Snow refers to forms of ice crystals that precipitate from the
atmosphere (usually from clouds) and undergo changes on the Earth's
surface. It pertains to frozen crystalline water throughout its
life cycle, starting when, under suitable conditions, the ice crystals
form in the atmosphere, increase to millimeter size, precipitate and
accumulate on surfaces, then metamorphose in place, and ultimately
melt, slide or sublimate away.
Snowstorms organize and develop by
feeding on sources of atmospheric moisture and cold air. Snowflakes
nucleate around particles in the atmosphere by attracting supercooled
water droplets, which freeze in hexagonal-shaped crystals. Snowflakes
take on a variety of shapes, basic among these are platelets, needles,
columns and rime. As snow accumulates into a snowpack, it may blow
into drifts. Over time, accumulated snow metamorphoses, by sintering,
sublimation and freeze-thaw. Where the climate is cold enough for
year-to-year accumulation, a glacier may form. Otherwise, snow
typically melts seasonally, causing runoff into streams and rivers and
Major snow-prone areas include the polar regions, the upper half of
Northern Hemisphere and mountainous regions worldwide with
sufficient moisture and cold temperatures. In the Southern Hemisphere,
snow is confined primarily to mountainous areas, apart from
Snow affects such human activities as transportation: creating the
need for keeping roadways, wings, and windows clear; agriculture:
providing water to crops and safeguarding livestock; sports such as
skiing, snowboarding, and snowmachine travel; and warfare. Snow
affects ecosystems, as well, by providing an insulating layer during
winter under which plants and animals are able to survive the cold.
1.1.1 Low pressure areas
Lake and ocean effects
1.3 Classification of snowflakes
3.1 Seasonal snowpack
5.1 Measurement and classification
5.2 Satellite data
6 Effects on human activity
Snow roads and runways
6.3.2 Utility lines
6.4 Sports and recreation
7 Effects on ecosystems
7.1 Plant life
7.2 Animal life
8 Extraterrestrial snow
9 See also
11 External links
Worldwide occurrence of snowfall.
Snow at reference above sea level
Below 500: annually.
Below 500: annually, but not in all of its territory.
500: above annually, below occasionally.
Above 500: annually.
Above 2,000: annually.
Any elevation: none.
Snow develops in clouds that themselves are part of a larger weather
system. The physics of snow crystal development in clouds results from
a complex set of variables that include moisture content and
temperatures. The resulting shapes of the falling and fallen crystals
can be classified into a number of basic shapes and combinations,
thereof. Occasionally, some plate-like, dendritic and stellar-shaped
snowflakes can form under clear sky with a very cold temperature
Snow clouds usually occur in the context of larger weather systems,
the most important of which is the low pressure area, which typically
incorporate warm and cold fronts as part of their circulation. Two
additional and locally productive sources of snow are lake-effect
(also sea-effect) storms and elevation effects, especially in
Low pressure areas
Main article: Extratropical cyclone
Extratropical cyclonic snowstorm, February 24, 2007
Mid-latitude cyclones are low pressure areas which are capable of
producing anything from cloudiness and mild snow storms to heavy
blizzards. During a hemisphere's fall, winter, and spring, the
atmosphere over continents can be cold enough through the depth of the
troposphere to cause snowfall. In the Northern Hemisphere, the
northern side of the low pressure area produces the most snow. For
the southern mid-latitudes, the side of a cyclone that produces the
most snow is the southern side.
Frontal snowsquall moving toward Boston, Massachusetts
A cold front, the leading edge of a cooler mass of air, can produce
frontal snowsqualls—an intense frontal convective line (similar to a
rainband), when temperature is near freezing at the surface. The
strong convection that develops has enough moisture to produce
whiteout conditions at places which line passes over as the wind
causes intense blowing snow. This type of snowsquall generally
lasts less than 30 minutes at any point along its path but the motion
of the line can cover large distances. Frontal squalls may form a
short distance ahead of the surface cold front or behind the cold
front where there may be a deepening low pressure system or a series
of trough lines which act similar to a traditional cold frontal
passage. In situations where squalls develop post-frontally it is not
unusual to have two or three linear squall bands pass in rapid
succession only separated by 25 miles (40 kilometers) with each
passing the same point in roughly 30 minutes apart. In cases where
there is a large amount of vertical growth and mixing the squall may
develop embedded cumulonimbus clouds resulting in lightning and
thunder which is dubbed thundersnow.
A warm front can produce snow for a period, as warm, moist air
overrides below-freezing air and creates precipitation at the
boundary. Often, snow transitions to rain in the warm sector behind
Lake and ocean effects
Main article: Lake-effect snow
Cold northwesterly wind over
Lake Superior and
Lake Michigan creating
Lake-effect snow is produced during cooler atmospheric conditions when
a cold air mass moves across long expanses of warmer lake water,
warming the lower layer of air which picks up water vapor from the
lake, rises up through the colder air above, freezes and is deposited
on the leeward (downwind) shores.
