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Snow comprises individual
ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , eve ...

ice
crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of 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.
Snowstorm A winter storm is an event in which wind coincides with varieties of precipitation that only occur at freezing temperatures, such as snow, Rain and snow mixed, mixed snow and rain, or freezing rain. In temperate continental climates, these stor ...

Snowstorm
s organize and develop by feeding on sources of atmospheric moisture and cold air.
Snowflake A snowflake is a single ice crystal Ice is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the mai ...

Snowflake
s
nucleate
nucleate
around particles in the atmosphere by attracting
supercooled Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isoc ...
water droplets, which freeze in hexagonal-shaped crystals. Snowflakes take on a variety of shapes, basic among these are platelets, needles, columns and
rime Rime may refer to: *Rime ice Rime ice forms when supercooled water liquid droplets freeze onto surfaces. Meteorology, Meteorologists distinguish between three basic types of ice forming on vertical and horizontal surfaces by deposition of superco ...
. As snow accumulates into a
snowpack Snowpack forms from layers of snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of f ...
, it may blow into drifts. Over time, accumulated snow metamorphoses, by
sintering Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction In materials science, liquefaction is a process t ...
, sublimation and
freeze-thaw Frost weathering is a collective term for several mechanical weathering Weathering is the breaking down of Rock (geology), rocks, soils, and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and b ...
. Where the climate is cold enough for year-to-year accumulation, a
glacier A glacier (; ) is a persistent body of dense ice Ice is into a state. Depending on the presence of such as particles of soil or bubbles of air, it can appear transparent or a more or less bluish-white color. In the , ice is abunda ...

glacier
may form. Otherwise, snow typically melts seasonally, causing runoff into streams and rivers and recharging
groundwater Groundwater is the water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known form ...

groundwater
. Major snow-prone areas include the
polar regions The polar regions, also called the frigid zones, of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent A continent ...
, the northernmost half of the
Northern Hemisphere The Northern Hemisphere is the half of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remain ...

Northern Hemisphere
and mountainous regions worldwide with sufficient moisture and cold temperatures. In the
Southern Hemisphere The Southern Hemisphere is the half (hemisphere Hemisphere may refer to: * A half of a sphere As half of the Earth * A hemispheres of Earth, hemisphere of Earth ** Northern Hemisphere ** Southern Hemisphere ** Eastern Hemisphere ** Western He ...

Southern Hemisphere
, snow is confined primarily to mountainous areas, apart from
Antarctica Antarctica ( or ) is Earth's southernmost continent. It contains the geographic South Pole and is situated in the Antarctic region of the Southern Hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Oc ...

Antarctica
. Snow affects such human activities as
transportation Transport (in British English British English (BrE) is the standard dialect of the English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medieval Engl ...

transportation
: creating the need for keeping roadways, wings, and windows clear;
agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Exercise trends, Increases in sedentary behaviors su ...

agriculture
: providing water to crops and safeguarding livestock;
sport Sport pertains to any form of Competition, competitive physical activity or game that aims to use, maintain or improve physical ability and Skill, skills while providing enjoyment to participants and, in some cases, entertainment to spectato ...

sport
s such as
skiing Skiing is the use of ski A ski is a narrow strip of semi-rigid material worn underfoot to glide over snow. Substantially longer than wide and characteristically employed in pairs, skis are attached to ski boot Ski boots are used in to p ...

skiing
,
snowboarding Snowboarding is a recreational and competitive activity that involves descending a snow-covered slope while standing on a snowboard Snowboards are boards where both feet are placed, and most times secured, to the same board, which are wider ...

snowboarding
, and
snowmachine
snowmachine
travel; and
warfare War is an intense armed conflict between states State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (new ...

warfare
. Snow affects
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

ecosystem
s, as well, by providing an insulating layer during winter under which plants and animals are able to survive the cold.


Precipitation

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 inversion present.


Cloud formation

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 mountains.


