Sea Ice Index products are derived from 2 data sets: the Near-Real-Time DMSP SSMIS Daily Polar Gridded Sea Ice Concentrations and the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data. These satellite passive microwave-derived data sets are used to generate the monthly images and numbers that comprise the Sea Ice Index record of sea ice extent and concentration from November 1978 to present. Median extents are calculated using a 30-year reference period of 1981 through 2010. The data are produced in a consistent way that makes the monthly index time-series appropriate for use when looking at long-term trends in sea ice cover.