The same effect also occurs over bodies of salt water, when it is
termed ocean-effect or bay-effect snow. The effect is enhanced when
the moving air mass is uplifted by the orographic influence of higher
elevations on the downwind shores. This uplifting can produce narrow
but very intense bands of precipitation, which deposit at a rate of
many inches of snow each hour, often resulting in a large amount of
The areas affected by lake-effect snow are called snowbelts. These
include areas east of the Great Lakes, the west coasts of northern
Kamchatka Peninsula in Russia, and areas near the Great
Salt Lake, Black Sea, Caspian Sea, Baltic Sea, and parts of the
northern Atlantic Ocean.
Precipitation types § Orographic
Orographic or relief snowfall is caused when masses of air pushed by
wind are forced up the side of elevated land formations, such as large
mountains. The lifting of air up the side of a mountain or range
results in adiabatic cooling, and ultimately condensation and
precipitation. Moisture is removed by orographic lift, leaving drier,
warmer air on the descending, leeward side. The resulting enhanced
productivity of snow fall and the decrease in temperature with
elevation means that snow depth and seasonal persistence of
snowpack increases with elevation in snow-prone areas.
Main article: Snowflake
Freshly fallen snowflakes
A snowflake consists of roughly 1019 water molecules, which are added
to its core at different rates and in different patterns, depending on
the changing temperature and humidity within the atmosphere that the
snowflake falls through on its way to the ground. As a result,
snowflakes vary among themselves, while following similar
Snow crystals form when tiny supercooled cloud droplets (about
10 μm in diameter) freeze. These droplets are able to remain
liquid at temperatures lower than −18 °C (0 °F), because
to freeze, a few molecules in the droplet need to get together by
chance to form an arrangement similar to that in an ice lattice. Then
the droplet freezes around this "nucleus". In warmer clouds an aerosol
particle or "ice nucleus" must be present in (or in contact with) the
droplet to act as a nucleus.
Ice nuclei are very rare compared to that
cloud condensation nuclei on which liquid droplets form. Clays, desert
dust and biological particles can be nuclei. Artificial nuclei
include particles of silver iodide and dry ice, and these are used to
stimulate precipitation in cloud seeding.
Once a droplet has frozen, it grows in the supersaturated
environment—one where air is saturated with respect to ice when the
temperature is below the freezing point. The droplet then grows by
diffusion of water molecules in the air (vapor) onto the ice crystal
surface where they are collected. Because water droplets are so much
more numerous than the ice crystals due to their sheer abundance, the
crystals are able to grow to hundreds of micrometers or millimeters in
size at the expense of the water droplets by the
Wegener–Bergeron–Findeisen process. The corresponding depletion of
water vapor causes the ice crystals to grow at the droplets' expense.
These large crystals are an efficient source of precipitation, since
they fall through the atmosphere due to their mass, and may collide
and stick together in clusters, or aggregates. These aggregates are
snowflakes, and are usually the type of ice particle that falls to the
ground. Although the ice is clear, scattering of light by the
crystal facets and hollows/imperfections mean that the crystals often
appear white in color due to diffuse reflection of the whole spectrum
of light by the small ice particles.
Classification of snowflakes
Snowflake § Classification
An early classification of snowflakes by Israel Perkins Warren
Micrography of thousands of snowflakes from 1885 onward, starting with
Wilson Alwyn Bentley, revealed the wide diversity of snowflakes within
a classifiable set of patterns. Closely matching snow crystals
have been observed.
Nakaya developed a crystal morphology diagram, relating crystal shapes
to the temperature and moisture conditions under which they formed,
which is summarized in the following table.
Crystal structure morphology as a function of temperature and water
Types of snow crystal
0 to −3.5
32 to 26
0.0 to 0.5
0.000 to 0.013
−3.5 to −10
26 to 14
0.5 to 1.2
0.013 to 0.032
−10 to −22
14 to −8
1.2 to 1.4
0.032 to 0.038
−22 to −40
−8 to −40
1.2 to 0.1
0.0324 to 0.0027
As Nakaya discovered, shape is also a function of whether the
prevalent moisture is above or below saturation. Forms below the
saturation line trend more towards solid and compact. Crystals formed
in supersaturated air trend more towards lacy, delicate and ornate.
Many more complex growth patterns also form such as side-planes,
bullet-rosettes and also planar types depending on the conditions and
ice nuclei. If a crystal has started forming in a column
growth regime, at around −5 °C (23 °F), and then falls
into the warmer plate-like regime, then plate or dendritic crystals
sprout at the end of the column, producing so called "capped
Magono and Lee devised a classification of freshly formed snow
crystals that includes 80 distinct shapes. They documented each with
An animation of seasonal snow changes, based on satellite imagery
Snow accumulates from a series of snow events, punctuated by freezing
and thawing, over areas that are cold enough to retain snow seasonally
or perennially. Major snow-prone areas include the arctic and
Antarctic, the Northern Hemisphere, and alpine regions. The liquid
equivalent of snowfall may be evaluated using a snow gauge or with
a standard rain gauge, adjusted for winter by removal of a funnel and
inner cylinder. Both types of gauges melt the accumulated snow and
report the amount of water collected. At some automatic weather
stations an ultrasonic snow depth sensor may be used to augment the
Snow flurry, snow storm and blizzard describe snow events of
progressively greater duration and intensity. A blizzard is a
weather condition involving snow and has varying definitions in
different parts of the world. In the United States, a blizzard occurs
when two conditions are met for a period of three hours or more: A
sustained wind or frequent gusts to 35 miles per hour (56 km/h),
and sufficient snow in the air to reduce visibility to less than 0.4
kilometers (0.25 mi). In
Canada and the United Kingdom, the
criteria are similar. While heavy snowfall often occurs during
blizzard conditions, falling snow is not a requirement, as blowing
snow can create a ground blizzard.