Low-pressure areas

Mid-latitude cyclones are
low-pressure area 250 px, This low-pressure system over Iceland Rights are law, legal, social, or ethics, ethical principles of Liberty, freedom or entitlement; that is, rights are the fundamental normative rules about what is allowed of people or owed to people ...
s which are capable of producing anything from cloudiness and mild to heavy
blizzard A blizzard is a severe Winter storm, snowstorm characterized by strong sustained winds of at least and lasting for a prolonged period of time—typically three hours or more. A ground blizzard is a weather condition where snow is not fallin ...

blizzard
s. During a hemisphere's
fall Autumn, also known as fall in American English and Canadian English, is one of the four temperate seasons. Outside the tropics, autumn marks the transition from summer to winter, in September (Northern Hemisphere) or March (Southern Hemisphe ...

fall
, winter, and spring, the atmosphere over continents can be cold enough through the depth of the
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere Planetary means relating to a planet A planet is an astronomical body orbiting a star or Stellar evo ...
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.


Fronts

A
cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough Trough may refer to: In science * Trough (geology), a long depression less steep than a tren ...

cold front
, the leading edge of a cooler mass of air, can produce frontal snowsqualls—an intense frontal
convective Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity Homogeneity and heterogeneity are concepts often used in the sciences and statistics Statistics ...
line (similar to a
rainband display A rainband is a cloud In meteorology Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. The study of meteorology dates bac ...
), when
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
is near freezing at the surface. The strong convection that develops has enough moisture to produce whiteout conditions at places which the 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 Trough may refer to: In science * Trough (geology), a long depression less steep than a trench * Trough (meteorology), an elongated region of low atmospheric pressure * Trough (physics), the lowest point on a wave * Trough level (medicine), the lo ...
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 separated only by 25 miles (40 kilometers), with each passing the same point 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 Thundersnow, also known as a winter thunderstorm or a thundersnowstorm, is an unusual kind of thunderstorm with snow falling as the primary precipitation instead of rain. It typically falls in regions of strong upward motion within the cold s ...
. A
warm front A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass upright=1.25, Different air masses which affect North America as well as other continents, tend to be separated by frontal boundaries In meteo ...

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 the front.


Lake and ocean effects

Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer
lake A lake is an area filled with water, localized in a basin, surrounded by land Land is the solid surface of Earth that is not permanently submerged in water. Most but not all land is situated at elevations above sea level (variable ove ...

lake
water, warming the lower layer of air which picks up
water vapor (99.9839 °C) , - , Boiling point The boiling point of a substance is the temperature at which the vapor pressure 280px, The ''pistol test tube'' experiment. The tube contains alcohol and is closed with a piece of cork. By heating th ...
from the lake, rises up through the colder air above, freezes, and is deposited on the
leeward 400px, Example image showing definitions of windward (upwind) and leeward (downwind) Windward () is the direction ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming. Leeward () is the direction ''dow ...
(downwind) shores. The same effect occurring over bodies of salt water 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 may deposit at a rate of many inches of snow each hour, often resulting in a large amount of total snowfall. The areas affected by lake-effect snow are called snowbelts. These include areas east of the
Great Lakes The Great Lakes also called the Great Lakes of North America or the Laurentian Great Lakes, is a series of large interconnected freshwater lake A lake is an area filled with water, localized in a basin, surrounded by land Land ...

Great Lakes
, the west coasts of northern Japan, the
Kamchatka Peninsula The Kamchatka Peninsula (, ''Poluostrov Kamchatka'', ) is a peninsula A peninsula ( la, paeninsula from ' "almost" and ' "island") is a landform surrounded by water on most of its border while being connected to a mainland from which it ex ...

Kamchatka Peninsula
in Russia, and areas near the
Great Salt Lake The Great Salt Lake, located in the northern part of the U.S. state of Utah, is the largest salt water lake in the Western Hemisphere, and the eighth-largest terminal lake in the world. In an average year the lake covers an area of approximat ...

Great Salt Lake
,
Black Sea , with the skyline of Batumi Batumi (; ka, ბათუმი ) is the second largest city of Georgia Georgia usually refers to: * Georgia (country) Georgia ( ka, საქართველო; ''Sakartvelo''; ) is a country locat ...

