The annual average areal extent of snow cover in the Northern Hemisphere is approximately 25x106 km2, with a minimum of 3.6x106 km2 in August and a maximum of 45.6x106 km2 in January. Individual monthly departures from normal of 3.0x106 km2 are not uncommon, and areal anomalies greater than 5.0x106 km2 have occasionally been observed in the 23 years of consistently observed satellite estimates of hemispheric snow cover (Fig. 1.20). The late 1980s to the present represents the longest interval of below-normal snow cover extent since the beginning of this record in 1972. However, longer term station-based snow cover observations suggest that the late 1970s may have been the snowiest period since the turn of the century (Robinson 1992).
For the eight-year period ending December 1994, hemispheric areal snow cover was above normal in only thirteen months. For the 16-month period ending in December 1994 hemispheric areal snow cover was above normal in only 5 months.
During the past decade the December - February (DJF) areal snow cover exhibited relatively little interannual variability (Fig. 1.21a) compared to the transition seasons (Figs. 1.21b, c). While the total hemispheric areal snow cover for DJF 1993/94 was only slightly below normal (Fig. 1.21a), above-normal areal snow cover was observed over the northern Plains of the United States, large portions of Manchuria, and east of the Caspian Sea (Fig. 1.22). These areas also experienced colder-than-normal surface temperatures during the winter season (see section 3). In contrast, below-normal areal snow cover extent was observed in the western United States, northern Europe, and portions of the Tibetan Plateau, areas where above-normal winter surface temperatures were observed.
Total Northern Hemisphere areal snow cover during March-May (MAM) was approximately 1.5x106 km2 below normal, making this the seventh consecutive MAM with below-average areal snow cover (Fig. 1.21b). The reduced extent of snow cover was most prevalent over the northwestern United States, western Canada, and Alaska, as well as large portions of northern Europe, northern Scandinavia, and southern Russia (Fig 1.23). Only isolated areas experienced a greater-than-normal frequency of snow cover during the season, i.e., portions of western Russia and portions of the eastern Tibetan Plateau.
Large interannual variability of total Northern Hemisphere snow cover area has been observed during September-November (SON) for the entire satellite-based record (Fig. 1.21c). During SON 1994, total snow coverage was approximately 17.7x106 km2, 1.1x106 km2 below normal, and nearly 3x106 km2 less coverage than observed in 1993. A reduced areal extent of snow cover was observed over much of eastern Canada, central Alaska, and large portions of central and western Siberia (Fig. 1.24). Above-normal snow cover areal extent was confined to the western United States during the season, and to scattered portions of southern Russia and western China.
Climate and Global Change Issues - Cryosphere: Sea Ice
Climate and Global Change Issues - Trace Gases: Methane
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