Classifications of snow

Classification of snow events

Snow-producing events

• Blizzard — Blizzards are 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 — A cold front is the leading edge of unstable cold air, replacing warmer, circulating around an extratropical cyclone, which may cause instability snow showers or squalls.
• Extratropical cyclone — A winter extratropical cyclone may cause snow, especially in its northwest quadrant where the wind comes from the northeast.
• Lake-effect snow — Lake-effect or ocean-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 — Snow flurries are an intermittent, light snowfall events of short duration with only a trace level of accumulation.
• Snowsquall — Snowsqualls are brief, but intense periods of moderate to heavy snowfall with strong, gusty surface winds and measurable snowfall.
• Thundersnow — 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 — Winter storms 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

Type

• Ice crystal — Ice crystals are manifest suspended in the atmosphere as needles, columns or plates at very low temperatures in a stable atmosphere.
• Ice pellet — Ice pellets have 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 — Hail forms in cumulonimbus clouds as irregular spheres of ice with a diameter of 5 mm or more.
• Snowflake — A snowflake grows from a single ice crystal and may have agglomerated with other crystals as it falls.
• Snow grain — Snow grains consist of 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 pellet — Snow pellets consist of 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 a hard surfaces upon landing.

  Intensity

• Light snow: visibility of or greater
• Moderate snow: visibility between and
• Heavy snow: visibility of less than

  Snow crystal classification

• 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, graupellike 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 snowpack material properties

• 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.

  Classifications of snowpack surface and structure

Wind-induced

• Cornice — Wind blowing over a ridge can create a compacted snowdrift with an overhanging top, called a cornice. Cornices present a hazard to mountaineers, because they are prone to break off.
• Finger drift — A finger drift is a narrow snow drift crossing a roadway. Several finger drifts in succession resemble the fingers of a hand.
• Pillow drift — A pillow drift is a snow drift crossing a roadway and usually 3 to 4.5 metres in width and 30 cm to 90 cm in depth.
• Sastrugi — Sastrugi are snow surface features sculpted by wind into ridges and grooves up to 3 meters high, with the ridges facing into the prevailing wind.
• Snowdrift — Snowdrifts are wind-driven accumulations of snow deposited downwind of obstructions.
• Wind crust — A layer of relatively stiff, hard snow formed by deposition of wind blown snow on the windward side of a ridge or other sheltered area. Wind crusts generally bond better to snowpack layers below and above them than wind slabs.
• Wind slab — A layer of relatively stiff, hard snow formed by deposition of wind blown snow on the leeward side of a ridge or other sheltered area. Wind slabs can form over weaker, softer freshly fallen powder snow, creating an avalanche hazard on steep slopes.

  Sun or temperature-induced

• Firn — Firn is dense, granular snow, which has been in place for multiple years but which has not yet consolidated into glacial ice.
• Névé — Névé is a young, granular type of snow which has been partially melted, refrozen and compacted, yet precedes the form of ice. This type of snow is associated with glacier formation through the process of nivation. Névé that survives a full season of ablation turns into firn, which is both older and slightly denser.
• Penitentes — Penitentes are snow formations, found at high elevations, which form of elongated, thin blades of hardened snow or ice up to 5 meters in height, closely spaced and pointing towards the general direction of the sun. They are evolved suncups.
• Suncups — Suncups are polygonal depressions in a snow surface that form patterns with sharp narrow ridges separating smoothly concave quasi-periodic hollows. They form during the ablation of snow from incident solar radiation in bright sunny conditions, sometimes enhanced by the insulating presence of dirt along the ridges.
• Yukimarimo — Yukimarimo are balls of fine frost, formed at low temperatures on the Antarctic Plateau during light or calm winds.

  Ski resort classification

• Base snow — Base snow refers to snow that has been thoroughly consolidated.
• Frozen granular — Frozen granular is snow whose granules have frozen together.
• Loose granular — Loose granular is snow with uncohesive granules.
• Machine-made — Machine-made snow is that produce by snow cannons and is typically denser than natural snow.
• New snow — New snow is that which has fallen since the previous day's report.
• Packed powder — Packed powder is powder snow that has been compressed by grooming or by ski traffic.
• Powder — Powder is freshly fallen, uncompacted snow. The density and moisture content of powder snow can vary widely; snowfall in coastal regions and areas with higher humidity is usually heavier than a similar depth of snowfall in an arid or continental region. Light, dry powder snow is prized by skiers and snowboarders. It is often found in the Rocky Mountains of North America and in most regions in Japan.
• Spring conditions — Spring conditions describe a variety of melting snow surfaces, including mushy powder or granular snow, which refreeze at night.
• Wet — Wet refers to warm snow with a high moisture content.

  Informal classification

• Corn snow — Corn snow is coarse, granular snow, subject to freeze-thaw.
• Crud — Crud covers varieties of snow that all but advanced skiers find impassable. Subtypes are windblown powder with irregularly shaped crust patches and ridges, heavy tracked spring snow re-frozen to leave a deeply rutted surface strewn with loose blocks, a deep layer of heavy snow saturated by rain.
• Packing snow — Packing snow is at or near the melting point, so that it can easily be packed into snowballs and thrown or used in the construction of a snowman, or a snow fort.
• Slush — Slush is substantially melted snow with visible water in it.
• Snirt — Snirt is an informal term for snow covered with dirt, especially where strong winds pick up topsoil from uncovered farm fields and blow it into nearby snowy areas. Also, dirty snow left over from plowing operations.
• Spring snow — Spring snow describes a variety of temperature and moisture conditions with corn snow.
• Watermelon snow — Watermelon snow is reddish pink, caused by a red-colored green algae called Chlamydomonas nivalis.

  Other cultures