Snow Cover over North America during the Winter of 2001-2002 (WMS)

The amount of snow covering the land has both short and long term effects on the environment. From season to season, snow coverage and depth affect soil moisture and water availability, which directly influence agriculture, wildfire occurrences, and drought. In the long term, the part of the Earth's surface covered by snow reflects up to 80 or 90 percent of the incoming solar radiation as opposed to the 10 or 20 percent that uncovered land reflects, and this has important consequences for the Earth's climate. Satellites identify the snow cover precisely by looking at the difference between light reflected off snow in the visible and the infrared wavelengths. This visualization shows the snow cover over North America from October, 2001, through April, 2002, as measured by the MODIS instrument on the Terra satellite. Since this instrument cannot measure snow cover through clouds, this visualization designates an area as covered by snow when the instrument takes a valid measurement showing greater than 50% snow coverage in that area. This area is assumed to be covered in snow until the instrument takes a valid measurement showing less than 40% coverage in that same area. In this animation, snow coverage is measured every 8 days.

See http://modis-snow-ice.gsfc.nasa.gov/intro.html