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Northern Hemisphere
Visible Satellite Charts
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Area of Snow Extent

Data Preparation

The 1° x 1° degree interpolated station data were produced using daily surface observations from the U.S. National Weather Service (NWS) cooperative station network (available from the National Climatic Data Center's TD3200 dataset) and Canadian daily surface observations (from R. Brown, Meteorological Service of Canada). The data were reformatted if necessary into TD3200 format so that all data were in a consistent format for further processing. Station lists were created with basic metadata for each site, including latitude and longitude, elevation, station name, and the state or province in which the station was located. The creation of this dataset was a combined effort of the University of Georgia Department of Geography (Dr. Thomas Mote) and the Rutgers University Global Snow Lab (Dr. David A. Robinson).

The National Climatic Data Center (NCDC) performs quality control procedures on all cooperative data, and the TD3200 format data gives two flag codes for each data entry regarding the accuracy and source of the information. The quality control procedures that the Canadian data were subject to were different from the US data. The data were strictly examined using criteria set forth by D. Robinson (Physical Geography, 10, 120-130, 1989). This was accomplished by adding an additional quality control flag to the data.

Once the quality control was complete, initial 0.25° latitude x 0.25° longitude grids were created using the station data. This task was accomplished by means of the Spheremap spatial interpolation program, developed at the Center for Climatic Research, Department of Geography, University of Delaware. This software was used because of its ability to create a 2-D grid in Cartesian space through input of an irregularly spaced point system where distances and station locations are not necessarily preserved. Spheremap uses a modified version of Shepard's (D. Shepard, Proceedings - 1968 ACM National Conference) procedure. The routine computes the distance and directional relationships among the input data points in spherical space, then uses them to interpolate from irregularly spaced data to the nodes of a spherical lattice. Within the program, the search radius changes according to the minimum and maximum number of data points allowed for the interpolation. For the dataset described here, a minimum of 5 points and a maximum of 25 points were set as the boundaries.

Upon completion of the 0.25° x 0.25° grids, the final 1° x 1° grids were prepared. This was done by utilizing the 16 individual 0.25° grid cells that comprise a 1° x 1° grid cell. In order to minimize file size and irrelevant data, a masking routine was developed and applied that effectively removed all 0.25° grid cells that did not directly fall over a land surface.

Pentad Snow Charts

Five-day snow charts were generated by examining the snow depth in each 1° x 1° cell for each day. A pentad cell was considered snow covered if three or more days in the pentad had a snow depth of more than 1.27 cm (one half inch). For leap years one day was added to pentad number 12 and the same rules were applied.