Black Sea
,
Caspian Sea The Caspian Sea (also known as Mazandaran Sea, Hyrcanian Ocean, or Khazar Sea), tk, Hazar deňzi, az, Xəzər Dənizi, russian: Каспийское море, script=Latn, fa, دریای مازندران، دریای خزر, script=Latn, tly, ...

Caspian Sea
,
Baltic Sea The Baltic Sea is an arm of the Atlantic Ocean, enclosed by Denmark Denmark ( da, Danmark, ) is a Nordic country The Nordic countries, or the Nordics, are a geographical and cultural region In geography, regions are areas that a ...

Baltic Sea
, and parts of the northern Atlantic Ocean.Thomas W. Schmidlin
Climatic Summary of Snowfall and Snow Depth in the Ohio Snowbelt at Chardon.
Retrieved on March 1, 2008.


Mountain effects

Orographic or relief snowfall is created when moist air is forced up the
windward 400px, Example image showing definitions of windward (upwind) and leeward (downwind) Windward () is the direction upwind from the point of reference, alternatively the direction from which the wind is coming. Leeward () is the direction downwin ...
side of
mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. A mountain differs from a plateau in having a limited summit area, and is larger than a hill, typically rising at least ...

mountain
ranges by a large-scale
wind Wind is the natural movement of air or other gases relative to a planet's surface. Wind occurs on a range of scales, from thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by th ...

wind
flow. The lifting of moist air up the side of a mountain range results in adiabatic cooling, and ultimately
condensation Condensation is the change of the state of matter In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and Plasma (physics), plasma. ...

condensation
and precipitation. Moisture is gradually removed from the air by this process, leaving drier and warmer air on the descending, or
leeward 400px, Example image showing definitions of windward (upwind) and leeward (downwind) Windward () is the direction ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming. Leeward () is the direction ''dow ...
, side.Physical Geography
CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes.
Retrieved on January 1, 2009.
The resulting enhanced snowfall, along with the decrease in temperature with elevation, combine to increase snow depth and seasonal persistence of snowpack in snow-prone areas.
Mountain waves In meteorology, lee waves are Earth's atmosphere, atmospheric stationary waves. The most common form is mountain waves, which are atmospheric internal gravity waves. These were discovered in 1933 by two German glider pilots, Hans Deutschmann and Wo ...

Mountain waves
have also been found to help enhance precipitation amounts downwind of mountain ranges by enhancing the lift needed for condensation and precipitation.


Cloud physics

A snowflake consists of roughly 1019 water
molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecule
s 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 differ from each other though they follow similar patterns. Snow crystals form when tiny ed cloud droplets (about 10  μm in diameter)
freeze Freeze may refer to: Liquids turning to solids *Freezing, the physical process of a liquid turning into a solid Cessation of movement or change *Freeze (b-boy move), the halting of all movement in a clever position *Freeze (command), freez ...

freeze
. These droplets are able to remain liquid at temperatures lower than , 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. 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 cloud condensation nuclei on which liquid droplets form. Clays, desert dust, and biological particles can be nuclei. Artificial nuclei include particles of
silver iodide Silver iodide is an inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. However, the distinction is not clearly defined; a ...
and
dry ice Dry ice is the solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is chara ...

dry ice
, and these are used to stimulate precipitation in
cloud seeding Cloud seeding is a type of weather modification Weather modification (also known as weather control) is the act of intentionally manipulating or altering the weather Weather is the state of the atmosphere, describing for example the de ...
. 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, 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 processThe Wegener–Bergeron–Findeisen process (after Alfred Wegener Alfred Lothar Wegener (; ; 1 November 1880 – November 1930) was a German polar researcher, geophysicist and meteorologist. During his lifetime he was primarily known for ...
. 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 Diffuse reflection is the reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflec ...

diffuse reflection
of the whole
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum Continuum may refer to: * Continuum (measurement) Continuum theories or models expla ...

spectrum
of
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

light
by the small ice particles.


