Daily snow charts were generated by degrading 1024 x 1024 NOAA Interactive Multisensor Snow and Ice Mapping System (IMS) charts to 128 x 128 resolution. Each 128 x 128 grid cell is considered snow covered if 42 percent or more of the IMS land cells within it are snow covered. Using charts produced on Mondays, this value most closely approximates the weekly snow product based on mean SCE area comparisons. An 88 x 88 cell subset is then charted, representing the northern hemisphere.
Climatology percentages for each cell are generated for each day using weekly satellite data from September 1970 - August 2000. Each day falls within the dates of a GSL weekly snow chart and is represented by this value. The percentage of days indicated as snow covered for a given cell represents the number of years the cell was snow covered on the week representing that day. For departure maps, cells that are 50 percent or more snow covered in the climatology are considered snow covered in the calculation of daily departures.
Daily products span from the first operational IMS chart (November 1, 1998) to present and are updated daily by 01:00 UTC. For questions regarding data manipulation or charting please use the online contact form.