B. Geerts and E. Linacre
Real-time snow cover data are now available for North America and the northern hemisphere. These weekly snow cover images are based largely on satellite imagery, both visible and microwave. Weekly satellite-based snow cover charts for the northern hemisphere have been produced since 1972 (1). The long-term average snow surface area is 25.3 million km2, i.e. 14.7 over Eurasia and 10.6 over North America and Greenland. The snow cover over the northern land areas is about 10 times larger in February than in August. The standard deviation of monthly values was 1.0 in Augusts and 2.9 in Octobers. The mean snow cover during 1972-85 was 25.9 106 km2, and then there was a reduction to an average of 24.2 106 km2 during 1988-98. The reduction is most remarkable in spring and summer. The extent of snow cover appears to be inversely related to hemispheric surface air temperature (2).
(1) Robinson, D.A. 1999. Northern hemisphere snow extent during the satellite era. Preprints to the 11th Conf. on Appl. Climatol., Dallas, Texas (Amer. Meteor. Soc.), 236-41.
(2) Robinson, D.A., K.F. Dewey and R.R. Heim 1993. Global snow cover monitoring: an update. Bull. Amer. Meteor. Soc., 74, 1689-96.