Time series of winter days with =7.5 cm of snow cover at four stations on the U.S. Great Plains. Also shown smoothed with a nine-point binomial filter, with only those points plotted where nine years are available for filtering (i.e., plotted year ±4 yr).

covered regions, which is currently not well understood, must also be taken into account when estimating snow cover. Because of the region-specific differences in land cover and snowpack properties, no single algorithm can adequately estimate snow cover across Northern Hemisphere lands.

Hemispheric Snow Products: NOAA Weekly Charts

In 1966, NOAA began to map snow cover over Northern Hemisphere lands on a weekly basis (Matson et al., 1986). That effort continues today, and remains the only such hemispheric product. NOAA charts are based on a visual interpretation of photographic copies of short-wave imagery by trained meteorologists. Up to 1972, the sub-point resolution of the meteorological satellites commonly used was around 4 km; since then it has become, and remained, close to 1.0 km. Charts show boundaries on the last day that the surface in a given region was seen. Since May 1982, dates when a region was last observed have been placed on the charts. An examination of these dates shows the charts 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 charts, especially during fall. Charting improved considerably in 1972 with the improvement of sensor resolution, and since then charting accuracy

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