Classification of snowflakes

Micrography Micrography (from Greek, literally small-writing – "Μικρογραφία"), also called microcalligraphy, is a Jewish form of calligrams developed in the 9th century, with parallels in Christianity and Islam,Wilson Alwyn Bentley, revealed the wide diversity of snowflakes within a classifiable set of patterns. Closely matching snow crystals have been observed.
Ukichiro Nakaya was a Japanese physicist and science essayist known for his work in glaciology and low-temperature sciences. He is credited with making the first artificial snowflakes. Life and research Nakaya was born near the Katayamazu onsen, hot springs in ...
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. Nakaya discovered that the 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 while crystals formed in supersaturated air trend more towards lacy, delicate, and ornate. Many more complex growth patterns also form, which include side-planes, bullet-rosettes, and planar types, depending on the conditions and ice nuclei. If a crystal has started forming in a column growth regime at around and then falls into the warmer plate-like regime, plate or dendritic crystals sprout at the end of the column, producing so called "capped columns". Magono and Lee devised a classification of freshly formed snow crystals that includes 80 distinct shapes. They documented each with micrographs.


Accumulation

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 The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Alaska (United States), Canada, Finland, Greenland (Danish Realm, ...

Arctic
and
Antarctic The Antarctic (US English or , UK English or and or ) is a around 's , opposite the region around the . The Antarctic comprises the continent of , the and other located on the or south of the . The Antarctic region includes the , wa ...

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 A rain gauge (also known as an udometer,pluvia metior, pluviometer, ombrometer, and hyetometer) is an instrument used by meteorologist A meteorologist is a scientist who studies and works in the field of meteorology Meteorology is a branch of ...
, 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 station Image:IMG 0430-aws-rothera 1200x900.jpg, An Antarctic Automatic Weather Stations Project AWS in Antarctica An automatic weather station (AWS) is an automated version of the traditional weather station, either to save human labour or to enable measur ...
s an ultrasonic snow depth sensor may be used to augment the precipitation gauge.


Events

Snow flurry A snow flurry is a light snowfall that results in little or no snow accumulation. The US National Weather Service The National Weather Service (NWS) is an agency Agency may refer to: * a governmental or other institution Institutions, a ...
, snow shower, snow storm and
blizzard A blizzard is a severe Winter storm, snowstorm characterized by strong sustained winds of at least and lasting for a prolonged period of time—typically three hours or more. A ground blizzard is a weather condition where snow is not fallin ...

blizzard
describe snow events of progressively greater duration and intensity. A
blizzard A blizzard is a severe Winter storm, snowstorm characterized by strong sustained winds of at least and lasting for a prolonged period of time—typically three hours or more. A ground blizzard is a weather condition where snow is not fallin ...

blizzard
is a weather condition involving snow and has varying definitions in different parts of the world. In the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

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 , and sufficient snow in the air to reduce visibility to less than . In
Canada Canada is a country in the northern part of North America North America is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, ...

Canada
and the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Britain as a synonym for the United Kingdom. Some prefer to use Britain as shorth ...

United Kingdom
, the criteria are similar. While heavy snowfall often occurs during blizzard conditions, falling snow is not a requirement, as
blowing snow Blowing snow is snow lifted from the surface by the wind, at eye level () or more, that will reduce visibility. Blowing snow can come from falling snow or snow that already accumulated on the ground but is picked up and blown about by strong winds. ...

blowing snow
can create a
ground blizzard Ground blizzard refers to a weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth Earth is the third planet from the Sun ...
. Snowstorm intensity may be categorized by visibility and depth of accumulation. Snowfall's intensity is determined by
visibility In meteorology Meteorology is a branch of the atmospheric sciences Atmospheric science is the study of the Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lowe ...

visibility
, as follows: * ''Light'': visibility greater than * ''Moderate'': visibility restrictions between * ''Heavy'': visibility is less than 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 Snowboards are boards where both feet are placed, and most times secured, to the same board, which are wider than skis, with the ability to glide on snow."snowboarding." Dictionary.com Unabridged (v 1.1). Random House, Inc. 17 Mar. 2009. . Snow ...
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 measuring snowfall.


