Snowfall at last brings weather systems to Skylines. Cosmetic rain and fog will be released as a free update and appear on existing maps to make your citizens' lives that little bit less pleasant.
If you buy the full expansion, you'll be expected to handle the misery yourself. On winter-themed maps, cities will have to be heated as the cold sets in, transport infrastructure will be expected to handle the ice, and you'll want snow ploughs to keep traffic moving. Hopefully winter parks and new landmarks will ease the winter misery.
There's no word on price yet, but the After Dark expansion costs $15/£11, so I'd expect something similar.