Talk:Snow/Sandbox

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Snow pertains to frozen crystalline water throughout its life cycle, starting when it precipitates from clouds and accumulates on surfaces, then metamorphoses in place, and ultimately melts, slides or sublimates away. Snowstorms organize develop by feeding on sources of atmospheric moisture and cold air. Snowflakes nucleate around particles in the atmosphere by attracting supercooled water droplets, which freeze in hexagonal-shaped crystals. Snowflakes take on a variety of shapes, basic among these are platelets, needles, columns and rime. As snow accumulates into a snowpack, it may blow into drifts. Over time, accumulated snow metamorphoses, by sintering, sublimation and freeze-thaw. Where the climate is cold enough for year-to-year accumulation, a glacier may form. Otherwise, snow typically melts, seasonally, and causes runoff into streams and rivers and recharging groundwater.

Scientists study snow at a wide variety of scales that include the physics of chemical bonds and clouds; the distribution, accumulation, metamorphosis, and ablation of snowpacks; and the contribution of snowmelt to river hydraulics and ground hydrology. In doing so, they employ a variety of instruments to observe and measure the phenomena studied. Their findings contribute to knowledge applied by engineers, who adapt vehicles and structures to snow, by agronomists, who address the availability of snowmelt to agriculture, and those, who design equipment for sporting activities on snow. Scientists develop and others employ snow classification systems that describe its physical properties at scales ranging from the individual crystal to the aggregated snowpack. A sub-specialty is avalanches, which are of concern to engineers and outdoors sports people, alike.

Snow affects such human activities as transportation: creating the need for keeping roadways, wings, and windows clear; agriculture: providing water to crops and safeguarding livestock, and such sports as skiing, snowboarding, and snowmachine travel. Snow affects ecosystems, 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. Types of snow can be designated by the shape of the flakes, the rate of accumulation, and the way the snow collects on the ground. Types that fall in the form of a ball due to melting and refreezing cycles, rather than a flake, are known as graupel, with ice pellets and snow pellets as types of graupel associated with wintry precipitation.

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
Miid-latitude cyclones are low pressure areas which are capable of producing anything from cloudiness and mild snow storms to heavy blizzards. During a hemisphere's fall, winter, and spring, the atmosphere over continents can be cold enough through the depth of the troposphere to cause snowfall. In the northern hemisphere, the northern side of the low pressure area produces the most snow. For the southern mid-latitudes, the side of a cyclone that produces the most snow is the southern side.

Fronts
A cold front, the leading edge of a cooler mass of air, can produce frontal snowsqualls—an intense frontal convective line (similar to a rainband), when temperature is near freezing at the surface. The strong convection that develops has enough moisture to produce whiteout conditions at places which line passes over as the wind causes intense blowing snow. This type of snowsquall generally lasts less than 30 minutes at any point along its path but the motion of the line can cover large distances. Frontal squalls may form a short distance ahead of the surface cold front or behind the cold front where there may be a deepening low pressure system or a series of trough lines which act similar to a traditional cold frontal passage. In situations where squalls develop post-frontally it is not unusual to have two or three linear squall bands pass in rapid succession only separated by 25 miles (40 kilometers) with each passing the same point in roughly 30 minutes apart. In cases where there is a large amount of vertical growth and mixing the squall may develop embedded cumulonimbus clouds resulting in lightning and thunder which is dubbed thundersnow.

A warm front can produce snow for a period, as warm, moist air overrides below-freezing air and creates precipitation at the boundary. Often, snow transitions to rain in the warm sector behind the front.

Lake effects


Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water, warming the lower layer of air which picks up water vapor from the lake, rises up through the colder air above, freezes and is deposited on the leeward (downwind) shores.

The same effect also occurs over bodies of salt water, when it is termed ocean-effect or bay-effect snow. The effect is enhanced when the moving air mass is uplifted by the orographic influence of higher elevations on the downwind shores. This uplifting can produce narrow but very intense bands of precipitation, which deposit at a rate of many inches of snow each hour, often resulting in a large amount of total snowfall.

The areas affected by lake-effect snow are called snowbelts. These include areas east of the Great Lakes, the west coasts of northern Japan, the Kamchatka Peninsula in Russia, and areas near the Great Salt Lake, Black Sea, Caspian Sea, Baltic Sea, and parts of the northern Atlantic Ocean.

Mountain effects
Orographic or relief snowfall is caused when masses of air pushed by wind are forced up the side of elevated land formations, such as large mountains. The lift of the air up the side of the mountain results in adiabatic cooling, and ultimately condensation and precipitation. Moisture is removed by orographic lift, leaving drier, warmer air on the descending, leeward side. The resulting enhanced productivity of snow fall and the decrease in temperature with elevation means that snow depth and seasonal persistence of snowpack increases with elevation in snow-prone areas.