Snowstorm intensity may be categorized by visibility and depth of
accumulation. Snowfall's intensity is determined by visibility, as
Light: visibility greater than 1 kilometer (0.6 mi)
Moderate: visibility restrictions between 0.5 and 1 kilometer (0.3 and
Heavy: visibility is less than 0.5 kilometers (0.3 mi)
The International Classification for Seasonal
Snow on the Ground
defines "height of new snow" as the depth of freshly fallen snow, in
centimeters as measured with a ruler, that accumulated on a snowboard
during an observation period of 24 hours, or other observation
interval. After the measurement, the snow is cleared from the board
and the board is placed flush with the snow surface to provide an
accurate measurement at the end of the next interval. Melting,
compacting, blowing and drifting contribute to the difficulty of
Glaciers with their permanent snowpacks cover about 10% of the earth's
surface, while seasonal snow covers about nine percent, mostly in
the Northern Hemisphere, where seasonal snow covers about
40 million square kilometres (15×10^6 sq mi),
according to a 1987 estimate. A 2007 estimate of snow cover over
Northern Hemisphere suggested that, on average, snow cover ranges
from a minimum extent of 2 million square kilometres
(0.77×10^6 sq mi) each August to a maximum extent of
45 million square kilometres (17×10^6 sq mi) each
January or nearly half of the land surface in that hemisphere.
A study of
Northern Hemisphere snow cover extent for the period
1972–2006 suggests a reduction of 0.5 million square kilometres
(0.19×10^6 sq mi) over the 35-year period.
The following are world records, regarding snowfall and snowflakes:
Highest seasonal total snowfall – The world record for the highest
seasonal total snowfall was measured in the
United States at Mt. Baker
Ski Area, outside of the town
Bellingham, Washington during the
1998–1999 season. Mount Baker received 2,896 cm (95.01 ft)
of snow, thus surpassing the previous record holder, Mount
Rainier, Washington, which during the 1971–1972 season received
2,850 cm (93.5 ft) of snow.
Highest seasonal average annual snowfall – The world record for the
highest average annual snowfall is 1,764 cm (57.87 ft),
measured in Sukayu Onsen, Japan for the period of 1981–2010.
Largest snowflakes –
Guinness World Records
Guinness World Records list the world's largest
snowflakes as those of January 1887 at Fort Keogh, Montana; allegedly
one measured 38 cm (15 in) wide.
Fresh snow beginning to metamorphose: The surface shows wind packing
and sastrugi. In the foreground are hoar frost crystals, formed by
refrozen water vapor emerging to the cold surface.
After deposition, snow progresses on one of two paths that determine
its fate, either ablation (mostly by melting) or transitioning from
firn (multi-year snow) into glacier ice. During this transition, snow
"is a highly porous, sintered material made up of a continuous ice
structure and a continuously connected pore space, forming together
the snow microstructure". Almost always near its melting temperature,
a snowpack is continually transforming these properties in a process,
known as metamorphism, wherein all three phases of water may coexist,
including liquid water partially filling the pore space. Starting
as a powdery deposition, snow becomes more granular when it begins to
compact under its own weight, be blown by the wind, sinter particles
together and commence the cycle of melting and refreezing. Water vapor
plays a role as it deposits ice crystals, known as hoar frost, during
cold, still conditions.
Snowpack and Névé
Over the course of time, a snowpack may settle under its own weight
until its density is approximately 30% of water. Increases in density
above this initial compression occur primarily by melting and
refreezing, caused by temperatures above freezing or by direct solar
radiation. In colder climates, snow lies on the ground all winter. By
late spring, snow densities typically reach a maximum of 50% of
Snow that persists into summer evolves into névé,
granular snow, which has been partially melted, refrozen and
Névé has a minimum density of 500 kilograms per cubic
metre (31 lb/cu ft), which is roughly half of the density of
Main article: Firn
Firn—metamorphosed multi-year snow
Firn is snow that has persisted for multiple years and has been
recrystallized into a substance denser than névé, yet less dense and
hard than glacial ice.
Firn resembles caked sugar and is very
resistant to shovelling. Its density generally ranges from 550
kilograms per cubic metre (34 lb/cu ft) to 830 kilograms per
cubic metre (52 lb/cu ft), and it can often be found
underneath the snow that accumulates at the head of a glacier. The
minimum altitude that firn accumulates on a glacier is called the firn
limit, firn line or snowline.
