SECTION 7: SUMMARY

Factors impacting snowpack development

  • At the global scale: Climate, elevation, latitude, terrain, solar variation
  • At regional and smaller scales: Geography, precipitation type, wind

Types of snowpack

  • Tundra: Thin, cold, windblown snow usually found above or north of tree line
  • Taiga: Thin to moderately deep, low-density snowpack found in forests in cold climates
  • Alpine: Intermediate to cold, deep snow cover, typically low density
  • Maritime: Warm, deep snow cover with coarse-grained snow due to wetting
  • Prairie: Typically thin, moderately cold snow cover with substantial wind drifting
  • Ephemeral: Thin, extremely warm snow cover that melts soon after being deposited

Snowpack density measurements

  • Snow water equivalent (SWE): The depth of water produced if the snowpack melted
  • Snow-to-liquid ratio (SLR): A unitless ratio of snow depth to SWE; high for light, powdery snowpack (up to about 40:1), low for snowpacks with older or drifted snow (as low as 6 or 7:1), even lower for very wet snow

Crystal types (habits)

  • Dendrites: Fastest-growing crystals; are produced at atmospheric temperatures from -11°C to -17°C (12°F to 1°F); tend to aggregate into larger snowflakes that result in low-density snowpack at cold temperatures
  • Columns, needles, and plates: Smaller crystals that tend to accumulate into a higher-density snow layer; needles form with atmospheric temperatures of -10°C to -3°C (14°F and 26°F); columns from -10°C to -3°C (14°F to 26°F) or colder than -22°C (-8°F)
  • Riming occurs when any type of ice crystal passes through super-cooled cloud (liquid water droplets below 0°C); rimed crystals are partially or completely coated in tiny frozen water droplets, and are associated with higher-density snow accumulation
  • Broken crystals: Crystals can fracture when they hit each other in the atmosphere or strike the ground; higher winds lead to more fractured crystals

Basic processes that affect snowpack development

  • Gravity: Pulls snowpack straight downwards on flat land, increasing its density; over sloped surfaces, a portion of the gravitational force is directed parallel to the slope; this increases with steeper slopes and is responsible for moving snow downhill
  • Conduction: The direct transfer of thermal energy from warmer to cooler substances that are in contact with each other; occurs at temperature gradients within a snowpack or at its top or bottom
  • Radiation:
    • Primarily responsible for inducing the melt/freeze process, which causes crusting and other types of crystal evolution
    • Two types are important for snowpack evolution
      • Incoming solar radiation: Snow is highly reflective (has a high albedo or ratio of reflected solar energy to incoming solar energy), with albedos typically 0.3 to 0.9; incoming solar radiation heats up the top 15 to 30 cm (6 to 12 in) of snowpack
      • Outgoing infrared radiation: Cools the very top few mm of the snowpack; the warmer and more vegetation-free the snowpack surface, the greater the rate of radiative loss; snow radiates heat very efficiently and is a very good insulator (prevents heat from rising through the pack)
  • Phase changes: Radiation and conduction transfer thermal energy and induce changes in snowpack between solid ice, liquid water, and water vapor. All phase changes induce either cooling or warming of the surrounding air depending the type. Evaporation and sublimation cool the immediate atmosphere while condensation warms it.
    • Melting: Phase change from solid ice to liquid water
    • Freezing: Phase change from liquid water to solid ice
    • Sublimation: Phase change from ice directly to water vapor
    • Microphysical vapor diffusion: Water molecules move from warm to cool in microscopic air pockets, attaching onto other snow grains in flat layers; this process usually occurs from the warmer ground upward toward the snow surface
  • Convection: Occurs in relatively porous snowpack when warm air at the bottom rises into the porous layers above, transporting small amounts of heat upwards

