Snow
Summary
Snow exerts a huge influence on climate, through its
high reflectivity, insulating properties, and cooling of
the atmosphere, and on surface hydrology, through its
effects on water resources in many parts of the world.
Mean monthly snow-cover extent in the Northern Hemi-
sphere has decreased at a rate of 1.3 per cent per dec- Annual maximum
ade during the last 40 years, with greatest losses in the
snow-cover extent in
both hemispheres
spring and summer months. Climate models project
significant decreases in snow cover by the end of this
Snow: frozen precipitation in the form of ice crystals that fall
in soft, white flakes.
century, with reductions of 60 to 80 per cent in snow wa-
ter equivalent (depth of water resulting from snow melt)
Snow on land (Northern Hemisphere) (annual minimum ~
in most mid-latitude regions. Increases are projected for
maximum):
the Canadian Arctic and Siberia. Higher temperatures
Area Covered (million square km) 1.9 ~ 45.2
and rises in snow line are projected for many mountain Potential Sea Level Rise (cm) 0.1 ~ 1
regions. Changes in snow cover, such as the formation
Source: IPCC 2007
1a
of ice layers in snow due to increased frequency of snow
thaw, have widespread impacts as snow is an important
ecological factor. Snow-cover changes also have impacts the surface characteristic responsible for the largest an-
on human well-being and economic activities, including nual and interannual differences in surface reflectivity
water resources, agriculture, animal husbandry, trans- (albedo) in the Northern Hemisphere (Figure 4.1). In
portation and winter recreation such as skiing. the Southern Hemisphere, excluding the 14.5 million
km
2
area of Antarctica, terrestrial snow covers a much
smaller area, mostly in the Andes and Patagonia, and it
Introduction to snow plays a smaller role in global climate. Limited summer
melt occurs in the Antarctic Peninsula and on the coasts
Snow occurs predominantly on the northern continents, of western Antarctica.
on the sea ice of the Arctic Ocean and on Antarctica.
On the Northern Hemisphere continents, snow covers Snow is an important climate variable. Due to its high
a maximum mean area of 45.2 million km
2
, typically in albedo, snow cover increases the amount of sunlight re-
January. The minimum snow-cover extent usually oc- flected from Earth’s surface. The low thermal conduc-
curs in August and covers a mean area of 1.9 million tivity of snow insulates the ground, and its cold, moist
km
2
, most of which is snow on the Greenland ice sheet surface affects the transfer of heat and moisture to and
and on mountain glaciers. As a result, snow cover is from the atmosphere. Thus, snow cover exerts a signifi-
40 GLOBAL OUTLOOK FOR ICE AND SNOW
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