Cloud physics
A snowflake consists of roughly 1019 water molecules, which are added to its core at different rates and in different patterns, depending on the changing temperature and humidity within the atmosphere that the snowflake falls through on its way to the ground. As a result, snowflakes vary among themselves, while following similar patterns.

Snow crystals form when tiny supercooled cloud droplets (about 10 μm in diameter) freeze. These droplets are able to remain liquid at temperatures lower than -18 °C, because to freeze, a few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice. Then the droplet freezes around this "nucleus". In warmer clouds an aerosol particle or "ice nucleus" must be present in (or in contact with) the droplet to act as a nucleus. Ice nuclei are very rare compared to that cloud condensation nuclei on which liquid droplets form. Clays, desert dust and biological particles may be effective, although to what extent is unclear. Artificial nuclei include particles of silver iodide and dry ice, and these are used to stimulate precipitation in cloud seeding.

Once a droplet has frozen, it grows in the supersaturated environment—one where air is saturated with respect to ice when the temperature is below the freezing point. The droplet then grows by diffusion of water molecules in the air (vapor) onto the ice crystal surface where they are collected. Because water droplets are so much more numerous than the ice crystals due to their sheer abundance, the crystals are able to grow to hundreds of micrometers or millimeters in size at the expense of the water droplets by the Wegener–Bergeron–Findeisen process. The corresponding depletion of water vapor causes the ice crystals to grow at the droplets' expense. These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their mass, and may collide and stick together in clusters, or aggregates. These aggregates are snowflakes, and are usually the type of ice particle that falls to the ground. Although the ice is clear, scattering of light by the crystal facets and hollows/imperfections mean that the crystals often appear white in color due to diffuse reflection of the whole spectrum of light by the small ice particles.

Classification of snowflakes


Micrography of thousands of snowflakes from 1885 onward, starting with Wilson Alwyn Bentley, revealed the wide diversity of snowflakes within a classifiable set of patterns. Closely matching snow crystals have been observed.

Nakaya developed a crystal morphology diagram, relating crystal shape to the temperature and moisture conditions under which they formed, which is summarized in the following table: As Nakaya discovered, shape is also a function of whether the prevalent moisture is above or below saturation. Forms below the saturation line trend more towards solid and compact. Many more complex growth patterns also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei. If a crystal has started forming in a column growth regime, at around -5 C, and then falls into the warmer plate-like regime, then plate or dendritic crystals sprout at the end of the column, producing so called "capped columns".

Magono and Lee devised a classification of freshly formed snow crystals that includes 80 distinct shapes. They are listed in the following main categories (with symbol): They documented each with micrographs.
 * Needle crystal (N) – Subdivided into: Simple and combination of needles
 * Columnar crystal (C) – Subdivided into: Simple and combination of columns
 * Plate crystal (P) – Subdivided into: Regular crystal in one plane, plane crystal with extensions, crystal with irregular number of branches, crystal with 12 branches, malformed crystal, radiating assemblage of plane branches
 * Combination of columnar and plate crystals (CP) – Subdivided into: Column with plane crystal at both ends, bullet with plane crystals, plane crystal with spatial extensions at ends
 * Columnar crystal with extended side planes (S) – Subdivided into: Side planes, scalelike side planes, combination of side planes, bullets and columns
 * Rimed crystal (R) – Subdivided into: Rimed crystal, densely rimed crystal, graupellike crystal, graupel
 * Irregular snow crystal (I) – Subdivided into: Ice particle, rimed particle, broken piece from a crystal, miscellaneous
 * Germ of snow crystal (G) – Subdivided into: Minute column, germ of skeleton form, minute hexagonal plate, minute stellar crystal, minute assemblage of plates, irregular germ

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 and antarctic, the northern hemisphere, and alpine regions. The liquid equivalent of snowfall may be evaluated using a snow gauge or with a standard rain gauge, adjusted for winter by removal of an funnel and inner cylinder. Both types of gauges melt the accumulated snow and report the amount of water collected. At some automatic weather stations an ultrasonic snow depth sensor may be used to augment the precipitation gauge.

Snow events
Snow flurry, snow storm and blizzard describe snow events of increasing duration and intensity. A blizzard is a weather condition involving snow and has varying definitions in different parts of the world. In the United States, a blizzard occurs when two conditions are met for a period of three hours or more: A sustained wind or frequent gusts to 35 mph, and sufficient snow in the air to reduce visibility to less than 0.4 km. In Canada and the United Kingdom, the criteria are similar. While heavy snowfall often occurs during blizzard conditions, falling snow is not a requirement, as blowing snow can create a ground blizzard.

