This data set is generated from brightness temperature data derived from the following sensors: SMMR, DMSP -F8, -F11 and -F13, SSM/Is, and SSMIS. The data are provided in the polar stereographic projection at a grid cell size of 25 x 25 km. This product is designed to provide a consistent time series of sea ice concentrations, the fraction or percentage of ocean area covered by sea ice, spanning the coverage of several passive microwave instruments.
Northern Hemisphere EASE-Grid 2.0 Weekly Snow Cover and Sea Ice Extent, Version 4
- Temporal coverage was extended to 29 June 2014.
- The projection changed to EASE-Grid 2.0, 720 x 720, at 25.000000 km
- Sensor change over dates:
- SMMR to SSM/I: 1987-07-03
- SSM/I F08 to F11: 1991-12-09
- SSM/I F11 to F13: 1995-05-08
- SSM/I F13 to SSMIS F17: 2008-01-01
- EASE-Grid Land Mask Source: Same as Version 3.1, regridded to EASE-Grid 2.0 (Brodzik et al. 2012).
- Sea Ice data set used: 1978-10-23 to present: Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data.
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Geographic Coverage
Parameter(s): |
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Spatial Coverage: |
N: 90, S: , E: 180, W: -180 |
Spatial Resolution: |
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Temporal Coverage: |
|
Temporal Resolution: | 7 day |
Data Format(s): |
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Platform(s) | DMSP, DMSP 5D-2/F11, DMSP 5D-2/F13, DMSP 5D-2/F8, DMSP 5D-3/F17, GOES, GOES-7, Nimbus-7 |
Sensor(s): | SMMR, SSM/I, SSMIS, VAS |
Version: | V4 |
Data Contributor(s): | Richard Armstrong, Mary Brodzik |
Metadata XML: | View Metadata Record |
Data Citation
As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.
Brodzik, M. J. and R. Armstrong. 2013. Northern Hemisphere EASE-Grid 2.0 Weekly Snow Cover and Sea Ice Extent, Version 4. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/P7O0HGJLYUQU. [Date Accessed].Detailed Data Description
Snow cover extent for this data set is based on the NOAA/NCDC Climate Data Record (CDR) of Northern Hemisphere (NH) Snow Cover Extent (SCE) by D. Robinson (2012) and regridded to the EASE-Grid. The NOAA/NCDC CDR of Northern Hemisphere Snow Cover Extent data were derived from the manual interpretation of AVHRR, GOES, and other visible-band satellite data (Helfrich et al. 2007). Sea ice extent is regridded to EASE-Grid from Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data.
Data are stored in flat binary files with arrays of 720 columns by 720 rows of 1-byte unsigned integers. Each data file contains one array.
Data files represent weekly Northern Hemisphere snow cover and sea ice extent.
Data are available on the HTTPS site in the https://daacdata.apps.nsidc.org/pub/DATASETS/nsidc0046_weekly_snow_seaice/ directory. Within this directory there are four folders: browse
, data
, metdata
, and stats
. Figure 1 shows an image of the directory structure.

Weekly Data, Browse, and Metadata Files
Weekly data and browse files are named according to the following convention, and as described in Tables 1a and 1b:
EASE2_N25km.snowice.yyyymmd1-yyyymmd2.vVV.bin
EASE2_N25km.snowice.yyyymmd1-yyyymmd2.vVV.png
Where:
Variable | Description |
---|---|
EASE2_N25km |
Indicates this is an EASE-Grid 2.0 Northern Hemisphere 25 km file |
snowice |
Indicates this is a snow and ice file |
yyyymmd1 |
4-digit year, 2-digit month, and 2-digit day of month indicating the beginning of the NOAA week (d1 ) |
yyyymmd2 |
4-digit year, 2-digit month, and 2-digit day of month indicating the end of the NOAA week (d2 ) |
vVV |
Version number (v04 ) |
.bin |
Indicates this is a binary data file |
.png |
Indicates this is a PNG browse file |
Metadata for each weekly data file is provided in an ASCII text file. The naming convention for this file varies only slightly from the above convention, for example:
EASE2_N25km.snowice.metadata.yyyy-yyyy.vVV.txt
Where:
Variable | Description |
---|---|
metadata |
Indicates this is a metadata file |
yyyy-yyyy |
Indicates 4-digit start and end years, such as 1966-2010 |
.txt |
Indicates this is an ASCII text file |
Monthly Climatology Files
Monthly climatology files containing probability of occurrence, average extent, or variance data are named according to the following conventions, and as described in Table 2:
EASE2_N25km.parm.cli.mm.yyyy-yyyy.vVV.bin
EASE2_N25km.parm.cli.yyyy-yyyy.vVV.png
EASE2_N25km.parm.cli.yyyy-yyyy.vVV.ps
Where:
Variable | Description |
---|---|
EASE2_N25km |
Indicates this is an EASE-Grid 2.0 Northern Hemisphere 25 km file |
parm |
Parameter (sno, ice ) |
cli |
Climatology file (frq: frequency/probability of occurrence; avg: average extent; var : variance) |
mm |
2-digit month |
yyyy-yyyy |
Indicates 4-digit start and end years, such as 1966-2005 |
vVV |
Version number (v04 ) |
.bin |
Indicates this is a binary data file |
.png |
Extension indicating a PNG file (viewable in any Web browser) |
.ps |
Extension indicating a color PostScript file (viewable in any Web browser) |
Data files are approximately 508 KB each. The browse image files are approximately 22 to 23 KB each.
