Snow comprises individual

ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...

crystals that grow while suspended in the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

—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 sto ...

s organize and develop by feeding on sources of atmospheric moisture and cold air.

Snowflake A snowflake is a single ice crystal that has achieved a sufficient size, and may have amalgamated with others, which falls through the Earth's atmosphere as snow.Knight, C.; Knight, N. (1973). Snow crystals. Scientific American, vol. 228, no. ...

nucleate In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that dete ...

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 Rime may refer to: *Rime ice, ice that forms when water droplets in fog freeze to the outer surfaces of objects, such as trees Rime is also an alternative spelling of "rhyme" as a noun: *Syllable rime, term used in the study of phonology in ling ...

. As snow accumulates into a

snowpack Snowpack forms from layers of snow that accumulate in geographic regions and high elevations where the climate includes cold weather for extended periods during the year. Snowpacks are an important water resource that feed streams and rivers as th ...

, 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 pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...

, sublimation and

freeze-thaw Frost weathering is a collective term for several mechanical weathering processes induced by stresses created by the freezing of water into ice. The term serves as an umbrella term for a variety of processes such as frost shattering, frost wedg ...

. Where the climate is cold enough for year-to-year accumulation, a

glacier A glacier (; ) 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#Glaciology, ablation over many years, often Century, centuries. It acquires dis ...

may form. Otherwise, snow typically melts seasonally, causing runoff into streams and rivers and recharging

groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...

. Major snow-prone areas include the

polar regions The polar regions, also called the frigid zones or polar zones, of Earth are the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitudes are dominated by float ...

, the northernmost half of the

Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...

and mountainous regions worldwide with sufficient moisture and cold temperatures. In the Southern Hemisphere, snow is confined primarily to mountainous areas, apart from

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

. Snow affects such human activities as

transportation Transport (in British English), or transportation (in American English), is the intentional movement of humans, animals, and goods from one location to another. Modes of transport include air, land (rail and road), water, cable, pipeline, ...

: creating the need for keeping roadways, wings, and windows clear;

agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to ...

: 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 specta ...

s such as

skiing Skiing is the use of skis to glide on snow. Variations of purpose include basic transport, a recreational activity, or a competitive winter sport. Many types of competitive skiing events are recognized by the International Olympic Committee (IO ...

snowboarding Snowboarding is a recreational and competitive activity that involves descending a snow-covered surface while standing on a snowboard that is almost always attached to a rider's feet. It features in the Winter Olympic Games and Winter Paralympi ...

, and snowmachine travel; and

warfare War is an intense armed conflict between states, governments, societies, or paramilitary groups such as mercenaries, insurgents, and militias. It is generally characterized by extreme violence, destruction, and mortality, using regular ...

. 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 syste ...

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 In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may com ...

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 Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...

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 In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather (such as cloudy, windy, with possible ...

s which are capable of producing anything from cloudiness and mild snow storms to heavy

blizzard A blizzard is a severe snowstorm characterized by strong sustained winds and low visibility, lasting for a prolonged period of time—typically at least three or four hours. A ground blizzard is a weather condition where snow is not falling b ...

s. During a hemisphere's

fall Autumn, also known as fall in American English and Canadian English, is one of the four temperate seasons on Earth. Outside the tropics, autumn marks the transition from summer to winter, in September (Northern Hemisphere) or March ( Southe ...

, 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, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...

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 middle latitudes (also called the mid-latitudes, sometimes midlatitudes, or moderate latitudes) are a spatial region on Earth located between the Tropic of Cancer (latitudes 23°26'22") to the Arctic Circle (66°33'39"), and Tropic of Caprico ...

, the side of a

cyclone In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anti ...

that produces the most snow is the southern side.

Fronts

Snowsquall line-Bourrasque neige frontal NOAA

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 of low pressure. It often forms behind an extratropical cyclone (to the west in the Norther ...

, 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 and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convect ...

line (similar to a

rainband A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands can be stratiform or convective, and are generated by differences in temperature. When noted on weather radar ima ...

), when

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

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 l ...

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 a kind of thunderstorm with snow falling as the primary precipitation instead of rain. It is considered a rare and unusual phenomenon. It typically falls in regions of s ...

. A

warm front A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold fro ...

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, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...

water, warming the lower layer of air which picks up

water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous pha ...

from the lake, rises up through the colder air above, freezes, and is deposited on the

leeward Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...

(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 Orography is the study of the topographic relief of mountains, and can more broadly include hills, and any part of a region's elevated terrain. Orography (also known as ''oreography'', ''orology'' or ''oreology'') falls within the broader discipl ...

