| Summary: | The U.S. National Ice Center (NIC) is an inter-agency sea ice analysis and forecasting center comprised of Department of Commerce/National Oceanic and Atmospheric Administration (NOAA), Department of Defense/U.S. Navy, and Department of Homeland Security/U.S. Coast Guard components. Since 1972, NIC has produced Arctic and Antarctic sea ice charts. This data set is an Arctic sea ice concentration climatology derived from the NIC weekly or biweekly operational ice chart time series. The charts used in the climatology are from 1972 through 2004, and the monthly climatology products are median, maximum, minimum, first quartile, and third quartile concentrations, as well as frequency of occurrence of ice at any concentration for 33 year, 10 year, and 5 year periods. These climatologies and the charts from which they are derived are provided in 25 km EASE-Grid (the NSIDC Equal-Area Scalable Earth Grid/gridded binary). Ice extent can be derived from concentration by summing the number of equal area EASE-Grid cells. GIF browse files are also provided. The climatologies are also made available in a Geographical Information System (GIS) compatible format. NIC charts are produced through the analyses of available in situ, remote sensing, and model data sources. They are generated primarily for mission planning and safety of navigation. NIC charts generally show more ice than do passive microwave derived sea ice concentrations, particularly in the summer when passive microwave algorithms tend to underestimate ice concentration. The record of sea ice concentration from the NIC series is believed to be more accurate than that from passive microwave sensors, especially from the mid-1990s on (see references in the documentation), but it lacks the consistency of some passive microwave time series.
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