Northern Hemisphere
Spatial Resolution
Source data are regridded to a Northern Hemisphere of 25 km resolution with Version 2.0 EASE-Grid.
Projection and Grid Description
Data are gridded using EASE-Grid 2.0 (Brodzik et al. 2012), which is defined with the WGS 84 ellipsoid rather than the spherical Earth model in the original EASE-Grid. The grid is 720 columns by 720 rows (0,719). For a detailed comparison between the two versions of EASE-Grid, refer to the EASE-Grid Data Versions page.
Snow cover data are provided from 03 October 1966 through 23 October 1978, and combined snow cover and sea ice extent from 23 October 1978 through 31 December 2017.
Weekly sea ice extent was derived from an average of three or four sea ice source files for the SMMR time period. Weekly sea ice extent was derived from an average of seven ice source files for the SSM/I–SSMIS time period.
Temporal Resolution
Each file represents weekly data corresponding with the weekly NOAA/NCDC snow cover extent CDR data. A week is defined here as seven days.
Missing Data
No files exist for the following weeks:
- Four weeks spanning 01 July – 28 July 1968
- 21 weeks spanning 02 June – 26 October 1969
- 12 weeks spanning 05 July – 26 September 1971
Files for the following weeks have no ice information:
- All weeks prior to 23 October 1978
- Five weeks spanning 07 December 1987 – 10 January 1988
Parameter Description
Snow Cover Extent
Binary, indicating snow or no snow
Sea Ice Extent
Binary, indicating presence or absence of sea ice when sea ice concentration is greater than 15 percent in a majority of a given week's sea ice files. The 15 percent ice concentration isopleth is a conventionally accepted cutoff for the ice edge (Johannessen et al. 1996).
Unit of Measurement
Binary presence or absence of snow or sea ice
Parameter Source
- Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I–SSMIS Passive Microwave Data
- NOAA/NCDC Climate Data Record (CDR) of Northern Hemisphere (NH) Snow Cover Extent (SCE) by D. Robinson et al. (2012), Rutgers University Global Snow Lab
Parameter Range
Weekly Data
Data files contain a binary array of unsigned byte (8-bit) data with the following data values:
Data Value | Parameter |
---|---|
0 | Snow-free land |
1 | Snow-covered land |
2 | Sea ice |
3 | QC sea ice: pixel set to sea ice via QC during regridding |
4 | QC ocean: pixel set to open ocean via QC during regridding |
5 | QC snow: pixel set to snow via QC during regridding |
6-252 | Not used |
253 | Unclassifiable water due to mask mismatches |
254 | Corner pixels (outside Northern Hemisphere) |
255 | Open ocean |
Metadata for Weekly Data
The metadata for each weekly data file is consolidated in one ASCII text file. Figure 2 shows a sample of the metadata file, which displays information such as pixel counts for two weekly data files in September 2008.

Monthly Probability of Occurrence
Monthly probability of occurrence is the percentage of time in a given month that each pixel was either snow or sea ice. Data in these files are 1 byte values, ranging from 0 to 100 percent.
Monthly Average Extent
Monthly average extent is the binary value for each pixel, indicating 50 percent or greater probability of occurrence of snow or sea ice. Data are 1 byte values of 0 (probability of occurrence was less than 50 percent) or 1 (probability of occurrence was greater than or equal to 50 percent).