Snowstorm intensity may be categorized by visibility and depth of accumulation. Snowfall's intensity is determined by visibility, as follows:
 * Light: visibility greater than 1 km
 * Moderate: visibility restrictions between 0.5 km and 1 km
 * Heavy: visibility is less than 0.5 km

Snowfall is defined by the U.S. National Weather Service as a being the maximum depth of snow on a snowboard (typically a piece of plywood painted white) observed during a six-hour period. At the end of the six-hour period, all snow is cleared from the measuring surface. For a daily total snowfall, four six-hour snowfall measurements are summed. Snowfall can be very difficult to measure due to melting, compacting, blowing and drifting.

Distribution
Seasonal snow covers about nine percent of the earth's surface, mostly in the northern hemisphere, where seasonal snow covers about 40 million km2.

Records
Highest seasonal total snowfall – The world record for the highest seasonal total snowfall was measured in the United States at Mount Baker Ski Area, outside of the town Bellingham, Washington during the 1998–1999 season. Mount Baker received 2896 cm of snow, thus surpassing the previous record holder, Mount Rainier, Washington, which during the 1971–1972 season received 2850 cm of snow.

Highest seasonal average annual snowfall – The world record for the highest average annual snowfall is 1764 cm, measured in Sukayu Onsen, Japan for the period of 1981–2010.

Largest snowflakes – Guinness World Records list the world's largest snowflakes as those of January 1887 at Fort Keogh, Montana; allegedly one measured 38 cm wide.

Metamorphosis
After deposition, snow progresses on one of two paths that determine its fate, either melting or turning 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 places 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 500 kg/m³, which is roughly half 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, yet less dense and hard than glacial ice. Firn resembles caked sugar and is very resistant to shovelling. Its density generally ranges from 550 kg/m³ to 830 kg/m³, and it can often be found underneath the snow that accumulates at the head of a glacier. The minimum altitude that firn accumulates on a glacier is called the firn limit, firn line or snowline.

Drifting
When powdery, snow drifts with the wind from the location where it originally fell, forming deposits with a depth of several meters in isolated locations. After attaching to hillsides, blown snow can evolve into a snow slab, which is an avalanche hazard on steep 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. They occur in three major mechanisms:
 * Slab avalanches occur in snow that has been deposited, or redeposited by wind. They have the characteristic appearance of a block (slab) of snow cut out from its surroundings by fractures. These account for most back-country fatalities.
 * Powder snow avalanches result from a deposition of fresh dry powder and generate a powder cloud, which overlies a dense avalanche. They can exceed speeds of 300 kph, and masses of 10000000 tonnes; 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 track surface. The low speed of travel is due to the friction between the sliding surface of the track and the water saturated flow. Despite the low speed of travel (~10–40 km/h), 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 a glacier forms is called a cirque (corrie or cwm), a typically armchair-shaped geological feature, which collects and compresses through gravity the snow which falls into it. This snow collects and is compacted by the weight of the snow falling above it forming névé. Further crushing of the individual snowflakes and squeezing the air from the snow turns it into 'glacial ice'. This glacial ice will fill the cirque until it 'overflows' through a geological weakness or vacancy, such as the gap between two mountains. When the mass of snow and ice is sufficiently thick, it begins to move due to a combination of surface slope, gravity and pressure. On steeper slopes, this can occur with as little as 15 m (50 ft) of snow-ice.

Measurement and classification
Snow scientists typically 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 and are quoted in the following table: All are formed in cloud, except for rime, which is formed 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 of snowpacks with satellites and other platforms typically includes multi-spectral collection of imagery. Sophisticated 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.

Models
The science lead to predictive models that include snow deposition, snow melt, global climate change, and snow hydrology.

Effects on human activity
Snow effects 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, and humans find recreation in snowy landscapes.

Transportation


Snow affects the rights of way of highways, airfields and railroads. They share a common tool for clearing snow, the snow plow. 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. However, the key to maintaining a roadway that can accommodate traffic during and after a snow event is an effective anti-icing program that employs both chemicals and plowing. The FHWA Manual of Practice for an Effective Anti-icing Program emphasizes "anti-icing" procedures that prevent the bonding of snow and ice to the road. Key aspects of the practice include: understanding anti-icing in light of the level of service to be achieved on a given roadway, the climatic conditions to be encountered, and the different roles of deicing, anti-icing, and abrasive materials and applications, and employing anti-icing "toolboxes", one for operations, one for decision-making and another for personnel. The elements to the toolboxes are: The manual offers matrices that address different types of snow and the rate of snowfall to tailor applications appropriately and efficiently.
 * 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.

