# Snow

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Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. The process of precipitation is called snowfall.

Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. The METAR code for snow is SN.

## Snowflakes

Animation of snowcover changing with the seasons
Trees covered with snow

Snow forms when water vapor condenses directly into ice crystals, usually in a cloud. Floating cloud particles ( ice nucleators, often of biological origin ) are needed in order for snowflakes to form at temperatures above -40C. 85% of these nuclei are airborne bacteria, with dust particles making up the rest. The ice crystals which form around the ice nucleators typically have a diameter of several milimetres and usually have six lines of symmetry. A snowflake is an aggregate of such ice crystals and may be several centimeters large. The term "snowflake" is also used below for the symmetrical ice crystals themselves. The individual ice crystals are clear but because of the amount of light the individual crystals reflect snowflakes appear white in colour unless contaminated by impurities.

### Geometry

Symmetry of an idealized snowflake
The first of a magnification series of a snow crystal ( view the entire series) using a low temperature scanning electron microscope

Large, well formed snowflakes are relatively flat and have six approximately identical arms, so that the snowflake nearly has the same 6-fold dihedral symmetry as a regular hexagon or hexagram. This symmetry arises from the hexagonal crystal structure of ordinary ice. However, the exact shape of the snowflake is determined by the temperature and humidity at which it forms.. Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes. Snowflakes are not perfectly symmetrical however. The most common snowflakes are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing.

Snowflakes can come in many different forms, including columns, needles, bricks and plates (with and without " dendrites" - the "arms" of some snowflakes). These different forms arise out of different temperatures and water saturation - among other conditions. Six petaled ice flowers grow in air between 0 °C (32 °F) and −3 °C (27 °F). The vapor droplets solidify around a dust particle. Between temperatures of −1 °C (30 °F) and −3 °C (27 °F), the snowflake will be in the form of a dendrite or a plate or the six petaled ice flower. As temperatures get colder, between −5 °C (23 °F) and −10 °C (14 °F), the crystals will form in needles or hollow columns or prisms. When the temperature becomes even colder from −10 °C (14 °F) to −22 °C (−8 °F) the ice flowers are formed again, and at temperatures below −22 °C (−8 °F), the vapors will turn into prisms again. If a crystal has started forming at around −5 °C (23 °F), and is then exposed to warmer or colder temperatures, a capped column may be formed which consists of a column-like design capped with a dendrite or plate-like design on each end of the column. At even colder temperatures, the snowflake design returns to the more common dendrite and plate. At temperatures approaching −20 °C (−4 °F), sectored plates are formed which appears as a dendrite, with each dendrite appearing flattened, like the design of a snowflake plate.

Snowflakes by Wilson Bentley, 1902

There are, broadly, two possible explanations for the symmetry of snowflakes. First, there could be communication or information transfer between the arms, such that growth in each arm affects the growth in each other arm. Surface tension or phonons are among the ways that such communication could occur. The other explanation, which appears to be the prevalent view, is that the arms of a snowflake grow independently in an environment that is believed to be rapidly varying in temperature, humidity and other atmospheric conditions. This environment is believed to be relatively spatially homogeneous on the scale of a single flake, leading to the arms growing to a high level of visual similarity by responding in identical ways to identical conditions, much in the same way that unrelated trees respond to environmental changes by growing near-identical sets of tree rings. The difference in the environment in scales larger than a snowflake leads to the observed lack of correlation between the shapes of different snowflakes. The sixfold symmetry happens because of the basic hexagonal crystalline structure from which the snowflake grows. The exact reason for the threefold symmetry of triangular snowflakes is still a mystery although trigonal symmetry is a subsymmetry of hexagonal.

There is a widely held belief that no two snowflakes are alike. Strictly speaking, it is extremely unlikely for any two macroscopic objects in the universe to contain an identical molecular structure; but there are, nonetheless, no known scientific laws that prevent it. In a more pragmatic sense, it's more likely—albeit not much more—that two snowflakes are virtually identical if their environments were similar enough, either because they grew very near one another, or simply by chance. The American Meteorological Society has reported that matching snow crystals were discovered in Wisconsin in 1988 by Nancy Knight of the National Centre for Atmospheric Research. The crystals were not flakes in the usual sense but rather hollow hexagonal prisms.

