My Oblivion install is somehow "only" 16GB. Of course I also have half a dozen backed-up Oblivion Data folders on a different partition should something go wrong.

For anyone who cares about my previous snow obsessions, I finally got an answer:

"Given its northern location and extreme elevations, Skyrim's climate is more prone to snowfall than Cyrodiil. To create realistic precipitation effects, Bethesda originally tried to use shaders and adjust their opacity and rim lighting, but once the artists built the models and populated the world the snow appeared to fall too evenly. To work around this problem, they built a new precipitation system that allows artists to define how much snow will hit particular objects. The program scans the geography, then calculates where the snow should fall to make sure it accumulates properly on the trees, rocks, and bushes. "

From http://www.gameinformer.com/games/the_elder_scrolls_v_skyrim/b/xbox360/archive/2011/01/17/the-technology-behind-elder-scrolls-v-skyrim.aspx. Totally off-topic as usual, but it is weather, I guess.