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 six or more hectares of area to be cleared.

Aircraft are able to fly through snowstorms under Instrument Flight Rules. Prior to takeoff, during snowstorms they require deicing fluid to prevent accumulation and freezing of snow and other precipitation on wings and fuselages, which may compromise the safety of the aircraft and its occupants. In flight, aircraft rely on a variety of mechanisms to avoid rime and other types of icing in clouds, these include pulsing pneumatic boots, electro-thermal areas that generate heat, and fluid deicers that bleed onto the 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) has 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

Agriculture
Snowfall can be beneficial to agriculture by serving as a thermal insulator, conserving the heat of the Earth and protecting crops from subfreezing weather. Some agricultural areas depend on an accumulation of snow during winter that will melt gradually in spring, providing water for crop growth. If it melts into water and refreezes upon sensitive crops, such as oranges, the resulting ice will protect the fruit from exposure to lower temperatures.

Structures
Snow is an important consideration for loads on structures. Europe employs Eurocode 1: Actions on structures - Part 1-3: General actions - Snow loads to address these. In North America, ASCE Minimum Design Loads for Buildings and Other Structures gives guidance on snow loads. Both standards employ method that translate maximum expected ground snow loads onto design loads for roofs.

Roofs
The Minimum Design Loads for Buildings and Other Structures gives guidance on how to translate the following factors into roof snow loads: 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.
 * Ground snow loads
 * Exposure of the roof
 * Thermal properties of the roof
 * Shape of the roof
 * Drifting
 * Importance of the building

Roofs must also be designed to avoid ice dams, which result from meltwater running under the snow on the roof and freezing at the eave. An ice dam is a problem of house and building maintenance on sloping roofs in cold climates. 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. Ice dams may resulti in damaged building materials or in damage or injury when the ice dam falls off or from attempts to remove ice dams. The melting results from heat passing through the roof under the highly insulating layer of snow. .

Utility lines
Utility lines on poles are typically not at risk to collapse due to snow loads on their structures. Instead, the are subject to damage from trees falling on them, damaged by heavy, wet snow. Snow with a snow-water equivalent (SWE) ratio of between 6:1 and 12:1 (in extreme cases, as heavy as 4:1) and a weight in excess of 10 pounds per square foot (~40 kg/m2) are likely to collapse trees—especially those with leaves on them.

Sport


Snow figures into many winter sports, including skiing and sledding. Common examples include cross-country skiing, Alpine skiing, snowboarding, snow shoeing, and snowmobiling. The design of the equipment used, typically relies on the bearing strength of snow, as with skis or snowboards and contends with the coefficient of friction of snow to allow sliding, often enhance by ski waxes.

By far the largest form of winter recreation is skiing. 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 of these 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 skier visits. The preponderant region for downhills skiing was Europe, followed by Japan and the US.

The ability of a ski or other runner to slide over snow 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.

Other
Snowfall can have a small negative effect on yearly yield from solar photovoltaic systems.

Effects on ecosystems
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 effects 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.

Forms
Once on the ground, snow can be categorized as powdery when light and fluffy, fresh when recent but heavier, granular when it begins the cycle of melting and refreezing, and eventually ice once it comes down, after multiple melting and refreezing cycles, into a dense mass called snowpack. When powdery, snow moves with the wind from the location where it originally landed, forming deposits called snowdrifts that may have a depth of several meters. After attaching itself to hillsides, blown snow can evolve into a snow slab—an avalanche hazard on steep slopes. The existence of a snowpack keeps temperatures lower than they would be otherwise, as the whiteness of the snow reflects most sunlight, and any absorbed heat goes into melting the snow rather than increasing its temperature. The water equivalent of snowfall is measured to monitor how much liquid is available to flood rivers from meltwater that will occur during the following spring. Snow cover can protect crops from extreme cold. If snowfall stays on the ground for a series of years uninterrupted, the snowpack develops into a mass of ice called glacier. Fresh snow absorbs sound, lowering ambient noise over a landscape because the trapped air between snowflakes attenuates vibration. These acoustic qualities quickly minimize and reverse, once a layer of freezing rain falls on top of snow cover. Walking across snowfall produces a squeaking sound at low temperatures.