Snowflakes

## Snow on the ground

Snow in the city of Rovaniemi, Finland
Record breaking snowfall March 2008 at Aubrey, Texas

Snow remains on the ground until it melts or sublimes. In colder climates this results in snow lying on the ground all winter; when the snow does not all melt in the summer it becomes glaciers.

This is often called snowpack, especially when it does persist a long time. The deepest snowpacks occur in mountainous regions. It is influenced by temperature and wind events which determine melting, accumulation and wind erosion.

The water equivalent of the snow is the thickness of a layer of water having the same content. For example, if the snow covering a given area has a water equivalent of 50 centimetres (20 in), then it will melt into a pool of water 50 centimetres (20 in) 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 between 5% and 15% of water. Snow that falls in maritime climates is usually denser than snow that falls in mid-continent locations because of the higher average temperatures over oceans than over land masses. 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 snow flake and this snow will therefore have a lower density, often referred to as "dry" snow. Conditions that create columnar or platelike crystals will have much less air space within the crystal and will therefore be more dense and feel "wetter".

First snow of winter, Truckee, California, United States

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 melting and refreezing, caused by temperatures above freezing or by direct solar radiation. By late spring, snow densities typically reach a maximum of 50% of water.

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.

Many rivers originating in mountainous or high-latitude regions have a significant portion of their flow from snowmelt. This often makes the river's flow highly seasonal resulting in periodic flooding. In contrast, if much of the melt is from glaciated or nearly glaciated areas, the melt continues through the warm season, mitigating that effect.

### Energy balance

The energy balance of the snowpack 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 longwave heat exchange takes place between the snowpack and its surrounding environment that includes overlaying air mass, tree cover and clouds. Convective (sensible) heat exchange between the snowpack and the overlaying air mass is governed by the temperature gradient and wind speed. Moisture exchange between the snowpack and the overlaying air mass is accompanied with latent heat transfer that is influenced by vapor pressure gradient and air wind. Rain on snow could induce significant heat input to the snowpack. A generally insignificant conductive heat exchange takes place between the snowpack and the underlying ground. That is the reason there is a small temperature rise after or before the snowfall.

### Effects on human society

Section of an icehouse

#### Activity

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. Basic infrastructures such as electricity, telephone lines, and gas supply can also fail. In addition, snow can make roads much harder to travel and cars attempting to traverse them can easily become stuck. The combined effects can lead to a " snow day" on which gatherings such as school, work, or church 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 ill-prepared to handle any amount of snow.

#### 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.

#### Conservation

In areas near mountains, people have harvested snow and stored it as layers of ice covered by straw or sawdust in icehouses. This allowed the ice to be used in summer for refrigeration or medical uses.

#### Damage

A mudslide, flash flood, or avalanche can occur when excessive snow has accumulated on a mountain and there is a sudden change of temperature. Large amounts of snow that accumulate on top of man-made structures can lead to structural failure.

## Records

The highest seasonal total snowfall ever measured was at Mount Baker Ski Area, outside of the town Bellingham, Washington in the United States during the 19981999 season. Mount Baker received 1,140 inches (29 m) of snow, thus surpassing the previous record holder, Mount Rainier, Washington, which during the 19711972 season received 1,122 in. (28.5 m) of snow. Guinness World Records list the world’s largest snowflakes as those of January 1887 at Fort Keogh, Montana;. allegedly one measured 15 inches (38 cm) wide.

## Recreation

Building a snowman.
• Many winter sports, such as skiing, snowboarding, snowmobiling and snowshoeing depend on 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 can play on a sled or ride in a sleigh.
• Snow can be sculptured into snowmen, used to trace the motion of a person's body ( snow angels), or formed into snowballs for throwing or for having snowball fights.
• Snow can be used to build defensive snow forts for outdoor games such as Capture the flag.
• 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.