Snow and ice cover the Earth's polar regions all year round. (In this data set, snow cover data are available only for the Northern Hemisphere, which is why Antarctica appears green). At mid-latitudes, the extent of snow and ice cover varies as the seasons change. In the fall and winter months (and occasionally into spring) snowstorms lay white carpets of snow across our world's landscapes as sea surface temperatures grow cold enough to freeze lake and sea water. It is important for scientists to measure the global extent of snow and ice because this helps them to determine how much radiant energy enters and leaves the Earth's system. Snow and ice are highly reflective and therefore raise our planet's albedo (sunlight reflected back to space). On a regional scale, scientists track snow and ice extent because many communities rely upon snow melt as a primary source of freshwater; alternatively, too much snow melt can lead to floods during the spring thaw. Sensor: DMSP/SSM/I, Nimbus 7/SMMR. Data Start Date: 1/1/81. Data End Date: 12/31/96.