The Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent Version 3 product and Version 3.1 update combine snow cover and sea ice extent at weekly intervals from 23 October 1978 through 24 June 2007, and snow cover alone from 3 October 1966 through 23 October 1978. The data set is the first representation of combined snow and sea ice measurements derived from satellite observations for the period of record. Snow cover extent is based on the digital NOAA-NESDIS Weekly Northern Hemisphere Snow Charts, revised by D. Robinson (Rutgers University) and regridded to the EASE-Grid. The original NOAA-NESDIS weekly snow charts are derived from the manual interpretation of AVHRR, GOES, and other visible-band satellite data. Sea ice extent is regridded to EASE-Grid from Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data. Designed to facilitate study of Northern Hemisphere seasonal fluctuations of snow cover and sea ice extent, the data set also includes monthly climatologies describing average extent, probability of occurrence, and variance. Data are provided in the Northern Hemisphere 25 km equal-area grid (NSIDC Nl EASE-Grid). Data are provided in flat, unsigned binary files and are available via FTP.