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

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 color unless contaminated by impurities.

Geometry

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. 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 and . The vapor droplets solidify around a dust particle. Between temperatures of and , the snowflake will be in the form of a dendrite or a plate or the six petaled ice flower. As temperatures get colder, between and , the crystals will form in needles or hollow columns or prisms. When the temperature becomes even colder from to the ice flowers are formed again, and at temperatures below , the vapors will turn into prisms again. If a crystal has started forming at around , 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.

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

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 1998–1999 season. Mount Baker received 1,140 inches (29 m) of snow, thus surpassing the previous record holder, Mount Rainier, Washington, which during the 1971–1972 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

• 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.

Types of snow

Falling snow

: A long-lasting snow storm with intense snowfall and usually high winds. Particularly severe storms can create whiteout conditions where visibility is reduced to less than 1 m.:A class of snow flakes that is shaped like a six sided column. One of the 4 classes of snow flakes.:A class of snow flakes that has 6 points, making it somewhat star shaped. The classic snow flake shape. One of the 4 classes of snow flakes.: A period of light snow with usually little accumulation with occasional moderate snowfall.: Supercooled rain that freezes on impact with a sufficiently cold surface. This can cover trees in a uniform layer of very clear, shiny ice – a beautiful phenomenon, though excessive accumulation can break tree limbs and utility lines, causing utility failures and possible property damage.

• Cold wave
• Frost
• Grit bin
• Igloo
• Lake effect snow
• Meltwater
• Snow gauge
• Snowbelt
• SnowCastle of Kemi
• Snow removal
• Snow sculpture
• Snow blindness
• Eskimo words for snow
• Wilson "Snowflake" Bentley
• Alberta Clipper

References

External links

• Snowpack energy and mass balance An article that contains detailed analysis of snowpack energy and mass balance.
• [ftp://198.77.171.17/pub/High%20resolution%20TIFF%20Snow%20Images%20from%20webpage Ultra-high resolution images] of snowflakes, hosted by the Electron Microscopy Unit of the USDA Beltsville Agricultural Research Center
• Snow at above freezing temperatures
• Kenneth G. Libbrecht - Snowflake FAQ
• United Nations Environment Programme: Global Outlook for Ice and Snow