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GLOBAL SNOWCOVER DISTRIBUTION
Once snow reaches the ground, it may accumulate into a
snowpack. The large-scale spatial extent of a snowpack is
commonly referred to as the snowcover.
Snowcover commonly occurs at higher latitudes and
altitudes since near or below freezing temperatures affect
both the frequency of occurrence of snowfall and the
probability of snowmelt; thus its duration is longest near
the poles and on high mountains.
Antarctica is a dominant feature since it covers an area of
approximately 14 million km2, making it a major factor in
the thermal and moisture balance of the earth.
Snowcover can be classified as perennial or seasonal. A
perennial snowcover is found over the Antarctic and
Greenland ice sheets, as well as on icefields and glaciers.
About 98% of the seasonal snowcover occurs in the
Northern Hemisphere. It ranges from a maximum of about
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45 million km2 in winter to a minimum of 3 million km2 in
summer.
The coefficient of variation in snowcover extent remains
nearly constant at 0.05 during winter but rapidly increases
during spring and summer to peak at 0.8 in all regions of
interest.
Generally the depths of snowcover in continental interiors
north of 50oN and in polar areas range between 40 to 80
cm. However, this is not universal since snowcover
depends on topography and geographical location, e.g., in
mountainous regions of Norway and Southern Alaska
average depths exceed 120 and 180 cm, respectively.
In addition to depth and density, the dates (time) of
occurrence and disappearance and the duration of
snowcover are important factors for human activities,
particularly in the Northern Hemisphere.
At most locations snowcover may form and disappear
several times a season. At high latitudes a long period of
winter snowcover is virtually assured (exceeding 182 days
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in continental areas north of 60oN), but at the onset and end
of winter, and even into summer, snowcover may form
briefly before melting. This indeterminate period occurs
throughout the winter in milder climates having small
seasonal accumulations.
REGIONAL SNOWCOVER DISTRIBUTION
Climate, physiography and vegetation interact in a complex
manner to govern snowcover accumulation and
distribution.
This procedure is often effective because the type of
vegetation is frequently indicative of climate. Simple
zonation by vegetation is often useful for interpreting
snowcover maps and data. Figure 5.6 presents the major
vegetation zones of the Northern Hemisphere.
Tundra
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In the tundra zone, with the exception of permanent snow
fields, the average period of snowcover on land surfaces
ranges from eight months to over ten months in high
latitude regions of Greenland and Ellesmere Island (see
Figs. 5.7 and 5.8). The actual mean dates of occurrence and
disappearance are difficult to define and measure and are
highly variable from year to year. Generally, snowcover
forms in late September or early October and disappears by
June while the maximum accumulation occurs in February,
March or April (see Fig. 5.9).
In Canada, the mean maximum depths of snowcover range
from 30 cm on the arid areas to 300 cm on mountain slopes
near open seas (see Fig. 5.10). Although the snowfall
distribution throughout the tundra zone may be considered
as being regionally uniform, the snow is quickly
redistributed by wind. Scour and sedimentation of graupel
and ice needles produces a cover which may be highly
variable in density and depth with numerous exposed areas,
drifts, dunes and sastrugi. The eroded snow accumulates
along the upwind sides of vaJleys and along the edges of
airflow obstacles, such as shrubs or rocks.
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The average density of snowcover in this zone is usually
taken to be 300 kg m$^{-3}$ (or greater) over most of the
season. The cold dry snow forms a finely structured surface
having a high bearing capacity. Major changes (increases)
in density do not occur until the time of active melt in late
Mayor June (see Fig. 5.11).
Taiga and Boreal forest
During October, the area of snow cover moves rapidly
southward to include most of the Taiga and Boreal Forest
except those parts influenced by a maritime climate. For
example, in Eurasia the snowcover is widespread
throughout Siberia in October, but does not extend to the
Baltic until December. In North America, the average
duration of snowcover in these zones is 200 to 240 days in
the far north and 120 days at their southern limits with the
maximum depths occurring in February and March.
However, the dates of occurrence of maximum depths
throughout the region vary widely from season to season
and between clearings and forests. Since most weather
stations are located in cleared areas, their reports probably
provide early estimates. In Canada the mean maximum
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depths of accumulation range from 50 to 150 cm depending
on the land form and proximity to the open sea.
