Melt onset data are stored year-by-year in a comma-delimited and as georeferenced tab-delimited ASCII-file. Data grid is the SSM/I standard grid with 332x316 grid points. Landmask and seamask points are denoted '500', Sea ice without detected melt is denoted 'NaN'.Melt onset dates range from 1 to 180 and represent the number of days after Oct, 1 for each summer respectively.Acknowledgements: SSM/I Pathfinder Data were kindly provided by the U.S. National Snow and Ice Data Center (NSIDC; http://www.nsidc.org). This study was funded by the German Research Foundation (DFG) under contract HA2724/4-1, BA2060/2-1.