Walsh and Chapman Northern Hemisphere Sea Ice Data Set

Mid-month values of sea ice concentration for the Arctic are digitized
on a standard 1-degree grid (cylindrical projection) to provide a
"relatively uniform set of sea ice extent for all longitudes, as a
basis for hemispheric scale studies of observed sea ice fluctuations"
(Walsh, 1978).

These data are a compilation of data from many sources integrated into
a single gridded product by John Walsh and Bill Chapman, University of
Illinois. The sources of data for each grid cell have changed over
the years from infrequent land/sea observations, to observationally
derived charts, to satellite data for the most recent decades.
Temporal and spatial gaps within observed data are filled with a
climatology or other statistically derived data.

Please note that large portions of the pre-1953, and almost all of the
pre-1900 data is either climatology or interpolated data and the user
is cautioned to use this data with care.

The data sources for the ice concentrations vary spatially and
temporally. There are eight basic data sources for the ice
concentrations:

1. Danish Meteorlogical Institute
2. Japan Meteorological Agency
3. Naval Oceanographic Office (NAVOCEANO)
4. Kelly ice extent grids (based upon Danish Ice Charts)
5. Walsh and Johnson/Navy-NOAA Joint Ice Center
6. Navy-NOAA Joint Ice Center Climatology
7. Temporal extension of Kelly data (see note below)
8. Nimbus-7 SMMR Arctic Sea Ice Concentrations or
DMSP SSM/I Sea Ice Concentrations using the NASA Team Algorithm