Monthly Variance
Monthly variance is the snow or sea ice probability of occurrence for each pixel, calculated as shown in Equation 1:

Where:
i = 1966 through current processing year for snow or 1978 through current processing year for ice
n = number of years= snow or sea ice probability of occurrence for data in this month for the period of record
= snow or sea ice probability of occurrence for data in this month for year i
Data are 1-byte values, ranging from 0 to 26, representing variances of 0.0 (least variance, i.e. always or never snow or ice covered) to 0.26 (most variance).
Sample data images are shown in Figures 3 through 6. Click on the thumbnail images for high-resolution images.
NSIDC observed and manually inspected selected files in this data set, and performed spot checks against the original hand-drawn NOAA/NESDIS snow charts and the original sea ice data. Please refer to the Quality Control Summary for more information about quality control procedures at NSIDC.
Software and Tools
Table 4 lists the software and tools that are available for this data set. Tools are accessible via direct FTP.
Tool Type | Tool File Name | Description | Access |
---|---|---|---|
Geocoordinate | EASE2_N25km.lats.720x720x1.double (Located within gridloc.EASE2_N25km.tgz) |
These are geolocation files containing double-precision (8-byte) floating point arrays (720 columns x 720 rows) representing latitude or longitude at the center of each cell in the EASE2_N25km grid. Cells in the grid that are not in the Northern Hemisphere are designated with the value -999.0. | FTP |
EASE2_N25km.gpd |
This is a Northern Hemisphere grid parameter definition file that contains projection and grid resolution information specifying a 25 km grid resolution in EASE-Grid 2.0. | FTP | |
easeconv.pro (Located within easeconv-0.3.tgz ) |
Easeconv.pro is an IDL program containing routines to convert between latitude/longitude and row/column coordinates. | FTP | |
Region Mask | EASE2_N25km.LOCImask_land50_coast0km.720x720.bin |
This is the EASE-Grid 2.0 land-ocean-coastline-ice (LOCI) mask (Brodzik et al. 2012); it contains a flat, binary array (dimensions 720x720 pixels) of 1-byte unsigned integers. See the EASE-Grid 2.0 Land-Ocean-Coastline-Ice Masks Derived from Boston University MODIS/Terra Land Cover Data documentation for more information. | FTP |
Data Acquisition and Processing
Snow Data
The snow data used for Version 4 are from the recently released NOAA/NCDC Climate Data Record (CDR) of Northern Hemisphere (NH) Snow Cover Extent (SCE). The snow cover extent CDR uses an improved algorithm to produce pseudo-weekly snow charts from the daily, 24 km IMS product (NOAA/NESDIS/OSDPD/SSD 2004; Ramsay 1998; Helfrich et al. 2007). The snow cover extent CDR corrects spatial inconsistencies in snow-covered cells between the original weekly product and the pseudo-weekly data, primarily in mountainous regions, including the Himalayas, during specific months of the year. For data during the 1970s and early 1980s, the snow cover extent CDR also eliminates cells from the weekly data that were previously identified as patchy snow. D. Robinson and colleagues have determined that the early, coarser product depicted too much snow cover in these areas (personal communication to M. J. Brodzik, 23 December 2012). Robinson's statistical comparisons indicate that these adjustments are small, influencing previously published assessments of continental snow extent variability, by at most a few percent in several years in the late 1970s and early 1980s. Recent analysis of the snow cover extent CDR indicates a 95% confidence interval uncertainty in snow cover extent of plus or minus 3–5% during Northern Hemisphere spring for 1966–2010 (Brown and Robinson 2011).
Sea Ice Data
The sea ice data set used for Version 4 is the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data.
The majority of data values were derived via nearest-neighbor interpolation from the respective source grids. Pixels with QC data values were determined by more complex algorithms as described in the Quality Control Summary and in the Processing Steps in the following section of this document.
- Define an EASE-Grid 2.0 Northern Hemisphere land/ocean/permanent ice mask.
- Identify source land mask mismatches with the EASE-Grid 2.0 Northern Hemisphere land mask.
- Define EASE-Grid 2.0 sea ice climatology mask.
- Combine sea ice data for the given week and regrid sea ice from source grid (polar stereographic projection) to EASE-Grid 2.0 via nearest-neighbor interpolation.
- Set one-neighbor category mismatch pixels to QC sea ice if at least one of the surrounding eight source grid pixels was sea ice.
- Set two-neighbor category mismatch pixels to QC sea ice if at least one of the surrounding twenty-four source grid pixels was sea ice.
- Set the others category mismatch pixels to unclassifiable water.
- Set all regridded open ocean pixels above 83 degrees north latitude to QC sea ice.
- Set all regridded sea ice pixels outside the defined sea ice climatology mask area to QC ocean.
