Image derivation is from the Sea
Ice Index data product, which relies on NASA-developed methods using passive-microwave data from the Defense Meteorological Satellite Program (DMSP) F13 Special Sensor Microwave/Imager (SSM/I). The basis for the Sea Ice Index is the data set, "Near Real-Time DMSP SSM/I Daily Polar Gridded Sea Ice Concentrations," and the NASA-produced "Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data."
The passive-microwave data used for the Sea Ice Index is especially helpful because it can "see" through clouds and deliver data even during frequently cloudy Arctic conditions. Some other satellites cannot penetrate clouds to take data, so the results are sporadic and dependent upon weather conditions. Still other satellites have the ability to see through clouds, but they do not pass over the North Pole every day, making near-real-time monitoring difficult. So, although NSIDC refers to additional satellite data in developing our analysis, we primarily rely on passive-microwave data for Arctic Sea Ice News & Analysis images and content, and for tracking long-term change.
The passive-microwave data records ice in 25 by 25 kilometer (km) areas, which is lower resolution than other satellite data. This means that the ice edge could be off by as much as 25 to 50 km in passive-microwave data compared to higher-resolution satellites. We define ice extent as anywhere with at least 15% ice. Regions with 15% ice will look quite open in higher-resolution satellite data, but will still count as ice in the passive-microwave extent fields.
Other reasons that passive-microwave data may show ice where none actually exists on the ground include signal variation along coastlines between land and water, the shift in albedo of actively melting ice, and atmospheric interference from rain or high winds over the ice-free ocean. In the daily extent data images, gaps (shown in dark gray in the extent map) are replaced with values interpolated from surrounding days, but temporary spurious results may occur.
Despite the limitations in passive-microwave data products, they still yield reasonable estimates for the overall extent pattern and values of the ice. Plus, the limitations are consistent, affecting the data this year in the same way they have affected it in previous years. While passive-microwave data products may not show as much detail or be as accurate "on the ground" as other satellite data, they provide a consistent timeseries to track sea ice extent going back to 1979. This type of long-term, consistent data is important to scientists who study whether or not change is taking place in a system.
To learn more about how scientists study sea ice, see All About Sea Ice: Studying. For more information about the data, contact User Services at nsidc@nsidc.org.