The energy balance of the snowpack itself is dictated by several heat exchange processes. The snowpack absorbs solar shortwave radiation that is partially blocked by cloud cover and reflected by snow surface. A long-wave heat exchange takes place between the snowpack and its surrounding environment that includes overlying air mass, tree cover and clouds. Heat exchange takes place by convection between the snowpack and the overlaying air mass, and it is governed by the temperature gradient and wind speed. Moisture exchange between the snowpack and the overlying air mass is accompanied by latent heat transfer that is influenced by vapor pressure gradient and air wind. Rain on snow can add significant amounts of thermal energy to the snowpack. A generally insignificant heat exchange takes place by conduction between the snowpack and the ground. The small temperature change from before to after a snowfall is a result of the heat transfer between the snowpack and the air. As snow degrades, its surface can develop characteristic ablation textures such as suncups or penitentes.

The term snow storm can describe a heavy snowfall, while a blizzard involves snow and wind, obscuring visibility. Snow shower is a term for an intermittent snowfall, while flurry is used for very light, brief snowfalls. Snow can fall more than a meter at a time during a single storm in flat areas, and meters at a time in rugged terrain, such as mountains. When snow falls in significant quantities, travel by foot, car, airplane and other means becomes severely restricted, but other methods of mobility become possible, such as the use of snowmobiles, snowshoes and skis. When heavy snow occurs early in the fall (or, on rarer occasions, late in the spring), significant damage can occur to trees still in leaf. Areas with significant snow each year can store the winter snow within an ice house, which can be used to cool structures during the following summer. A variation on snow has been observed on Venus, though composed of metallic compounds and occurring at a substantially higher temperature.

Cause


Extratropical cyclones can bring cold and dangerous conditions with heavy rain and snow with winds exceeding 119 km/h, (sometimes referred to as windstorms in Europe). The band of precipitation that is associated with their warm front is often extensive, forced by weak upward vertical motion of air over the frontal boundary, which condenses as it cools off and produces precipitation within an elongated band, which is wide and stratiform, meaning falling out of nimbostratus clouds. When moist air tries to dislodge an arctic air mass, overrunning snow can result within the poleward side of the elongated precipitation band. In the Northern Hemisphere, poleward is towards the North Pole, or north. Within the Southern Hemisphere, poleward is towards the South Pole, or south.

Within the cold sector, poleward and west of the cyclone center, small scale or mesoscale bands of heavy snow can occur within a cyclone's comma head pattern. The cyclone's comma head pattern is a comma-shaped area of clouds and precipitation found around mature extratropical cyclones. These snow bands typically have a width of 20 to 50 mi. These bands in the comma head are associated with areas of frontogenesis, or zones of strengthening temperature contrast.

Southwest of extratropical cyclones, curved cyclonic flow bringing cold air across the relatively warm water bodies can lead to narrow lake-effect snow bands. Those bands bring strong localized snowfall, which can be understood as follows: Large water bodies such as lakes efficiently store heat that results in significant temperature differences (larger than 13 °C [23 °F]) between the water surface and the air above. Because of this temperature difference, warmth and moisture are transported upward, condensing into vertically oriented clouds (see satellite picture) that produce snow showers. The temperature decrease with height and cloud depth are directly affected by both the water temperature and the large-scale environment. The stronger the temperature decrease with height, the deeper the clouds get, and the greater the precipitation rate becomes.

In mountainous areas, heavy snowfall accumulates when air is forced to ascend the mountains, and any precipitation usually falls along their windward slopes, which in cold conditions, falls in the form of snow. Because of the ruggedness of terrain, forecasting the location of heavy snowfall remains a significant challenge.

Snowflakes
A snowflake consists of roughly 1019 water molecules, which are added to its core at different rates and in different patterns, depending on the changing temperature and humidity within the atmosphere that the snowflake falls through on its way to the ground. As a result, snowflakes vary among themselves, while following similar patterns. Micrography of thousands of snowflakes from 1885 onward, starting with Wilson Alwyn Bentley, revealed the wide diversity of snowflakes within a classifiable set of patterns. Closely matching snow crystals have been observed.

Snow crystals form when tiny supercooled cloud droplets (about 10 μm in diameter) freeze. These droplets are able to remain liquid at temperatures lower than -18 °C, because to freeze, a few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice. Then the droplet freezes around this "nucleus". In warmer clouds an aerosol particle or "ice nucleus" must be present in (or in contact with) the droplet to act as a nucleus. Ice nuclei are very rare compared to that cloud condensation nuclei on which liquid droplets form. Clays, desert dust and biological particles may be effective, although to what extent is unclear. Artificial nuclei include particles of silver iodide and dry ice, and these are used to stimulate precipitation in cloud seeding.

