This data set includes yearly snow melt onset dates over Arctic sea ice derived from brightness temperatures from the Scanning Multichannel Microwave Radiometer (SMMR), the Special Sensor Microwave/Imager (SSM/I), and the Special Sensor Microwave Imager/Sounder (SSMIS). The introduction of liquid water to snow results in a sharp increase in the emissivity and hence brightness temperature of the snowpack. Snow melt onset is defined as the point in time when microwave brightness temperatures increase sharply due to the presence of liquid water in the snowpack. Data span the years 1979 through 2012 and are in a polar stereographic grid at 25 km resolution. The data are flat binary, 1-byte integer files and GIF images. Value-added data sets include the following for each pixel: mean melt onset date; latest (maximum) melt onset date; earliest (minimum) melt onset date; range of melt onset dates (the difference between maximum and minimum -- an index of variability); and standard deviation of melt onset date (another index of variability). Graphical representations of value-added data are also available.