- Regrid digitized snow extent data from source (polar stereographic projection) grid to EASE-Grid 2.0 via nearest-neighbor interpolation.
- Set lower latitude snow neighbor category mismatch pixels to QC snow if at least one of the adjacent lower latitude source grid pixels was snow.
- Set any snow neighbor category mismatch pixels to QC snow if any of the adjacent source grid pixels were snow.
The EASE-Grid land-ocean-coastline-ice (LOCI) mask was derived from the Boston University version of global 1 km land cover from MODIS 2001 (Knowles 2004), courtesy of K. Knowles, National Snow and Ice Data Center.
NSIDC used American National Standards Institute (ANSI) C and Research Systems' Interactive Data Language (IDL) Version 8.2 to develop software for this data set.
Table 5 summarizes the cumulative differences between versions of this product.
- Version 1
- Version 2
- Version 3
- Version 3.1
- Version 4
- Version 4.1
Version 1 | |
---|---|
NSIDC Data Use Recommendations | |
Projection, Dimensions, and Spatial Resolution | EASE-Grid 721 x 721 25.067525 km |
Temporal Coverage | 1971-01-04 to 1995-09-03 |
Missing Weeks |
|
Weeks with No Ice |
|
Weeks with Incomplete Ice |
|
Sensor Changeover Dates |
|
Sea Ice Data Set Used | N/A |
Ice Climatology | Static climatology, a "best guess" at likely sea ice locations, bounded by lat/lon boxes |
Polar Stereographic Maps Earth Model | Spherical, would have caused slight errors in regridded ocean/ice locations; the model error would cause geolocation errors of up to 8 km, which would result in errors of at most one pixel when sea ice was regridded using nearest-neighbor to the NL EASE-Grid |
EASE-Grid Land Mask Source | Original NSIDC EASE-Grid mask, all of Greenland was mapped to QC snow. |
Manual Snow Chart Corrections |
|
Version 2 | |
---|---|
NSIDC Data Use Recommendations | |
Projection, Dimensions, and Spatial Resolution | EASE-Grid 721 x 721 25.067525 km |
Temporal Coverage | 1966-10-03 to 2001-06-24 |
Missing Weeks |
|
Weeks with No Ice |
Same as Version 1 |
Weeks with Incomplete Ice |
|
Sensor Changeover Dates |
|
Sea Ice Data Set Used | N/A |
Ice Climatology | Monthly climatology, derived from historical record of SMMR and SSM/I sea ice data from 1978 to 1999 |
Polar Stereographic Maps Earth Model | Elliptical, this is the correct Earth model for the input sea ice data |
EASE-Grid Land Mask Source | Original NSIDC EASE-Grid mask, all of Greenland was mapped to QC snow. |
Manual Snow Chart Corrections |
|
Version 3 | |
---|---|
NSIDC Data Use Recommendations | NSIDC believes that those time periods using the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data should replace previous versions and updates using near-real-time and preliminary sea ice input data. Extensive quality-control, reducing the number of pixels misclassified as sea ice due to weather effects and/or coastal contamination as well as the long-term calibrated time-series extending back to the SMMR era makes this input data a higher quality product. |
Projection, Dimensions, and Spatial Resolution | EASE-Grid 721 x 721 25.067525 km |
Temporal Coverage | 1966-10-03 to 2005-06-05 |
Missing Weeks | Same as Version 2 |
Weeks with No Ice |
|
Weeks with Incomplete Ice | None |
Sensor Changeover Dates |
|
Sea Ice Data Set Used |
|
Ice Climatology | Monthly climatology, derived from historical record of SMMR and SSM/I sea ice data from 1978 to 2003 |
Polar Stereographic Maps Earth Model | Same as Version 2 |
EASE-Grid Land Mask Source | EASE-Grid land-ocean-permanent ice mask derived from BU-MODIS land cover (Knowles 2004) |
Manual Snow Chart Corrections | Rutgers corrected all previously noted irregularities. However, a previously undetected irregularity was found and manually corrected in the chart for 1972-04-03. |
Version 3.1 | |
---|---|
NSIDC Data Use Recommendations | Users already making use of the original v03 data would not likely see significant changes in the overlap period in the new data because the processing differences are small. New users of these data are recommended to use v03.1 files for the overlap period 27 December 2003 to 05 June 2005. NSIDC believes that those time periods using the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data should replace previous versions and updates using near-real-time and preliminary sea ice input data. Extensive quality control, reducing the number of pixels misclassified as sea ice due to weather effects and/or coastal contamination as well as the long-term calibrated time-series extending back to the SMMR era makes this input data a higher quality product. |
