Classifications of snow


Classifications of snow describe and categorize the attributes of snow-generating weather events, including the individual crystals both in the air and on the ground, and the deposited snow pack as it changes over time. Snow can be classified by describing the weather event that is producing it, the shape of its ice crystals or flakes, how it collects on the ground, and thereafter how it changes form and composition. Depending on the status of the snow in the air or on the ground, a different classification applies.
Snowfall arises from a variety of events that vary in intensity and cause, subject to classification by weather bureaus. Some snowstorms are part of a larger weather pattern. Other snowfall occurs from lake effects or atmospheric instability near mountains. Falling snow takes many different forms, depending on atmospheric conditions, especially vapor content and temperature, as it falls to the ground. Once on the ground, snow crystals metamorphose into different shapes, influenced by wind, freeze-thaw and sublimation. Snow on the ground forms a variety of shapes, formed by wind and thermal processes, all subject to formal classifications both by scientists and by ski resorts. Those who work and play in snowy landscapes have informal classifications, as well.
There is a long history of northern and alpine cultures describing snow in their different languages, including Inupiat, Russian and Finnish. However, the lore about the multiplicity of Eskimo words for snow originates from controversial scholarship on a topic that is difficult to define, because of the structures of the languages involved.

Classification of snow events

Snow events reflect the type of storm that generates them and the type of precipitation that results. Classification systems use rates of deposition, types of precipitation, visibility, duration and wind speed to characterize such events.

Snow-producing events

The following terms are consistent with the classifications of United States National Weather Service and the Meteorological Service of Canada:
  • Blizzard – Characterized by sustained wind or frequent gusts of or greater and falling or blowing snow that frequently lowers visibility to less than over a period of 3 hours or longer.
  • Cold front – The leading edge of unstable cold air, replacing warmer, circulating around an extratropical cyclone, which may cause instability snow showers or squalls.
  • Extratropical cyclone – May cause snow in the winter, especially in its northwest quadrant where the wind comes from the northeast.
  • Lake-effect snow – Occurs when relatively cold air flows over warm lake water to cause localized, convective snow bands.
  • Mountain snow – Orographic lift causes moist air to rise upslope on mountains to where freezing temperatures cause orographic snow.
  • Snow flurry – An intermittent, light snowfall event of short duration with only a trace level of accumulation.
  • Snowsquall – A brief but intense period of moderate to heavy snowfall with strong, gusty surface winds and measurable snowfall.
  • Thundersnow – Occurs when a snowstorm generates lightning and thunder. It may occur in areas that are prone to a combination of wind and moisture triggers that promote instability, often downwind of lakes or in mountainous terrain. It may occur with intensifying extratropical cyclones. Such events are often associated with intense snowfall.
  • Warm front – Snow may fall as warm air initially over-rides cold in a warm front, circulating around an extratropical cyclone.
  • Winter storm – May constitute any combination of sleet, snow, ice, and wind that accumulates or more of snow in 12 hours or less; or or more in 24 hours or of ice.

    Precipitation

Precipitation may be characterized by type and intensity.

Type

Frozen precipitation includes snow, snow pellets, snow grains, ice crystals, ice pellets, and hail. Falling snow comprises ice crystals, growing in a hexagonal pattern and combining as snowflakes. Ice crystals may be "any one of a number of macroscopic, crystalline forms in which ice appears, including hexagonal columns, hexagonal platelets, dendritic crystals, ice needles, and combinations of these forms". Terms that refer to falling snow particles include:
  • Ice crystals – Suspended in the atmosphere as needles, columns or plates at very low temperatures in a stable atmosphere.
  • Ice pellets – Two manifestations, sleet and small hail, that result in irregular spherical particles, which typically bounce upon impact. Sleet comprises grains of ice that form from refreezing of largely melted snowflakes when falling through into a frozen layer of air near the surface. Small hail forms from snow pellets encased in a thin layer of ice caused either by accretion of droplets or by refreezing of each particle's surface.
  • Hail – Forms in cumulonimbus clouds as irregular spheres of ice with a diameter of 5 mm or more.
  • Snowflake – Grows from a single ice crystal and may have agglomerated with other crystals as it falls.
  • Snow grain – Flattened and elongated agglomerations of crystals, typically less than 1 mm diameter, that include a range of crystal sizes and complexities to include a rime core and glaze coating. They typically originate in stratus clouds or from fog and fall in small quantities, not in showers.
  • Snow pellets – Spherical or conical ice particles, based on a snowlike structure, with diameters between 2 mm and 5 mm. They form by accretion of supercooled droplets near or slightly below the freezing point and rebound off hard surfaces upon landing.