Distribution

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 , according to a 1987 estimate. A 2007 estimate of snow cover over the Northern Hemisphere suggested that, on average, snow cover ranges from a minimum extent of each August to a maximum extent of 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 over the 35-year period.


Records

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 city of
Bellingham, Washington Bellingham ( ) is the most populous city in and county seat A county seat is an administrative centerAn administrative centre is a seat of regional administration or local government, or a county town, or the place where the central administ ...
during the 1998–1999 season. Mount Baker received of snow, thus surpassing the previous record holder,
Mount Rainier Mount Rainier (), also known as Tahoma,Tacoma or Tacobet, is a large active stratovolcano in the Cascade Range of the Pacific Northwest, located in Mount Rainier National Park about south-southeast of Seattle. With a summit elevation of , it is ...

Mount Rainier
, Washington, which during the 1971–1972 season received of snow. * ''Highest seasonal average annual snowfall'' – The world record for the highest average annual snowfall is , measured in , Japan for the period of 1981–2010. * ''Largest snowflake'' – According to ''
Guinness World Records ''Guinness World Records'', known from its inception in 1955 until 1999 as ''The Guinness Book of Records'' and in previous United States editions as ''The Guinness Book of World Records'', is a reference book A reference work is a work such ...
,'' the world's largest snowflake fell in January 1887 outside present-day ,
Montana Montana () is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspaper), a daily newspaper ...

Montana
. It measured in diameter.


Metamorphosis

After deposition, snow progresses on one of two paths that determine its fate, either ''ablation'' (mostly by melting) or transitioning from
firn Firn (; from Swiss German Swiss German (Standard German Standard High German (SHG), less precisely Standard German or High German (not to be confused with High German The High German languages or High German dialects (german: hochde ...
(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 Frost is a thin layer of ice on a solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic e ...
, during cold, still conditions.


Seasonal snowpack

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 water. Snow that persists into summer evolves into
névé Névé is a young, granular type of snow Snow comprises individual ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all know ...
, granular snow, which has been partially melted, refrozen and compacted. Névé has a minimum
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

density
of , which is of the density of liquid water.


Firn

Firn is snow that has persisted for multiple years and has been recrystallized into a substance denser than
névé Névé is a young, granular type of snow Snow comprises individual ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all know ...
, yet less dense and hard than glacial
ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , eve ...

ice
. Firn resembles caked sugar and is very resistant to shovelling. Its density generally ranges from to , and it can often be found underneath the snow that accumulates at the head of a
glacier A glacier (; ) is a persistent body of dense ice Ice is into a state. Depending on the presence of such as particles of soil or bubbles of air, it can appear transparent or a more or less bluish-white color. In the , ice is abunda ...

glacier
. The minimum altitude that firn accumulates on a glacier is called the ''firn limit'', ''firn line'' or ''snowline''.


Movement

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 glacier ice.


Drifting

When powdery, snow drifts with the
wind Wind is the natural movement of air or other gases relative to a planet's surface. Wind occurs on a range of scales, from thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by th ...

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 slopes.


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 In fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodynamics (the study of air and other gases in ...
. They occur in three major mechanisms:McClung, David and Shaerer, Peter: The Avalanche Handbook, The Mountaineers: 2006. * ''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 avalanche image:2007-02-15-CLB-Couloir2-1c.JPG, thumbnail, Example of powder snow avalanche A powder snow avalanche is a type of avalanche where the snow grains are largely or completely suspended by fluid turbulence. They are particle-laden gravity currents ...
s'' result from a deposition of fresh dry powder and generate a powder cloud, which overlies a dense avalanche. They can exceed speeds of , and masses of ; 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 (~), wet snow avalanches are capable of generating powerful destructive forces, due to the large mass, and density.


Snowmelt

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 A flood is an overflow of water that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide Tides are the rise and fall of s caused by the combined effects of the fo ...

flooding
during the spring months and at least in dry mountainous regions like the mountain West of the US or most of
Iran Iran ( fa, ایران ), also called Persia, and officially the Islamic Republic of Iran, is a country in Western Asia Western Asia, West Asia, or Southwest Asia, is the westernmost subregion A subregion is a part of a larger regio ...

