The raw NOAA gridded 89x89 snow cover data set/charts and the RUCL reanalysis gridded data are both utilized in creating a unique Northern Hemisphere snow cover product. Using Perl software created at the RUCL, weekly and monthly 89x89 grid cell charts are generated. In this procedure, weekly areas are calculated from digitized snow files, and monthly values are calculated by weighting the weekly areas according to the number of days of a map week falling in the given month. During this process, the raw NOAA 89x89 grid data, as well as the reanalysis gridded data, are subjected to filtering through the corrected land mask created here at the RUCL. The result is an accurate grid cell product which details Northern Hemisphere snow cover data over the last 36 years. (from RUCL Web Site)

In 1966, NOAA began to map the snow and ice areas in the Northern Hemisphere on a weekly basis. That effort continues today, and remains the only such hemispheric product. NOAA maps are based on a visual interpretation of photographic copies of shortwave imagery by trained meteorologists. Up to 1972, the subpoint resolution of the meteorological satellites commonly used was around 4 km. Beginning in October 1972, the Very High Resolution Radiometer (VHRR) provided imagery with a spatial resolution of 1.0 km, which in November 1978, with the launching of the Advanced VHRR (AVHRR), was reduced slightly to 1.1 km. Maps show boundaries on the last day that the surface in a given region is seen. Since May 1982, dates when a region was last observed have been placed on the maps. An examination of these dates shows the maps to be most representative of the fifth day of the week.

It is recognized that in early years the snow extent was underestimated on the NOAA maps, especially during Fall. Mapping improved considerably in 1972 with the deployment of the VHRR sensor, and since then mapping accuracy is such that this product is considered suitable for continental-scale climate studies.

Despite the shortwave limitations mentioned earlier, the NOAA maps are quite reliable at many times and in many regions. These include regions where, 1) skies are frequently clear, commonly in Spring near the snowline, 2) solar zenith angles are relatively low and illumination is high, 3) the snow cover is reasonably stable or changes slowly, and 4) pronounced local and regional signatures are present owing to the distribution of vegetation, lakes and rivers. Under these conditions, the satellite-derived product will be superior to maps of snow extent gleaned from station data, particularly in mountainous and sparsely inhabited regions. Another advantage of the NOAA snow maps is their portrayal of regionally-representative snow extent, whereas maps based on ground station reports may be biased, due to the preferred position of weather stations in valleys and in places affected by urban heat islands, such as airports.

The NOAA maps are digitized on a weekly basis using the National Meteorological Center Limited-Area Fine Mesh grid. This is an 89 x 89 cell Northern Hemisphere grid, with cell resolution ranging from 16,000 sq. km to 42,000 sq. km. If a cell is interpreted to be at least fifty percent snow covered it is considered to be completely covered, otherwise it is considered to be snowfree.

Dave Robinson (from RUCL website)

Groisman, P. Ya., T.R. Karl and R.W. Knight, 1994: Observed impact of snow cover on the heat balance and the rise of continental spring temperatures. Science, 263, 198-200.

Kukla, G. and D.A. Robinson, 1981: Accuracy of operational snow and ice charts. 1981 IEEE Interna. Geosci. Remote Sensing Symp. Digest, 974-987.

Matson, M., C.F. Ropelewski and M.S. Varnadore, 1986: An Atlas of Satellite-Derived Northern Hemisphere Snow Cover Frequency. NOAA Atlas, 75pp.

Walsh, J.E., and B. Ross, 1988: Sensitivity of 30-day dynamical forecasts to snow-cover. J. Climate, 1, 739-754.

Robinson, D.A., 1993: Monitoring Northern Hemisphere snow cover. Snow Watch ‘92: Detection Strategies for Snow and Ice. Glaciological Data Report, GD-25, 1-25.

Robinson, D.A., K.F. Dewey and R.R. Heim, Jr., 1993: Global snow cover monitoring: an update. Bull. Am. Met. Soc., 74, 1689-1696.

Robinson, D.A. & A. Frei (2000) Seasonal variability of northern hemisphere snow extent using visible satellite data. Professional Geographer, 51, 307-314.

Updated: 1/10/03
Maintained by asphilli@ucar.edu