This data set is held at International Satellite Land-Surface Climatology Project (ISLSCP) and is derived from the National Snow and Ice Data Center (NSIDC) Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent product. The NSIDC product combines snow cover and sea ice extent at weekly intervals for October 1978 through June 2001, and snow cover alone from 1966 through June 2001. (Sea ... ice data were not available prior to October 23, 1978.) Designed to facilitate study of Northern Hemisphere seasonal fluctuations of snow cover and sea ice extent, the original NSIDC data set includes monthly climatologies describing average extent, probability of occurrence, and variance. This data set shows the extent of snow on the land at a variety of scales (1 degree, 0.5 degrees, and 0.25 degrees). The values represent the percentage of days in each month where snow was present. 100 means 100% of the month, 80 means 80% of the month, etc. These data are available from the ISLSCP data archive.