Once a droplet has frozen, it grows in the supersaturated environment—one where air is saturated with respect to ice when the temperature is below the freezing point. The droplet then grows by diffusion of water molecules in the air (vapor) onto the ice crystal surface where they are collected. Because water droplets are so much more numerous than the ice crystals due to their sheer abundance, the crystals are able to grow to hundreds of micrometers or millimeters in size at the expense of the water droplets by the Wegener–Bergeron–Findeisen process. The corresponding depletion of water vapor causes the ice crystals to grow at the droplets' expense. These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their mass, and may collide and stick together in clusters, or aggregates. These aggregates are snowflakes, and are usually the type of ice particle that falls to the ground. Guinness World Records list the world's largest snowflakes as those of January 1887 at Fort Keogh, Montana; allegedly one measured 38 cm wide. Although the ice is clear, scattering of light by the crystal facets and hollows/imperfections mean that the crystals often appear white in color due to diffuse reflection of the whole spectrum of light by the small ice particles.

The shape of the snowflake is determined broadly by the temperature and humidity at which it is formed. The most common snow particles are visibly irregular. Planar crystals (thin and flat) grow in air between 0 C and -3 C. Between -3 C and -8 C, the crystals will form needles or hollow columns or prisms (long thin pencil-like shapes). From -8 C to -22 C the shape reverts to plate-like, often with branched or dendritic features. At temperatures below -22 C, the crystal development becomes column-like, although many more complex growth patterns also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei. If a crystal has started forming in a column growth regime, at around -5 C, and then falls into the warmer plate-like regime, then plate or dendritic crystals sprout at the end of the column, producing so called "capped columns".

Types
Types of snow can be designated by the shape of the flakes, the rate of accumulation, and the way the snow collects on the ground. Types that fall in the form of a ball due to melting and refreezing cycles, rather than a flake, are known as graupel, with ice pellets and snow pellets as types of graupel associated with wintry precipitation. Once on the ground, snow can be categorized as powdery when fluffy, granular when it begins the cycle of melting and refreezing, and eventually ice once it packs down into a dense drift after multiple melting and refreezing cycles. When powdery, snow drifts with the wind from the location where it originally fell, forming deposits with a depth of several meters in isolated locations. Snow fences are constructed in order to help control snow drifting in the vicinity of roads, to improve highway safety. After attaching to hillsides, blown snow can evolve into a snow slab, which is an avalanche hazard on steep slopes. A frozen equivalent of dew known as hoar frost forms on a snowpack when winds are light and there is ample low-level moisture over the snowpack.

Snowfall's intensity is determined by visibility. When the visibility is over 1 km, snow is considered light. Moderate snow describes snowfall with visibility restrictions between 0.5 and 1 km. Heavy snowfall describes conditions when visibility is less than 0.5 km. Steady snows of significant intensity are often referred to as "snowstorms". When snow is of variable intensity and short duration, it is described as a "snow shower". The term snow flurry is used to describe the lightest form of a snow shower.

A blizzard is a weather condition involving snow and has varying definitions in different parts of the world. In the United States, a blizzard occurs when two conditions are met for a period of three hours or more: A sustained wind or frequent gusts to 35 mph, and sufficient snow in the air to reduce visibility to less than 0.4 km. In Canada and the United Kingdom, the criteria are similar. While heavy snowfall often occurs during blizzard conditions, falling snow is not a requirement, as blowing snow can create a ground blizzard.



Density
Snow remains on the ground until it melts or sublimates. Sublimation of snow directly into water vapor is most likely to occur on a dry and windy day such as when a strong downslope wind, such as a Chinook wind, exists.

Once the snow is on the ground, it will settle under its own weight (largely due to differential evaporation) 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. When the snow does not all melt in the summer it evolves into firn, where individual granules become more spherical in nature, evolving into a glacier as the ice flows downhill.

Snow water equivalent
The snow water equivalent is the product of snow depth and the snow bulk density. It is a quantity of type columnar mass density, having units of area density (kg/m2), though it is usually reported normalized by the volumetric density of liquid water (units kg/m3), thus being expressed in units of length (e.g., millimeter or inches). It corresponds to the depth of a layer of water that would accumulate in an area, if all the snow and ice were melted in that given area. For example, if the snow covering a given area has a water equivalent of 50 cm, then it will melt into a pool of water 50 cm deep covering the same area. This is a much more useful measurement to hydrologists than snow depth, as the density of cool freshly fallen snow widely varies. New snow commonly has a density of around 8% of water. This means that 33 cm of snow melts down to 2.5 cm of water. Cloud temperatures and physical processes in the cloud affect the shape of individual snow crystals. Highly branched or dendritic crystals tend to have more space between the arms of ice that form the snowflake and this snow will therefore have a lower density, often referred to as "dry" snow. Conditions that create columnar or plate-like crystals will have much less air space within the crystal and will therefore be denser and feel "wetter".