There are four main mechanisms for movement of deposited snow:
drifting of unsintered snow, avalanches of accumulated snow on steep
slopes, snowmelt during thaw conditions, and the movement of glaciers
after snow has persisted for multiple years and metamorphosed into
Snow drifts forming around downwind obstructions
When powdery, snow drifts with the wind from the location where it
originally fell, forming deposits with a depth of several meters
in isolated locations. After attaching to hillsides, blown snow
can evolve into a snow slab, which is an avalanche hazard on steep
Main article: Avalanche
A powder snow avalanche
An avalanche (also called a snowslide or snowslip) is a rapid flow of
snow down a sloping surface. Avalanches are typically triggered in a
starting zone from a mechanical failure in the snowpack (slab
avalanche) when the forces on the snow exceed its strength but
sometimes only with gradually widening (loose snow avalanche). After
initiation, avalanches usually accelerate rapidly and grow in mass and
volume as they entrain more snow. If the avalanche moves fast enough
some of the snow may mix with the air forming a powder snow avalanche,
which is a type of gravity current. They occur in three major
Slab avalanches occur in snow that has been deposited, or redeposited
by wind. They have the characteristic appearance of a block (slab) of
snow cut out from its surroundings by fractures. These account for
most back-country fatalities.
Powder snow avalanches result from a deposition of fresh dry powder
and generate a powder cloud, which overlies a dense avalanche. They
can exceed speeds of 300 kilometres per hour (190 mph), and
masses of 10,000,000 tonnes (9,800,000 long tons; 11,000,000 short
tons); their flows can travel long distances along flat valley bottoms
and even uphill for short distances.
Wet snow avalanches are a low-velocity suspension of snow and water,
with the flow confined to the surface of the pathway. The low
speed of travel is due to the friction between the sliding surface of
the pathway and the water saturated flow. Despite the low speed of
travel (~10 to 40 kilometres per hour (6 to 25 mph)), wet snow
avalanches are capable of generating powerful destructive forces, due
to the large mass, and density.
Snowmelt-induced flooding of the
Red River of the North
Red River of the North in 1997
Many rivers originating in mountainous or high-latitude regions
receive a significant portion of their flow from snowmelt. This often
makes the river's flow highly seasonal resulting in periodic
flooding during the spring months and at least in dry mountainous
regions like the mountain West of the US or most of
Afghanistan, very low flow for the rest of the year. In contrast, if
much of the melt is from glaciated or nearly glaciated areas, the melt
continues through the warm season, with peak flows occurring in mid to
Main article: Glacier
Glaciers form where the accumulation of snow and ice exceeds ablation.
The area in which an alpine glacier forms is called a cirque (corrie
or cwm), a typically armchair-shaped geological feature, which
collects snow and where the snowpack compacts under the weight of
successive layers of accumulating snow, forming névé. Further
crushing of the individual snow crystals and reduction of entrapped
air in the snow turns it into 'glacial ice'. This glacial ice will
fill the cirque until it 'overflows' through a geological weakness or
vacancy, such as the gap between two mountains. When the mass of snow
and ice is sufficiently thick, it begins to move due to a combination
of surface slope, gravity and pressure. On steeper slopes, this can
occur with as little as 15 m (50 ft) of snow-ice.
Scientists study snow at a wide variety of scales that include the
physics of chemical bonds and clouds; the distribution, accumulation,
metamorphosis, and ablation of snowpacks; and the contribution of
snowmelt to river hydraulics and ground hydrology. In doing so, they
employ a variety of instruments to observe and measure the phenomena
studied. Their findings contribute to knowledge applied by engineers,
who adapt vehicles and structures to snow, by agronomists, who address
the availability of snowmelt to agriculture, and those, who design
equipment for sporting activities on snow. Scientists develop and
others employ snow classification systems that describe its physical
properties at scales ranging from the individual crystal to the
aggregated snowpack. A sub-specialty is avalanches, which are of
concern to engineers and outdoors sports people, alike.
Snow science addresses how snow forms, its distribution, and processes
affecting how snowpacks change over time. Scientists improve storm
forecasting, study global snow cover and its effect on climate,
glaciers, and water supplies around the world. The study includes
physical properties of the material as it changes, bulk properties of
in-place snow packs, and the aggregate properties of regions with snow
cover. In doing so, they employ on-the-ground physical measurement
techniques to establish ground truth and remote sensing techniques to
develop understanding of snow-related processes over large areas.
Measurement and classification
In the field snow scientists often excavate a snow pit within which to
make basic measurements and observations. Observations can describe
features caused by wind, water percolation, or snow unloading from
trees.Water percolation into a snowpack can create flow fingers and
ponding or flow along capillary barriers, which can refreeze into
horizontal and vertical solid ice formations within the snowpack.
Among the measurements of the properties of snowpacks that the
International Classification for Seasonal
Snow on the Ground includes
are: snow height, snow water equivalent, snow strength, and extent of
snow cover. Each has a designation with code and detailed description.