Factors involved in snowpack evolution

  • Vegetation: Impacts snowfall distribution/redistribution, with more snow (20 to 45%) accumulating in clearings than adjoining forests
  • Daytime solar energy: Heats the upper surface of the snowpack
  • Aspect (the direction a tilted surface faces): Melting is strongest on sun-facing sides (south in the Northern Hemisphere)
  • Radiative cooling after sunset: Quickly cools a few millimeters of the top of the pack
  • Crusting: Freezing of a snowpack surface that has previously melted due to either solar radiation or warm temperatures; often occurs just after sunset 
  • Radiative recrystallization: Occurs when an intense vertical thermal gradient transports moisture from the lower part of the snowpack to the top layer, causing ice grains in the ~top 5 cm (1.9 in) to grow
  • Hoar: Large, rounded, feathery crystals with flat edges that grow rapidly
    • Surface hoar: Forms when snow at the top of the pack cools rapidly overnight; a strong, upward temperature gradient develops along with a vapor pressure gradient that drives water vapor out of the snow and into the atmosphere; the water vapor freezes and forms hoar   
    • Depth hoar: Takes several days of strong temperature gradients in the snowpack to form; causes highly faceted hoar crystals to grow on the edges of existing snow grains; although the crystals are bonded, they form a weak, brittle structure that can cause a fracture or slide
  • Wind: Can transport snow when speeds are over 5 m/s (~10 kt); weaker winds can move low-density snow whereas older, hardened snow surfaces may only start moving with much stronger winds
  • Dust storms: Created by very strong low-level winds moving across arid regions; the dust moves downstream in the atmosphere and resettles on snowpack, causing the top to melt significantly during daytime; when the pack melts, the top dust layer merges with other dust layers, forming a strong, thick, ‘dirty’ layer
  • High-density snow over low-density snow: Can lead to an unstable snowpack, one prone to collapse
  • Precipitation types
    • Dry snow: Typically decreases snowpack density and increases snowpack depth
    • Wet snow: Generally increases snowpack density and depth, but if a thin layer of wet snow falls on a thin layer of dry snow, the depth can actually decrease due to compaction
    • Sleet (ice pellets): Adds a high-density layer of ice to the very top of the snowpack; increases the density of a relatively dense snowpack while decreasing its depth
    • Freezing rain: Freezes upon contact with the snowpack, forming a thin layer of dense, hard ice on top; typically increases snowpack density and decreases snowpack depth
    • Freezing drizzle: Adds an ultra-thin, high-density layer to the top of the snowpack
    • Graupel: High-density frozen precipitation that typically increases snowpack density and depth
    • Rain: Creates a wet snow layer in the top few cm of the snowpack; can lead to melting and refreezing in the top of the pack; increases snowpack density but decreases snowpack depth
  • Springtime snowmelt
    • The warmer the atmospheric temperature, the greater the rate of melting (adds liquid to the top of the pack, increasing the density of the upper layer)
    • Most liquid from melting and rain moves down through vertical channels in the snowpack unless it encounters an ice layer, in which case it will pool above it (freezing if temperatures fall) and move horizontally
    • Water moves parallel to snow layers when there’s a capillary barrier (a gradient of pore sizes, from smaller to larger) that causes water to run through the upper layer rather than draining into the lower one
    • At night, when the air temperature is below freezing, the top layer freezes into a crust whose density is near that of water (many times higher than snowpack)
    • The cycling of melting and freezing can occur diurnally or with the passage of storm systems

Aspects of snowpack evolution specific to mountainous terrain

  • Snowpack is often unstable in both flat and sloped terrain but the impacts are far more severe in mountainous areas
  • Gravity has a greater impact in sloped terrain since it can cause layers to detach and slide
  • Friction is the primary factor that lets snowpack build up on sloped surfaces rather than just sliding downslope
  • Terrain affects the type and amount of snowfall, with upslope areas typically receiving more precipitation than surrounding locations
  • Wind can significantly redistribute snowpack in the mountains where high wind speeds are common; snow depths can be at least 50% higher in redistributed areas; the weight of the additional snow can destabilize formerly stable snowpacks
  • Snow layers move by:
    • Gliding, where the entire snowpack detaches at the bed and moves slowly down the slope; the same process occurs with avalanches but at a much faster pace
    • By creep, the slow, differential movement of a slab down the slope, with the upper portion traveling faster than the lower portion; although creep occurs slowly, it can eventually produce a slide
  • Avalanches form on slopes mild enough for snow to accumulate but steep enough for it to slide; the steepness threshold generally ranges from 30 and 45 degrees (up to 60 in maritime areas)

Snowpack assessment

  • Two primary methods: Onsite (in-situ) and remotely-sensed (satellite-based); both measure snow depth, SWE, density, temperature, and the nature of the layers throughout the snowpack to provide a complete profile analysis
  • Onsite measurements (snow courses)
    • Taken at fixed sites at regular intervals throughout the cold season; taken at particular sites as needed
    • Manual observations
      • Taken in vertical snow pits dug out with shovels, with straight, vertical columns sampled
      • Snowpack wetness: Characterizes snowpack as dry, moist, wet, very wet, or slush; to measure it, squeeze a handful of snow and observe the amount of water; or cut out a volume of snow and weigh its density, then divide the measured density by the density of water
      • Snowpack depth: Use a ruler to measure the thickness of a layer or the snowpack; take multiple measurements to get representative samples
      • Snowpack hardness: Gently press a fist, four fingers, one finger, pencil, knife into the snow and determine the largest object that can penetrate it; or use a ram penetrometer to determine resistance as it’s thrust into a layer
      • Snow grain type and size: Use a simple magnifier to assess the type of snow grain within a layer; hoar and other facetized grains are typically large and reduce snowpack stability
      • Snowpack temperature: Use a thermometer in the shade; take measurements at several depths to estimate temperature gradients
      • Shear quality (the resistance of layers when an amount of pressure is applied to one layer): Exert increasing amounts of vertical pressure on the top of the snowpack until the top layer begins to move down the slope; the Rutschblock test is done by a skier who cuts out a u-shaped trench and applies pressure on it (from gently stepping on it to jumping up and down) to see when it fails; the Stuffblock test is more quantifiable since it uses pre-set amounts of weight; the Extended Column Test identifies layers that are likely to both initiate and propagate a fracture, helping to focus attention on unstable areas where an avalanche might really occur
    • SNOTELs: An extensive, automated system for collecting snowpack and related climatic data in the Western U.S., including Alaska; measures solar radiation, RH, snow depth, SWE and snow weight, air temperature, wind direction/speed, soil moisture, soil temperature, precipitation, barometric pressure
  • Remote sensing
    • Covers large areas with near-uniform resolution, retrieves data from remote regions
    • Microwave wavelengths are best for assessing snow properties because their energy penetrates snowpack and is reflected/emitted from the surface and deeper within the pack; are sensitive to snow depth, SWE, snowpack temperature, snow crystal type, wet-dry state, soil conditions; penetrate cloud cover; operate day and night
    • Products: SWE, snow cover, snow depth, snow top temperature, snowmelt

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