Acoustic properties
Newly fallen snow acts as a sound-absorbing material, which minimizes sound over its surface. This is due to the trapped air between the individual crystalline flakes, trapping sound waves and dampening vibrations. Once it is blown around by the wind and exposed to sunshine, snow hardens and its sound-softening quality diminishes. Snow cover as thin as 2 cm thick changes the acoustic properties of a landscape. Studies concerning the acoustic properties of snow have revealed that loud sounds, such as from a pistol, can be used to measure snow cover permeability and depth. Within motion pictures, the sound of walking through snow is simulated using cornstarch, salt, or cat litter. When the temperature falls below -10 C, snow will squeak when walked upon due to the crushing of the ice crystals within the snow. If covered by a layer of freezing rain, the hardened frozen surface acts to echo sounds, similar to concrete.

From under water, snowfall has a unique sound when compared to other forms of precipitation, and the sound varies little with differences in the snowflakes' size and shape.



Snowfall measurement


Snowfall is defined by the U.S. National Weather Service as a being the maximum depth of snow on a snowboard (typically a piece of plywood painted white) observed during a six-hour period. At the end of the six-hour period, all snow is cleared from the measuring surface. For a daily total snowfall, four six-hour snowfall measurements are summed. Snowfall can be very difficult to measure due to melting, compacting, blowing and drifting.

The liquid equivalent of snowfall may be evaluated using a snow gauge or with a standard rain gauge having a diameter of 100 mm (4 in; plastic) or 200 mm (8 in; metal). Rain gauges are adjusted to winter by removing the funnel and inner cylinder and allowing the snow/freezing rain to collect inside the outer cylinder. Antifreeze liquid may be added to melt the snow or ice that falls into the gauge. In both types of gauges once the snowfall/ice is finished accumulating, or as its height in the gauge approaches 300 mm, the snow is melted and the water amount recorded.

Another type of gauge used to measure the liquid equivalent of snowfall is the weighing precipitation gauge. The wedge and tipping bucket gauges will have problems with snow measurement. Attempts to compensate for snow/ice by warming the tipping bucket meet with limited success, since snow may sublimate if the gauge is kept much above the freezing temperature. Weighing gauges with antifreeze should do fine with snow, but again, the funnel needs to be removed before the event begins. At some automatic weather stations an ultrasonic snow depth sensor may be used to augment the precipitation gauge.

Spring snow melt is a major source of water supply to areas in temperate zones near mountains that catch and hold winter snow, especially those with a prolonged dry summer. In such places, water equivalent is of great interest to water managers wishing to predict spring runoff and the water supply of cities downstream. Measurements are made manually at marked locations known as snow courses, and remotely using special scales called snow pillows.

When a snow measurement is made, various networks exist across the United States and elsewhere where rainfall measurements can be submitted through the Internet, such as CoCoRAHS or GLOBE. If a network is not available in the area where one lives, the nearest local weather office will likely be interested in the measurement.

Records
The world record for the highest seasonal total snowfall was measured in the United States at Mount Baker Ski Area, outside of the town Bellingham, Washington during the 1998–1999 season. Mount Baker received 2896 cm of snow, thus surpassing the previous record holder, Mount Rainier, Washington, which during the 1971–1972 season received 2850 cm of snow.

The world record for the highest average yearly snowfall is 1764 cm, measured in Sukayu Onsen, Japan for the period of 1981–2010.

Relation to river flow
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.

Effects on human society


Substantial snowfall can disrupt public infrastructure and services, slowing human activity even in regions that are accustomed to such weather. Air and ground transport may be greatly inhibited or shut down entirely. Populations living in snow-prone areas have developed various ways to travel across the snow, such as skis, snowshoes, and sleds pulled by horses, dogs, or other animals and later, snowmobiles. Basic utilities such as electricity, telephone lines, and gas supply can also fail. In addition, snow can make roads much harder to travel and vehicles attempting to use them can easily become stuck. Snowfall can have a small negative effect on yearly yield from solar photovoltaic systems.

The combined effects can lead to a "snow day" on which gatherings such as school or work are officially canceled. In areas that normally have very little or no snow, a snow day may occur when there is only light accumulation or even the threat of snowfall, since those areas are unprepared to handle any amount of snow. In some areas, such as some states in the United States, schools are given a yearly quota of snow days (or "calamity days"). Once the quota is exceeded, the snow days must be made up. In other states, all snow days must be made up. For example, schools may extend the remaining school days later into the afternoon, shorten spring break, or delay the start of summer vacation.