    Intensity

In the US, the intensity of snowfall is characterized by visibility through the falling precipitation, as follows:
  • Light snow: visibility of or greater
  • Moderate snow: visibility between and
  • Heavy snow: visibility of less than

    Snow crystal classification

Ice approximates hexagonal symmetry in most of its atmospheric manifestations of a crystal lattice as snow. Temperature and vapor pressure determine the growth of the hexagonal crystal lattice in different forms that include columnar growth in the axis perpendicular to the hexagonal plane to form snow crystals. Ukichiro Nakaya developed a crystal morphology diagram, relating crystal shape to the temperature and moisture conditions under which they formed. Magono and Lee devised a classification of freshly formed snow crystals that includes 80 distinct shapes. They are summarized in the following principal snow crystal categories :
  • Needle : Snow crystals may be simple or a combination of needles.
  • Column : Snow crystals may be simple or a combination of columns.
  • Plate : Snow crystals may be a regular crystal in one plane, a plane crystal with extensions, a crystal with irregular number of branches, crystal with 12 branches, malformed crystal, radiating assemblage of plane branches.
  • Column and plate combination : Snow crystals may be a column with plane crystal at both ends, a bullet with plane crystals, a plane crystal with spatial extensions at ends.
  • Side plane : Snow crystals may have extended side planes, some scalelike side planes, and some a combination of side planes, bullets, and columns.
  • Rime : Rimed crystals may be densely rimed crystals, graupel-like crystals, or graupel.
  • Irregular : Snow crystals include ice particles, rimed particles, broken pieces from a crystal, and miscellaneous crystals.
  • Germ : Crystals may be a minute column, hexagonal plate, stellar crystal, assemblage of plates, irregular germ, or other skeletal form.

    Classifications of snow on the ground

Classification of snow on the ground comes from two sources: the science community and the community of those who encounter it in their daily lives. Snow on the ground exists both as a material with varying properties and as a variety of structures, shaped by wind, sun, temperature, and precipitation.

Classification of snowpack material properties

The International Classification for Seasonal Snow on the Ground describes snow crystal classification, once it is deposited on the ground, that include grain shape and grain size. The system also characterizes the snowpack, as the individual crystals metamorphize and coalesce. It uses the following characteristics to describe deposited snow: microstructure, grain shape, grain size, snow density, snow hardness, liquid water content, snow temperature, impurities, and layer thickness. The grain shape is further characterized, using the following categories : precipitation particles, machine-made snow, decomposing and fragmented precipitation particles, rounded grains, faceted crystals, depth hoar, surface hoar, melt forms, and ice formations. Other measurements and characteristics are used as well, including a snow profile of a vertical section of the snowpack. Some snowpack features include:
  • Crust – A variety of processes can create a crust, a layer of snow on the surface of the snowpack that is stronger than the snow below, which may be powder snow. Crusts often result from partial melting of the snow surface by direct sunlight or warm air followed by re-freezing, but can also be created by wind or by surface water. Snow travelers consider the thickness and resulting strength of a crust to determine whether it is "unbreakable", meaning that they will support the weight of the traveler or "breakable", meaning that it will not.
  • Depth hoar – Depth hoar comprises faceted snow crystals, usually poorly or completely unbonded to adjacent crystals, creating a weak zone in the snowpack. Depth hoar forms from metamorphism of the snowpack in response to a large temperature gradient between the warmer ground beneath the snowpack and the surface. The relatively high porosity, relatively warm temperature, and unbonded weak snow in this layer can allow various organisms to live in it.
  • Machine-made – Machine-made artificial snow has two classifications: round, polycrystalline particles, which are produced by the freezing of water droplets expelled from a snow cannon, and shard-like ice plates, which are produced by the shaving of ice.
  • Surface hoar – Surface hoar is manifest as striated, usually flat, sometimes needle-like crystals, usually deposited as frost on a snow surface that is colder than the air. Crystals grow rapidly by transfer of moisture from the atmosphere onto the snow surface, which is cooled below ambient temperature by radiational cooling. Subsequent snowfall can bury layers of surface hoar, incorporating them into the snowpack where they can form a weak layer.