The extent of snow and ice that covers the earth's surface in the northern hemisphere grows and shrinks with the seasons. This animations shows the average snow and ice cover for a given month over a 24-year period, 1979 - 2002. It shows how often a particular point is covered with snow in a given month. The SVS Image Server gives each particular image in the animation the last date for which the data was used in creating that image, even though each of the images covers a span of years for a particular month.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 18.104.22.168.0