Projection, Dimensions, and Spatial Resolution | EASE-Grid 721 x 721 25.067525 km |
Temporal Coverage | 2003-12-27 to 2007-06-24 (v3.1 files whose dates overlap v3.0 dates should replace them.) |
Missing Weeks | Same as Version 3 |
Weeks with No Ice |
Same as Version 3 |
Weeks with Incomplete Ice | Same as Version 3 |
Sensor Changeover Dates | Same as Version 3 |
Sea Ice Data Set Used |
|
Ice Climatology | Same as Version 3 |
Polar Stereographic Maps Earth Model | Same as Version 3 |
EASE-Grid Land Mask Source | Same as Version 3 |
Manual Snow Chart Corrections | During data set quality control procedures, NSIDC noted a NOAA pixel at the western end of Baffin Island that became snow-free beginning with the week of 31 May to 06 June 1999, and that remained snow-free for the remainder of the current time series (through May 2005). |
Version 4 | |
---|---|
NSIDC Data Use Recommendations | NSIDC believes the improvements to the snow CDR data and the update to EASE-Grid 2.0 represent the best available analyses and presentation of these data sets, and warrant the complete reprocessing of this product. Refer to the Data Source section of this document. Users of previous versions are recommended to move to Version 4 data. |
Projection, Dimensions, and Spatial Resolution | EASE-Grid 2.0 720 x 720 25.000000 km |
Temporal Coverage | 1966-10-03 to 2014-06-29 |
Missing Weeks | Same as Version 3.1 |
Weeks with No Ice |
Same as Version 3.1 |
Weeks with Incomplete Ice | Same as Version 3.1 |
Sensor Changeover Dates |
|
Sea Ice Data Set Used |
|
Ice Climatology | Same as Version 3.1, regridded to EASE-Grid 2.0 (Brodzik et al. 2012) |
Polar Stereographic Maps Earth Model | Same as Version 3.1 |
EASE-Grid Land Mask Source | Same as Version 3.1, regridded to EASE-Grid 2.0 (Brodzik et al. 2012) |
Manual Snow Chart Corrections | Rutgers corrected all previously noted irregularities. However, NSIDC has discovered an irregularity in one file starting 03 April 1972 (EASE2_N25km.snowice.19720403- 19720409.v04.bin); a number of pixels in the Great Lakes region are showing a snow-free pattern. This has been reported to Rutgers. |
Version 4.1 | |
---|---|
NSIDC Data Use Recommendations | The input data to this data set, Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data, NSIDC-0051, was updated causing a minor update, 4.1, to this data set. The temporal coverage for this data set was also updated to 31 December 2015. Users of previous versions are recommended to move to Version 4.1 data. |
Projection, Dimensions, and Spatial Resolution | EASE-Grid 2.0 720 x 720 25.000000 km |
Temporal Coverage | 03 October 1966 to 31 December 2015 |
Missing Weeks | Same as Version 3.1 |
Weeks with No Ice |
Same as Version 3.1 |
Weeks with Incomplete Ice | Same as Version 3.1 |
Sensor Changeover Dates |
|
Sea Ice Data Set Used |
|
Ice Climatology | Same as Version 3.1, regridded to EASE-Grid 2.0 (Brodzik et al. 2012) |
Polar Stereographic Maps Earth Model | Same as Version 3.1 |
EASE-Grid Land Mask Source | Same as Version 3.1, regridded to EASE-Grid 2.0 (Brodzik et al. 2012) |
Manual Snow Chart Corrections | Rutgers corrected all previously noted irregularities. However, NSIDC discovered an irregularity in one file starting 03 April 1972 (EASE2_N25km.snowice.19720403-19720409.v04.bin); a number of pixels in the Great Lakes region are showing a snow-free pattern. This was reported to Rutgers. |
These data are most appropriate for large area studies. Users should note that source grid snow pixels range in size from approximately 16,000 km2 to 42,000 km2, and source grid ice concentration pixels range in size from approximately 485 km2 to 664 km2.
Pixels with values of unclassifiable water are undetermined because of mismatches between the various source and target land masks.
References and Related Publications
Contacts and Acknowledgments
Richard Armstrong
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309–0449 USA
Mary Jo Brodzik
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309–0449 USA
The National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA) provided funding for the production of this data set.
Document Information
DOCUMENT CREATION DATE
January 1997
DOCUMENT REVISION DATE
January 2013
September 2007
January 2005
March 2002