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

snowbelt The Snowbelt is the region near the Great Lakes in North America where heavy snowfall in the form of lake-effect snow is particularly common. Snowbelts are typically found downwind of the lakes, principally off the eastern and southern shores. Ca ...

s. These include areas east of the

Great Lakes The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lakes ...

, the west coasts of northern Japan, the

Kamchatka Peninsula The Kamchatka Peninsula (russian: полуостров Камчатка, Poluostrov Kamchatka, ) is a peninsula in the Russian Far East, with an area of about . The Pacific Ocean and the Sea of Okhotsk make up the peninsula's eastern and we ...

in Russia, and areas near the

Great Salt Lake The Great Salt Lake is the largest saltwater lake in the Western Hemisphere and the eighth-largest terminal lake in the world. It lies in the northern part of the U.S. state of Utah and has a substantial impact upon the local climate, particula ...

Black Sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Roma ...

Caspian Sea The Caspian Sea is the world's largest inland body of water, often described as the world's largest lake or a full-fledged sea. An endorheic basin, it lies between Europe and Asia; east of the Caucasus, west of the broad steppe of Central Asia ...

Baltic Sea The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain. The sea stretches from 53°N to 66°N latitude and from ...

, 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 Orography is the study of the topographic relief of mountains, and can more broadly include hills, and any part of a region's elevated terrain. Orography (also known as ''oreography'', ''orology'' or ''oreology'') falls within the broader discipl ...

relief Relief is a sculptural method in which the sculpted pieces are bonded to a solid background of the same material. The term ''relief'' is from the Latin verb ''relevo'', to raise. To create a sculpture in relief is to give the impression that the ...

snowfall is created when moist air is forced up the

windward Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...

side of

mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited Summit (topography), summit area, and ...

ranges by a large-scale

wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...

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 from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

and precipitation. Moisture is gradually removed from the air by this process, leaving drier and warmer air on the descending, or

, 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 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 Wolf Hirth, above t ...

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 a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

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

supercool Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its melting point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crystal ...

ed cloud droplets (about 10 μm in diameter) 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 with the formula Ag I. The compound is a bright yellow solid, but samples almost always contain impurities of metallic silver that give a gray coloration. The silver contamination arises because AgI is hig ...

and

dry ice Dry ice is the solid form of carbon dioxide. It is commonly used for temporary refrigeration as CO2 does not have a liquid state at normal atmospheric pressure and sublimates directly from the solid state to the gas state. It is used primarily a ...

, and these are used to stimulate precipitation in

cloud seeding Cloud seeding is a type of weather modification that aims to change the amount or type of precipitation that falls from clouds by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysical p ...

. 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 A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...

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 process The Wegener–Bergeron–Findeisen process (after Alfred Wegener, Tor Bergeron and ), (or "cold-rain process") is a process of ice crystal growth that occurs in mixed phase clouds (containing a mixture of supercooled water and ice) in regions wher ...

. 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 A snowflake is a single ice crystal that has achieved a sufficient size, and may have amalgamated with others, which falls through the Earth's atmosphere as snow.Knight, C.; Knight, N. (1973). Snow crystals. Scientific American, vol. 228, no. ...

, 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 reflection (physics), reflection of light or other radiation, waves or particles from a surface such that a ray (optics), ray incident on the surface is scattering, scattered at many angles rather than at just one angle ...

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. The word was first used scientifically in optics to describe the rainbow of colors i ...

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

by the small ice particles.

Classification of snowflakes

Snowflakeschapte00warriala-p11-p21-p29-p39

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 hot springs in Kag ...

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 Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...

and

Antarctic The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...

, the Northern Hemisphere, and alpine regions. The liquid equivalent of snowfall may be evaluated using a

snow gauge A snow gauge is a type of instrument used by meteorologists and hydrologists to gather and measure the amount of solid precipitation (as opposed to liquid precipitation, which is measured by a rain gauge) over a set period of time. History The ...

or with a standard

rain gauge A rain gauge (also known as udometer, pluvia metior, pluviometer, ombrometer, and hyetometer) is an instrument used by meteorologists and hydrologists to gather and measure the amount of liquid precipitation over a predefined area, over a period o ...

, 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 An automatic weather station (AWS) is an automated version of the traditional weather station, either to save human labour or to enable measurements from remote areas. An AWS will typically consist of a weather-proof enclosure containing the data ...

s an ultrasonic snow depth sensor may be used to augment the precipitation gauge.