The classification extends the prior classifications of Nakaya and his
successors to related types of precipitation and are quoted in the
Snow pit on the surface of a glacier, profiling snow properties where
the snow becomes increasingly dense with depth as it metamorphoses
Frozen precipitation particles, related to snow crystals
Heavily rimed particles, spherical, conical,
hexagonal or irregular in shape
Heavy riming of particles by
accretion of supercooled water droplets
Laminar internal structure, translucent
or milky glazed surface
Growth by accretion of
supercooled water, size: >5 mm
mostly small spheroids
Freezing of raindrops or refreezing of largely melted snow crystals or
Graupel or snow pellets encased in thin ice layer (small hail). Size:
both 5 mm
Irregular deposits or longer cones and
needles pointing into the wind
Accretion of small, supercooled fog droplets frozen in place.
Thin breakable crust forms on snow surface if process continues long
All are formed in cloud, except for rime, which forms on objects
exposed to supercooled moisture.
It also has a more extensive classification of deposited snow than
those that pertain to airborne snow. The categories include both
natural and man-made snow types, descriptions of snow crystals as they
metamorphose and melt, the development of hoar frost in the snow pack
and the formation of ice therein. Each such layer of a snowpack
differs from the adjacent layers by one or more characteristics that
describe its microstructure or density, which together define the snow
type, and other physical properties. Thus, at any one time, the type
and state of the snow forming a layer have to be defined because its
physical and mechanical properties depend on them. Physical properties
include microstructure, grain size and shape, snow density, liquid
water content, and temperature.
Remote sensing of snowpacks with satellites and other platforms
typically includes multi-spectral collection of imagery. Multi-faceted
interpretation of the data obtained allows inferences about what is
observed. The science behind these remote observations has been
verified with ground-truth studies of the actual conditions.
Satellite observations record a decrease in snow-covered areas since
the 1960s, when satellites when satellite observations began. In some
areas, including China, snow cover has increased. In some regions such
as China, a trend of increasing snow cover has been observed from 1978
to 2006. These changes are attributed to global climate change, which
may lead to earlier melting and less aea coverage. However, in some
areas there may be an increase in snow depth because of higher
temperatures for latitudes north of 40°. For the Northern Hemisphere
as a whole the mean monthly snow-cover extent has been decreasing by
1.3% per decade.
The most frequently used methods to map and measure snow extent, snow
depth and snow water equivalent employ multiple inputs on the
visible–infrared spectrum to deduce the presence and properties of
snow. The National
Ice Data Center (NSIDC) uses the
reflectance of visible and infrared radiation to calculate a
normalized difference snow index, which is a ratio of radiation
parameters that can distinguish between clouds and snow. Other
researchers have developed decision trees, employing the available
data to make more accurate assessments. One challenge to this
assessment is where snow cover is patchy, for example during periods
of accumulation or ablation and also in forested areas.
inhibits optical sensing of surface reflectance, which has led to
other methods for estimating ground conditions underneath clouds. For
hydrological models, it is important to have continuous information
about the snow cover. Passive microwave sensors are especially
valuable for temporal and spatial continuity because they can map the
surface beneath clouds and in darkness. When combined with reflective
measurements, passive microwave sensing greatly extends the inferences
possible about the snowpack.
Snowfall and snowmelt are parts of the Earth's water cycle.
Snow science often leads to predictive models that include snow
deposition, snow melt, and snow hydrology—elements of the Earth's
water cycle—which help describe global climate change.
Global climate change
Global climate change models (GCMs) incorporate snow as a factor in
their calculations. Some important aspects of snow cover include its
albedo (reflectivity of incident radiation, including light) and
insulating qualities, which slow the rate of seasonal melting of sea
ice. As of 2011, the melt phase of GCM snow models were thought to
perform poorly in regions with complex factors that regulate snow
melt, such as vegetation cover and terrain. These models typically
derive snow water equivalent (SWE) in some manner from satellite
observations of snow cover. The International Classification for
Snow on the Ground defines SWE as "the depth of water that
would result if the mass of snow melted completely".
Given the importance of snowmelt to agriculture, hydrological runoff
models that include snow in their predictions address the phases of
accumulating snowpack, melting processes, and distribution of the
meltwater through stream networks and into the groundwater. Key to
describing the melting processes are solar heat flux, ambient
temperature, wind, and precipitation. Initial snowmelt models used a
degree-day approach that emphasized the temperature difference between
the air and the snowpack to compute snow water equivalent, SWE. More
recent models use an energy balance approach that take into account
the following factors to compute Qm, the energy available for melt.
This requires measurement of an array of snowpack and environmental
factors to compute six heat flow mechanisms that contribute to Qm.
Effects on human activity
Snow affects human activity in four major areas, transportation,
agriculture, structures, and sports. Most transportation modes are
impeded by snow on the travel surface.
Agriculture often relies on
snow as a source of seasonal moisture. Structures may fail under snow
loads. Humans find a wide variety of recreational activities in snowy
See also: Snowplow
Snow affects the rights of way of highways, airfields and railroads.
They share a common tool for clearing snow, the snowplow. However, the
application is different in each case—whereas roadways employ
anti-icing chemicals to prevent bonding of ice, airfields may not;
railroads rely on abrasives to enhance traction on tracks.