The snowcover within the evergreen forest differs from that
in the tundra and grasslands. Forest cover intercepts falling
snow, serves as a wind break and shelters the snowcover
from solar radiation, thereby extending its duration, and
resulting in less compacted formations. The average
densities of snowcover in these regions are much lower
than that in tundra, viz. 170 to 210 kg m$^{-3}$ during
most of the non-melt period.
Grassland and Steppes
Snowcover usually forms on the colder, continental
grassland plains and steppes in November. In the Soviet
Union cast of the Urals a permanent or semi-permanent
cover does not form until December, whereas in the
southern Great Plains of North America, the cover becomes
permanent in December or January. Estimates of the date of
formationand the probability of permanent snowcover are
quite unreliable in the southern extremity of this zone.
These characteristics are largely dependent on air
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temperature. Over the cold northern grasslands snowcover
persists for 120 to 160 days. In Oklahoma and the Southern
Ukraine it lasts from 30 to 60 days, but over central Texas
only for a few days.
The grassland snowcover is fairly shallow and well-drifted,
but nevertheless is rather uniform spatially, being broken
by local variations in topography and vegetation. Shelter
belts and buildings may cause massive drifts whereas
adjacent fields may be relatively free of snow. Also,
depressions and eroded areas fill with well-packed snow.
The mean annual accumulated depths of snowcover in this
zone mainly fall in the range from 20 to 50 cm.
The density of the prairie snowcover is ~ 200 kg m$^{-3}$
throughout most of the winter increasing slowly with time
because of metamorphic changes up to the time of active
melt (about March 15) when the changes are marked as
melt progresses.
A climatological feature of particular interest in both the
grasslands and tundra zones is the blizzard. In Canada, for a
storm to be categorized as a blizzard requires the co7
existence of 40 km h$^{-1}$ winds, an air temperature
equal to or lower than -12$^{\circ}$C and visibilities less
than 0.8 km which last for a duration of at least six hours.
Such conditions can lead to massive, hard drifts.
Mixed forest
The most extensive mixed forest zones are in eastern North
America, south of 35$^{\circ}$N; along the Pacific Coast;
and in regions of Europe west of 10$^{\circ}$E with low
elevations. Snowcover usually forms in late November and
December and recedes in two directions; from the south in
mid-February, and from the north in late March or early
April. Predictions of snowcover duration are unreliable
since the cover does not remain on the ground for long
periods.
In this zone, because of its mild climate and sheltering
effect against wind action, the snowcover tends to be moist,
except at high elevations and during periods when cold
outbreaks follow snow storms. The average snowcover
density is ~ 200 kg m$^{-3}$ in mid-winter increasing
progressively over the winter to ~ 300 kg m$^{-3}$ by late
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March. Freeze-thaw cycling of air temperature, freezing
rain and drizzle are common and produce hard crusts, ice
layers and crystalline changes within the snowcover.
One feature of the Great Lakes Forest climate is the ``lake
effect’’ storm which deposits massive amounts of snow
over fairly small areas downwind of a lake. For example,
the areas near Buffalo and Oswego, N.Y. are seriously
affected by such storms which originate from Lake Ontario
and are also marked by gale force winds and blowing snow
(see Fig. 5.12 for a typical deposition pattern). According
to Sykes (1965) 257 cm of snow which accumulated over a
five-day period in 1966 at Oswego was due to lake effect.
The snow belts which occur to the south and east of each of
the Great Lakes can be largely attributed to lake effects.
Mountain areas
Snow exists on high mountain tops every month of the
year, at elevations which vary with latitude and climate.
Table 5.9 (Geiger, 1961) shows the increase of snowcover
duration with altitude in the Swiss Alps. The snowcover on
rugged terrain above the tree line is highly heterogeneous
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because of its exposure to slides and wind action. At the
higher elevations tundra conditions prevail and the
snowcover undergoes severe erosion and wind packing
which may result in the formation of slabs. With decreasing
elevation the type of forest changes gradually from
coniferous to deciduous in phase with the climate. The
snow cover pattern in forested areas is governed by the
species of trees, stand density, land features and topography
(e.g., steppes).
Other zones
The other major zones where snowcover occurs include the
mid-latitude deserts, the Mediterranean area and the
warmer climatic regions of North America and Asia. In
these, snow cover does not normally present major
problems to man's activities because it is shallow and has a
short duration and low frequency of occurrence. The
characteristics of these snowcovers can generally be
determined by analogy with the snowcover in adjacent
areas although allowances must be made for the
continentality of the climate.
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