Events

Winter Storm Jonas 2016 NYC Pershing Square

Snow flurry A snow flurry is a light snowfall that results in little or no snow accumulation. The US National Weather Service defines snow flurries as intermittent light snow that produces no measurable precipitation (trace amounts). In contrast, bursts of s ...

snow shower A shower is a mode of precipitation characterized by an abrupt start and end and by rapid variations in intensity. Often strong and short-lived, it comes from convective clouds, like cumulus congestus. A shower will produce rain if the temperatu ...

snow storm A winter storm is an event in which wind coincides with varieties of precipitation that only occur at freezing temperatures, such as snow, mixed snow and rain, or freezing rain. In temperate continental climates, these storms are not necessar ...

and

describe snow events of progressively greater duration and intensity. A

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 primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

, 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 North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

and the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...

, 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. ...

can create a

ground blizzard Ground blizzard refers to a weather condition where loose snow or ice on the ground is lifted and blown by strong winds. This can occur in the absence of precipitation, and can even occur when the sky is clear. This is in contrast to "ordinary" b ...

. Snowstorm intensity may be categorized by visibility and depth of accumulation. Snowfall's intensity is determined by

visibility The visibility is the measure of the distance at which an object or light can be clearly discerned. In meteorology it depends on the transparency of the surrounding air and as such, it is unchanging no matter the ambient light level or time of ...

, 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 the user places both feet, usually secured, to the same board. The board itself is wider than most skis, with the ability to glide on snow."snowboarding." Dictionary.com Unabridged (v 1.1). Random House, Inc. 17 Mar ...

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 Mt. Baker Ski Area is a ski resort in the Pacific Northwest, northwest United States, located in Whatcom County, Washington, at the end of Washington State Route 542, State Route 542. The base elevation is at , while the peak of the resort is at ...

, outside of the city of

Bellingham, Washington Bellingham ( ) is the most populous city in, and county seat of Whatcom County in the U.S. state of Washington. It lies south of the U.S.–Canada border in between two major cities of the Pacific Northwest: Vancouver, British Columbia (locat ...

during the 1998–1999 season. Mount Baker received of snow, thus surpassing the previous record holder,

Mount Rainier Mount Rainier (), indigenously known as Tahoma, Tacoma, Tacobet, or təqʷubəʔ, 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 s ...

, 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

Sukayu Onsen is an ''onsen'' (hot spring) in the Hakkōda Mountains in the city of Aomori, Aomori Prefecture in Japan. It is known for its "Sen-nin-buro" or "1,000-person bath", a large mixed gender public bath. Climate Sukayu Onsen is the snowiest inhabit ...

, 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 published annually, listing world ...

,'' the world's largest snowflake fell in January 1887 outside present-day Miles City,

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...

. It measured in diameter. The cities (more than 100,000 inhabitants) with the highest annual snowfall are Aomori (792 cm), Sapporo (485 cm) and Toyama, Toyama, Toyama (363 cm) in Japan, followed by St. John's, Newfoundland and Labrador, St. John's (332 cm) and Quebec City (315 cm) in

Metamorphosis

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.

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, granular snow, which has been partially melted, refrozen and compacted. Névé has a minimum density of , which is Density#Water, roughly half of the density of liquid water.

Firn

Firn is snow that has persisted for multiple years and has been Recrystallization (chemistry), recrystallized into a substance denser than névé, yet less dense and hard than glacial

. 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

. 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

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 Entrainment (physical geography), 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 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 avalanches'' 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 during the spring months and at least in dry mountainous regions like the mountain West of the US or most of Iran and 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 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 of chemical bonds and Cloud physics, 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

, 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 following table: Taku glacier firn ice sampling

''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. When it comes to measuring snow cover on the ground, typically three variables are measured: the snow cover extent (SCE) — the land area covered by snow, snow cover duration (SD) — how long a particular area is covered by snow, and the snow accumulation, often expressed as snow water equivalent (SWE), which expresses how much water the snow would be if it were all melted: this last one is a measurement of the volume of the snowpack. To measure these variables a variety of techniques are used: surface observations, remote sensing, Land surface models (climate), land surface models and Atmospheric reanalysis, reanalysis products. These techniques are often combined to form the most complete datasets.

Satellite data

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. Satellite measurements show that snow cover has been decreasing in many areas of the world since 1978.

Models

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. 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 ''Q_m'', 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 ''Q_m''.

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

Cars covered in Snow on Lake Shore Drive Chicago Feb 2 2011 storm

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 ''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. 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

and sledding. Common examples include cross-country skiing, Alpine skiing,

, 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. 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, United States 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.

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