Traffic stranded in a 2011
In the late 20th century, an estimated $2 billion was spent annually
in North America on roadway winter maintenance, owing to snow and
other winter weather events, according to a 1994 report by Kuemmel.
The study surveyed the practices of jurisdictions within 44 US states
and nine Canadian provinces. It assessed the policies, practices, and
equipment used for winter maintenance. It found similar practices and
progress to be prevalent in Europe.
The dominant effect of snow on vehicle contact with the road is
diminished friction. This can be improved with the use of snow tires,
which have a tread designed to compact snow in a manner that enhances
traction. However, the key to maintaining a roadway that can
accommodate traffic during and after a snow event is an effective
anti-icing program that employs both chemicals and plowing. The
FHWA Manual of Practice for an Effective Anti-icing Program emphasizes
"anti-icing" procedures that prevent the bonding of snow and ice to
the road. Key aspects of the practice include: understanding
anti-icing in light of the level of service to be achieved on a given
roadway, the climatic conditions to be encountered, and the different
roles of deicing, anti-icing, and abrasive materials and applications,
and employing anti-icing "toolboxes", one for operations, one for
decision-making and another for personnel. The elements to the
Operations – Addresses the application of solid and liquid
chemicals, using various techniques, including prewetting of
chloride-salts. It also addresses plowing capability, including types
of snowplows and blades used.
Decision-making – Combines weather forecast information with road
information to assess the upcoming needs for application of assets and
the evaluation of treatment effectiveness with operations underway.
Personnel – Addresses training and deployment of staff to
effectively execute the anti-icing program, using the appropriate
materials, equipment and procedures.
The manual offers matrices that address different types of snow and
the rate of snowfall to tailor applications appropriately and
Snow fences, constructed upwind of roadways control snow drifting by
causing windblown, drifting snow to accumulate in a desired place.
They are also used on railways. Additionally, farmers and ranchers use
snow fences to create drifts in basins for a ready supply of water in
Ice protection system
Deicing an aircraft during a snow event
In order to keep airports open during winter storms, runways and
taxiways require snow removal. Unlike roadways, where chloride
chemical treatment is common to prevent snow from bonding to the
pavement surface, such chemicals are typically banned from airports
because of their strong corrosive effect on aluminum aircraft.
Consequently, mechanical brushes are often used to complement the
action of snow plows. Given the width of runways on airfields that
handle large aircraft, vehicles with large plow blades, an echelon of
plow vehicles or rotary snowplows are used to clear snow on runways
and taxiways. Terminal aprons may require 6 hectares (15 acres) or
more to be cleared.
Properly equipped aircraft are able to fly through snowstorms under
Instrument flight rules. Prior to takeoff, during snowstorms they
require deicing fluid to prevent accumulation and freezing of snow and
other precipitation on wings and fuselages, which may compromise the
safety of the aircraft and its occupants. In flight, aircraft rely
on a variety of mechanisms to avoid rime and other types of icing in
clouds, these include pulsing pneumatic boots, electro-thermal
areas that generate heat, and fluid deicers that bleed onto the
Railroads have traditionally employed two types of snow plows for
clearing track, the wedge plow, which casts snow to both sides, and
the rotary snowplow, which is suited for addressing heavy snowfall and
casting snow far to one side or the other. Prior to the invention of
the rotary snowplow ca. 1865, it required multiple locomotives to
drive a wedge plow through deep snow. Subsequent to clearing the track
with such plows, a "flanger" is used to clear snow from between the
rails that are below the reach of the other types of plow. Where icing
may affect the steel-to-steel contact of locomotive wheels on track,
abrasives (typically sand) have been used to provide traction on
Railroads employ snow sheds—structures that cover the track—to
prevent the accumulation of heavy snow or avalanches to cover tracks
in snowy mountainous areas, such as the
Alps and the Rocky
Snowplows for different transportation modes
Trucks plowing snow on a highway in Missouri
Airport snow-clearing operations include plowing and brushing
Swiss low-profile, train-mounted snowplow
Snow roads and runways
Snow can be compacted to form a snow road and be part of a winter road
route for vehicles to access isolated communities or construction
projects during the winter.
Snow can also be used to provide the
supporting structure and surface for a runway, as with the Phoenix
Airfield in Antarctica. The snow-compacted runway is designed to
withstand approximately 60 wheeled flights of heavy-lift military
aircraft a year.
Satellite view of the Indus River, showing snow in the Himalayas,
which feeds it, and green areas that draw on it for irrigation
Snowfall can be beneficial to agriculture by serving as a thermal
insulator, conserving the heat of the Earth and protecting crops from
subfreezing weather. Some agricultural areas depend on an accumulation
of snow during winter that will melt gradually in spring, providing
water for crop growth, both directly and via runoff through streams
and rivers, which supply irrigation canals. The following are
examples of rivers that rely on meltwater from glaciers or seasonal
snowpack as an important part of their flow on which irrigation
depends: the Ganges, many of whose tributaries rise in the Himalayas
and which provide much irrigation in northeast India, the Indus
River, which rises in Tibet and provides irrigation water to
Pakistan from rapidly retreating Tibetan glaciers, and the
Colorado River, which receives much of its water from seasonal
snowpack in the Rocky Mountains and provides irrigation water to
some 4 million acres (1.6 million hectares).