Accumulated snow is removed to make travel easier and safer, and to decrease the long-term impact of a heavy snowfall. This process utilizes shovels, snowplows and snow blowers and is often assisted by sprinkling salt or other chloride-based chemicals, which reduce the melting temperature of snow. In some areas with abundant snowfall, such as Yamagata Prefecture, Japan, people harvest snow and store it surrounded by insulation in ice houses. This allows the snow to be used through the summer for refrigeration and air conditioning, which requires far less electricity than traditional cooling methods.

Agriculture
Persistent snow and glaciers represent important sources of water feeding rivers in Africa, Americas and the Eurasian continent, which become the source of irrigation in agriculture. Snowfall can be beneficial to agriculture by serving as a thermal insulator, conserving the heat of the Earth and protecting crops from subfreezing weather. Some agricultural areas depend on an accumulation of snow during winter that will melt gradually in spring, providing water for crop growth. If it melts into water and refreezes upon sensitive crops, such as oranges, the resulting ice will protect the fruit from exposure to lower temperatures.

Recreation
Many winter sports, such as skiing, snowboarding, snowmobiling, and snowshoeing depend upon snow. Where snow is scarce but the temperature is low enough, snow cannons may be used to produce an adequate amount for such sports. Children and adults can play on a sled or ride in a sleigh. Although a person's footsteps remain a visible lifeline within a snow-covered landscape, snow cover is considered a general danger to hiking since the snow obscures landmarks and makes the landscape itself appear uniform.

One of the recognizable recreational uses of snow is in building snowmen. A snowman is created by making a man shaped figure out of snow – often using a large, shaped snowball for the body and a smaller snowball for the head which is often decorated with simple household items – traditionally including a carrot for a nose, and coal for eyes, nose and mouth; occasionally including old clothes such as a top hat or scarf.

Snow can be used to make snow cones, also known as snowballs, which are usually eaten in the summer months. Flat areas of snow can be used to make snow angels, a popular pastime for children.

Snow can be used to alter the format of outdoor games such as capture the flag, or for snowball fights. The world's biggest snowcastle, the SnowCastle of Kemi, is built in Kemi, Finland every winter. Since 1928 Michigan Technological University in Houghton, Michigan has held an annual winter carnival in mid-February, during which a large snow sculpture contest takes place between various clubs, fraternities, and organizations in the community and the university. Each year there is a central theme, and prizes are awarded based on creativity. Snowball softball tournaments are held in snowy areas, usually using a bright orange softball for visibility, and burlap sacks filled with snow for the bases.

Damage
When heavy, wet snow with a snow-water equivalent (SWE) ratio of between 6:1 and 12:1 (in extreme cases, as heavy as 4:1) and a weight in excess of 10 pounds per square foot (~40 kg/m2) piles onto trees or electricity lines – particularly if the trees have full leaves or are not adapted to snow – significant damage may occur on a scale usually associated with hurricanes. An avalanche can occur upon a sudden thermal or mechanical impact upon snow that has accumulated on a mountain, which causes the snow to rush downhill en masse. Preceding an avalanche is a phenomenon known as an avalanche wind caused by the approaching avalanche itself, which adds to its destructive potential. Large amounts of snow which accumulate on top of man-made structures can lead to structural failure. During snowmelt, acidic precipitation which previously fell into the snowpack is released, which harms marine life.

Design of structures considering snow load
The designs of all structures and buildings use the ground snow load determined by professional engineers and designers. Data on ground snow in the U.S.A. are provided by the American Society of Civil Engineers (ASCE7-latest edition) for most jurisdictions. This load is typically the governing design factor on roofs and structural elements exposed to the effects of snow in the northern United States. Closer to the Equator, the snow load becomes less important and may or may not be the governing factor.

Extraterrestrial snow
Very light snow is known to occur at high latitudes on Mars. A "snow" of hydrocarbons is also theorized to occur on Saturn's moon Titan.

While there is little or no water on Venus, there is a phenomenon which is quite similar to snow. The Magellan probe imaged a highly reflective substance at the tops of Venus's highest mountain peaks which bore a strong resemblance to terrestrial snow. This substance arguably formed from a similar process to snow, albeit at a far higher temperature. Too volatile to condense on the surface, it rose in gas form to cooler higher elevations, where it then fell as precipitation. The identity of this substance is not known with certainty, but speculation has ranged from elemental tellurium to lead sulfide (galena).