The average monthly extent of snow cover in the Northern Hemisphere has declined 1.3% per decade, with the greatest losses in the spring and summer, over the last 40 years. In the last 50 years, the U.S. snow season has become 16 days shorter, with peak run off over a week earlier.

Source: United Nations Environment Program (2007). Global Outlook for Ice and Snow. United Nations Environment Program. See Ch. 4: Snow.