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
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the Antarctic and Greenland ice sheets, as well as
on Arcitc and alpine icefields and glaciers.
About 98% of the seasonal snowcover occurs in
the Northern Hemisphere. It ranges from a
maximum of about 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.
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Mean maximum snow depth in Canada is > 160
cm in parts of Newfoundland and Labrador, with
even higher values (> 320 cm) in B.C.’s Coastal,
Columbia, & Rocky Mountains.
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; in other words,
the snowpack is intermittent.
At high latitudes a long period of winter
snowcover is virtually assured (exceeding 182
days in continental areas north of 60oN), but at
the onset and end of winter, and even into
summer, snowcover may form briefly before
melting.
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This indeterminate period occurs throughout the
winter in milder climates having small seasonal
accumulations.
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Regional Snowcover Distribution
Climate, physiography and vegetation interact in
a complex manner to govern snowcover
accumulation and distribution.
Simple zonation by vegetation is often useful for
interpreting snowcover maps and data.
This procedure is often effective because the type
of vegetation is frequently indicative of climate.
Tundra
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.
The actual mean dates of occurrence and
disappearance are difficult to define and measure
and are highly variable from year to year.
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Generally, snowcover forms in late September or
early October and disappears by June while the
maximum accumulation occurs in February,
March or April.
Although the snowfall distribution throughout the
tundra zone may be considered as being
regionally uniform, the snow is quickly
redistributed by wind (“drifting and blowing
snow”).
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 valleys and along the edges of airflow
obstacles, such as shrubs or rocks.
The average density of snowcover in this zone is
usually taken to be 300 kg m-3 (or greater) over
most of the season.
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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.
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.
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In Canada the mean maximum 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.
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In Russia, a permanent or semi-permanent
snowcover does not form until December in the
Urals, whereas in the southern Great Plains of
North America, the cover becomes permanent in
December or January.
Estimates of the date of formation and the
probability of permanent snowcover are quite
unreliable in the southern extremity of this zone.
These characteristics are largely dependent on air
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.
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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 co-existence of 40 km
h-1 winds, an air temperature equal to or lower
than -12oC (or high windchills of 1600 W m-2)
and visibilities < 1 km which last for a duration
of several hours. Such conditions can lead to
massive, hard drifts.
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Mixed forest
The most extensive mixed forest zones are in
eastern North America, south of 35oN; along the
Pacific Coast; and in regions of Europe west of
10oE 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 March.
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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.
One extreme event occurred in 1966 which saw
an accumulation of 257 cm of snow over a 5-day
period in Oswego, New York
The snow belts which occur to the south and east
of each of the Great Lakes can be largely
attributed to lake effects.
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Mountain areas
Snow exists on high mountain tops every month
of the year, at elevations which vary with latitude
and climate.
The snowcover on rugged terrain above the tree
line is highly heterogeneous because of its
exposure to avalanches 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.
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