Snow accumulation on building roofs
Snow is an important consideration for loads on structures. To address
these, European countries employ Eurocode 1: Actions on structures -
Part 1-3: General actions -
Snow loads. In North America, ASCE
Minimum Design Loads for Buildings and Other Structures gives guidance
on snow loads. Both standards employ method that translate maximum
expected ground snow loads onto design loads for roofs.
Icings resulting from water leaking through the roof due to an ice
dam, caused by snowmelt on the roof
Snow loads and icings are two principal issues for roofs.
are related to the climate in which a structure is sited. Icings are
usually a result of the building or structure generating heat that
melts the snow that is on it.
Snow loads – The Minimum Design Loads for Buildings and Other
Structures gives guidance on how to translate the following factors
into roof snow loads:
Ground snow loads
Exposure of the roof
Thermal properties of the roof
Shape of the roof
Importance of the building
It gives tables for ground snow loads by region and a methodology for
computing ground snow loads that may vary with elevation from nearby,
measured values. The Eurocode 1 uses similar methodologies, starting
with ground snow loads that are tabulated for portions of Europe.
Icings – Roofs must also be designed to avoid ice dams, which result
from meltwater running under the snow on the roof and freezing at the
Ice dams on roofs form when accumulated snow on a sloping roof
melts and flows down the roof, under the insulating blanket of snow,
until it reaches below freezing temperature air, typically at the
eaves. When the meltwater reaches the freezing air, ice accumulates,
forming a dam, and snow that melts later cannot drain properly through
Ice dams may result in damaged building materials or in
damage or injury when the ice dam falls off or from attempts to remove
ice dams. The melting results from heat passing through the roof under
the highly insulating layer of snow.
In areas with trees, utility distribution lines on poles are less
susceptible to snow loads than they are subject to damage from trees
falling on them, felled by heavy, wet snow. Elsewhere, snow can
accrete on power lines as "sleeves" of rime ice. Engineers design for
such loads, which are measured in kg/m (lb/ft) and power companies
have forecasting systems that anticipate types of weather that may
cause such accretions. Rime ice may be removed manually or by creating
a sufficient short circuit in the affected segment of power lines to
melt the accretions.
Sports and recreation
Snow figures into many winter sports and forms of recreation,
including skiing and sledding. Common examples include cross-country
skiing, Alpine skiing, snowboarding, snowshoeing, and snowmobiling.
The design of the equipment used, typically relies on the bearing
strength of snow, as with skis or snowboards and contends with the
coefficient of friction of snow to allow sliding, often enhance by ski
Skiing is by far the largest form of winter recreation. As of 1994, of
the estimated 65–75 million skiers worldwide, there were
approximately 55 million who engaged in Alpine skiing, the rest
engaged in cross-country skiing. Approximately 30 million skiers (of
all kinds) were in Europe, 15 million in the US, and 14 million in
Japan. As of 1996, there were reportedly 4,500 ski areas, operating
26,000 ski lifts and enjoying 390 million skier visits per year. The
preponderant region for downhill skiing was Europe, followed by Japan
and the US.
Increasingly, ski resorts are relying on snowmaking, the production of
snow by forcing water and pressurized air through a snow gun on ski
Snowmaking is mainly used to supplement natural snow at
ski resorts. This allows them to improve the reliability of their
snow cover and to extend their ski seasons from late autumn to early
spring. The production of snow requires low temperatures. The
threshold temperature for snowmaking increases as humidity decreases.
Wet-bulb temperature is used as a metric since it takes air
temperature and relative humidity into account.
Snowmaking is a
relatively expensive process in its energy consumption, thereby
limiting its use.
Ski wax enhances the ability of a ski or other runner to slide over
snow, which depends on both the properties of the snow and the ski to
result in an optimum amount of lubrication from melting the snow by
friction with the ski—too little and the ski interacts with solid
snow crystals, too much and capillary attraction of meltwater retards
the ski. Before a ski can slide, it must overcome the maximum value
static friction. Kinetic (or dynamic) friction occurs when the ski is
moving over the snow.
Main article: Cold-weather warfare
See also: Ski warfare
Snow affects warfare conducted in winter, alpine environments or at
high latitudes. The main factors are impaired visibility for acquiring
targets during falling snow, enhanced visibility of targets against
snowy backgrounds for targeting, and mobility for both mechanized and
infantry troops. Snowfall can severely inhibit the logistics of
supplying troops, as well.
Snow can also provide cover and
fortification against small-arms fire. Noted winter warfare
campaigns where snow and other factors affected the operations
The French invasion of Russia, where poor traction conditions for
ill-shod horses made it difficult for supply wagons to keep up with
troops. That campaign was also strongly affected by cold, whereby
the retreating army reached
Neman River in December 1812 with only
10,000 of the 420,000 that had set out to invade
Russia in June of the
Winter War, an attempt by the
Soviet Union to take territory in
Finland in late 1939 demonstrated superior winter tactics of the
Finnish Army, regarding over-snow mobility, camouflage, and use of the
The Battle of the Bulge, a German counteroffensive during World War
II, starting December 16, 1944, was marked by heavy snowstorms that
hampered allied air support for ground troops, but also impaired
German attempts to supply their front lines. On the Eastern Front
with the Nazi invasion of
Russia in 1941, Operation Barbarossa, both
Russian and German soldiers had to endure terrible conditions during
the Russian winter. While use of ski infantry was common in the Red
Army, Germany formed only one division for movement on skis.