Iran
and
Afghanistan Afghanistan (; Pashto Pashto (,; / , ), sometimes spelled Pukhto or Pakhto, is an Eastern Iranian language The Eastern Iranian languages are a subgroup of the Iranian languages The Iranian or Iranic languages are a branch of t ...

Afghanistan
, very low flow for the rest of the year. In contrast, if much of the melt is from
glaciated A glacier ( or ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. Glaciers slowly deform and flow under str ...
or nearly glaciated areas, the melt continues through the warm season, with peak flows occurring in mid to late summer.


Glaciers

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 an escape route, 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.


Science

Scientists study snow at a wide variety of scales that include the
physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Physical scie ...

physics
of
chemical bonds A chemical bond is a lasting attraction between atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyda ...
and
clouds In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
; the distribution, accumulation, metamorphosis, and ablation of snowpacks; and the contribution of snowmelt to river
hydraulics Hydraulics (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ap ...
and ground
hydrology Hydrology (from Ancient Greek, Greek wikt:ὕδωρ, ὕδωρ, ''hýdōr'' meaning "water" and wikt:λόγος, λόγος, ''lógos'' meaning "study") is the scientific study of the movement, distribution, and management of water on Earth and ...
. In doing so, they employ a variety of instruments to observe and measure the phenomena studied. Their findings contribute to knowledge applied by
engineer Engineers, as practitioners of engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of enginee ...

engineer
s, who adapt vehicles and structures to snow, by
agronomist Agronomy is the science and technology of producing and using plant Plants are mainly multicellular organisms, predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, co ...

agronomist
s, who address the availability of snowmelt to
agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Exercise trends, Increases in sedentary behaviors su ...

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
avalanche An avalanche (also called a snowslide) is a rapid flow of snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo fur ...

avalanche
s, 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 Ground truth is a term used in various fields to refer to information provided by direct observation (i.e. empirical evidence Empirical evidence is the information Information can be thought of as the resolution of uncertainty; it ans ...
and
remote sensing image of Death Valley colored using polarimetry. Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied e ...

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 following table: ''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.


Satellite data

Remote sensing image of Death Valley colored using polarimetry. Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied e ...

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 satellite observations began. In some regions such as China, a trend of increasing snow cover was observed from 1978 to 2006. These changes are attributed to global climate change, which may lead to earlier melting and less coverage area. 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 Snow and 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. Cloud cover 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.


Models

Snow science often leads to predictive models that include snow deposition, snow melt, and snow hydrology—elements of the Earth's
water cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships ...

water cycle
—which help describe
global climate change Climate change includes both global warming driven by human emissions of greenhouse gas A greenhouse gas (sometimes abbreviated GHG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radia ...
. Climate model, 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 Seasonal 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 landscapes.


Transportation

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.


Highway

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 Snowplow, plowing. The Federal Highway Administration, 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 toolboxes are: * ''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 efficiently. 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 the spring.


Aviation

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 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 deicing boot, pneumatic boots, electro-thermal areas that generate heat, and fluid deicers that bleed onto the surface.


Rail

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 steeper uphills. 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 Mountains.
;Snowplows for different transportation modes
File:TowPLow front view2.JPG, Trucks plowing snow on a highway in Missouri File:Winter Operations @ Brussels Airport January 2013 (8387468508).jpg, Airport snow-clearing operations include plowing and brushing File:RhB ABe 8-12 Allegra mit Spurpflug bei Ospizio Bernina.jpg, 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, winter road route for vehicles to access isolated communities or construction projects during the winter.Abele, G., 1990. Snow roads and runways, U.S. Army Cold Regions Research and Engineering Laboratory, Monograph 90-3, Washington, D.C. 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.


Agriculture

Snowfall can be beneficial to agriculture by serving as a thermal insulation, thermal insulator, conserving the heat of the Earth and protecting agriculture, 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).


Structures

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 methods that translate maximum expected ground snow loads onto design loads for roofs.


