This animation from NASA's Earth Observatory shows the extent of snow cover in the Northern Hemisphere every month since February 2000. The gray disk in the center that appears, grows, and shrinks every winter is the extent of the Arctic's 24-hour darkness, which prevents NASA's satellite from collecting snow-cover data.
I love how this animation reveals the mountain ranges of the middle latitudes every winter: first the Rockies and the Alps, then the Caucasus, Appalachians, and Atlas ranges. The Ural Mountains lead the vanguard of winter snow from the Arctic Ocean towards the Caspian Sea. The snow is like an ebbing and flowing tide that hides and reveals islands of cold.
Click here for the original, more detailed animation from NASA.