Korean War which lasted from June 25, 1950, until an armistice in
July 27, 1953, began when
North Korea invaded South Korea. Much of the
fighting occurred during winter conditions, involving snow,
notably during the Battle of Chosin Reservoir, which was a stark
example of cold affecting military operations, especially vehicles
Military operations in snow
Bivouac of Napoleon's Grande Armée, during the winter retreat from
Finnish ski troops during the invasion of
Finland by the Soviet Union
Army vehicles coping with snow during the
Battle of the Bulge
Battle of the Bulge of World
Norwegian military preparations during the 2009
Cold Response exercise
Navy SEALs training for winter warfare at Mammoth Mountain,
Effects on ecosystems
Algae, Chlamydomonas nivalis, that thrive in snow form red areas in
the suncups on this snow surface
Both plant and animal life endemic to snow-bound areas develop ways to
adapt. Among the adaptive mechanisms for plants are dormancy, seasonal
dieback, survival of seeds; and for animals are hibernation,
insulation, anti-freeze chemistry, storing food, drawing on reserves
from within the body, and clustering for mutual heat.
Snow interacts with vegetation in two principal ways, vegetation can
influence the deposition and retention of snow and, conversely, the
presence of snow can affect the distribution and growth of vegetation.
Tree branches, especially of conifers intercept falling snow and
prevent accumulation on the ground.
Snow suspended in trees ablates
more rapidly than that on the ground, owing to its greater exposure to
sun and air movement. Trees and other plants can also promote snow
retention on the ground, which would otherwise be blown elsewhere or
melted by the sun.
Snow affects vegetation in several ways, the
presence of stored water can promote growth, yet the annual onset of
growth is dependent on the departure of the snowpack for those plants
that are buried beneath it. Furthermore, avalanches and erosion from
snowmelt can scour terrain of vegetation.
Arctic fox, a predator of smaller animals that live beneath the snow
Snow supports a wide variety of animals both on the surface and
beneath. Many invertebrates thrive in snow, including spiders, wasps,
beetles, snow scorpionflys and springtails. Such arthropods are
typically active at temperatures down to −5 °C (23 °F).
Invertebrates fall into two groups, regarding surviving subfreezing
temperatures: freezing resistant and those that avoid freezing because
they are freeze-sensitive. The first group may be cold hardy owing to
the ability to produce antifreeze agents in their body fluids that
allows survival of long exposure to sub-freezing conditions. Some
organisms fast during the winter, which expels freezing-sensitive
contents from their digestive tracts. The ability to survive the
absence of oxygen in ice is an additional survival mechanism.
Small vertebrates are active beneath the snow. Among vertebrates,
alpine salamanders are active in snow at temperatures as low as
−8 °C (18 °F); they burrow to the surface in springtime
and lay their eggs in melt ponds. Among mammals, those that remain
active are typically smaller than 250 grams (8.8 oz). Omnivores
are more likely to enter a torpor or be hibernators, whereas
herbivores are more likely to maintain food caches beneath the snow.
Voles store up to 3 kilograms (6.6 lb) of food and pikas up to 20
kilograms (44 lb). Voles also huddle in communal nests to benefit
from one another's warmth. On the surface, wolves, coyotes, foxes,
lynx, and weasels rely on these subsurface dwellers for food and often
dive into the snowpack to find them.
Extraterrestrial "snow" includes water-based precipitation, but also
precipitation of other compounds prevalent on other planets and moons
in the Solar System. Examples are:
On Mars, observations of the Phoenix
Mars lander reveal that
water-based snow crystals occur at high latitudes. Additionally,
carbon dioxide precipitates from clouds during the Martian winters at
the poles and contributes to a seasonal deposit of that compound,
which is the principal component of that planet's ice caps.
On Venus, observations from the Magellan spacecraft reveal the
presence a metallic substance, which precipitates as "
Venus snow" and
leaves a highly reflective substance at the tops of Venus's highest
mountain peaks resembling terrestrial snow. Given the high
temperatures on Venus, the leading candidates for the precipitate are
lead sulfide and bismuth(III) sulfide.
On Saturn's moon, Titan,
Cassini–Huygens spacecraft observations
suggest the presence of methane or some other form of
hydrocarbon-based crystalline deposits.
Eskimo words for snow
The wrong type of snow
Notable snow events
2007 Siberian orange snow
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United Nations Environment Programme: Global Outlook for
Ice and Snow
Institute of Low
Temperature Science, Hokkaido University
Swiss Federal Institute for Forest,
Snow and Landscape Research
Ice Data Center of the United States
American Society of Civil Engineers ground snow loads interactive map
for the continental US