Roofs

Snow loads and icings are two principal issues for roofs. Snow loads 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 * Drifting * 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 dam (roof), ice dams, which result from meltwater running under the snow on the roof and freezing at the eave. 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 the dam.Paul Fisette, "Preventing Ice Dams", ''Roofing, flashing & waterproofing''. Newtown, CT: Taunton Press, 2005. 54. Ice dams may result in Water damage, 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.


Utility lines

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 Skiing is the use of ski A ski is a narrow strip of semi-rigid material worn underfoot to glide over snow. Substantially longer than wide and characteristically employed in pairs, skis are attached to ski boot Ski boots are used in to p ...

skiing
and sledding. Common examples include cross-country skiing, Alpine skiing,
snowboarding Snowboarding is a recreational and competitive activity that involves descending a snow-covered slope while standing on a snowboard Snowboards are boards where both feet are placed, and most times secured, to the same board, which are wider ...

snowboarding
, snowshoeing, and Snowmobile, snowmobiling. The design of the equipment used, e.g. skis and snowboards, typically relies on the bearing strength of snow and contends with the coefficient of friction bearing on snow. 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 Snowmaking#Snowmaking guns, snow gun on ski slopes. 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 by reducing its coefficient of friction, 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.


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 infantry, mechanized and infantry troops. Snowfall can severely inhibit the Military logistics, 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 include: * 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 same year.The Wordsworth Pocket Encyclopedia, p. 17, Hertfordshire 1993. * 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 terrain. * 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 1st Ski Division (Germany), one division for movement on skis. * The Korean War which lasted from June 25, 1950, until an armistice on 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 and weapons.
;Military operations in snow
File:Night Bivouac of Great Army.jpg, Bivouac of Napoleon's Grande Armée, during the winter retreat from Moscow File:Finn ski troops.jpg, Finnish ski troops during the invasion of Finland by the Soviet Union File:Army vehicles on a road in Belgium.jpg, Army vehicles coping with snow during the Battle of the Bulge of World War II. File:Cold Response DV dag.jpg, Norwegian military preparations during the 2009 Cold Response exercise File:Navy Seals Winter warfare at Mammoth Mountain, California, in December 2014.jpg, Navy SEALs training for winter warfare at Mammoth Mountain, California.


Effects on ecosystems

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.


Plant life

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.


Animal life

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 . 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 Fasting, 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 ; they burrow to the surface in springtime and lay their eggs in melt ponds. Among mammals, those that remain active are typically smaller than . Omnivores are more likely to enter a torpor or be Hibernate, hibernators, whereas herbivores are more likely to maintain food caches beneath the snow. Voles store up to of food and pikas up to . 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.


Outside of Earth

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 (spacecraft), ''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 Martian polar ice caps, planet's ice caps. * On Venus, observations from the Magellan (spacecraft), ''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 (moon), Titan, Cassini–Huygens, ''Cassini–Huygens'' spacecraft observations suggest the presence of methane or some other form of hydrocarbon-based crystalline deposits.


See also

Lexicon * Eskimo words for snow * The wrong type of snow Notable snow events * 2007 Siberian orange snow * Alberta clipper * List of blizzards * List of snowiest places in the United States by state Recreation * Skiing * Sled * Snow angel * Snow cannon * Snowman * Snowmobiling * Winter sport Related concepts * Freezing rain * Frost * Graupel * Hail * Ice * Ice pellets * Hard rime, Rime * Rain and snow mixed, Sleet * Snowbelt Science and scientists * Snow hydrology * Timeline of snowflake research *
Ukichiro Nakaya was a Japanese physicist and science essayist known for his work in glaciology and low-temperature sciences. He is credited with making the first artificial snowflakes. Life and research Nakaya was born near the Katayamazu onsen, hot springs in ...
Snow structures * Igloo * Quinzhee * Snow cave * Snow grooming


References


External links


United Nations Environment Programme: Global Outlook for Ice and Snow




* [http://nsidc.org National Snow and Ice Data Center of the United States]
American Society of Civil Engineers ground snow loads interactive map for the continental US
{{Authority control Snow, Articles containing video clips Forms of water Water ice Precipitation Weather hazards