partly cloudy, then some clouds will roll in, then it’s drizzling, then you’re caught in a heavy thunderstorm. We use a dual-layered system in Oblivion to more realistically transition between different types of weather and to better model the way clouds move in the sky.
I don't think that whole dual-layered bit has been discussed before. Sounds like weather transition will be quite realistic.
Credit to Kaleem of the forums again for spotting this before us.