Snow Metamorphism “Change of the snowpack over time” Metamorphism at and near the surface Metamorphism within the snowpack Photo: Scott Schell When snow falls from the sky, it looks like a “snow flake.” E. Greene slide As it falls, and piles up and sits on the ground, it changes. How it changes affects the avalanche conditions. Once snow is on the ground, the grains that make up the layers change over time. Layers undergo continual change Photo: Scott Schell At the surface and below the surface This process is called metamorphism Snow Metamorphism “Change of the snowpack over time” Factors changed by metamorphism: •Shape and size of grains •Bonds between grains •Density •Temperature, Reflectivity of radiant energy (albedo) •Hardness •Porosity •Deformation properties •Shear and tensile strength •Thermal conductivity Weather affects snow at and near the surface • Wind • Temperature • Solar Radiation • Rain dif f usion suspension more than 100 more than 15 Blowing Snow Definitions This graph should change to: Type of Moveme nt Snow particle Me chan is m Creep Near surf ace rolling Saltation B ouncing <1m height Turbulent dif f usion Airborn suspension W ind spe e d (Kph) Light: less than 18 Moderate: 20-40 Strong: >50 Re-distribution of snow Little: Less than 10% Most: about 80% Little: less than 10% less t Wind Crust Wind Affected Wind Slab Photo: B. Pritchett What do the Grains Look Like in a Wind Slab? Temperature Affected Solar Affected Photo by E. Wengli Solar and Temperature Affected HEATS UP DURING DAY – REFREEZES AT NIGHT MELT FREEZE METAMORPHISM Photo: T.Carter Rain Affected Surface Hoar Metamorphism Within the Snowpack Vapor pressure gradients Vapor movement Created and affected by: •Temperature gradients •Grain size •Radius and curvature Factors that Drive Change within the Snowpack • Air temp •Height of snow •Ground temp Photo: Gallatin NF Metamorphism Within the Snowpack Faceting Rounding Wet Metamorphism • Liquid water present • Temperature near 0° C Corn Snow Corn Snow Slush Melt-freeze Crust Conditions that Promote Melt-freeze: • • • • • • • • High daytime temperatures Strong solar radiation Cold night time temperatures Recurring cycle of melting and freezing High density wet snow Rain Sunny aspects Steeper slopes Dry Metamorphism • No liquid water present • Temperatures less than 0° C Result of vapor movement Vapor movement is driven by vapor pressure gradient, controlled by: – Temperature – Grain size – Radius and curvature Why temps are important Temperature is only important because vapor pressure decreases with ice temperature !!! Temperature gradient “The change in temperature over height” Primary factors: Air temperature Ground temperature Snow height Air Colder < 0 °C Snowpack Warmer ~0 °C } Snowpack height Ground Amount of change in temperature between the ground and snow surface influences metamorphism Big Change = High Gradient Small Change= Low Gradient Calculated Temperature Gradient Tsurf – Tgnd -------------------- = cTG HS • Tsurf is temperature of the snow at the surface • Tgnd is temperature of the ground • HS is the height of snow in centimeters (/10cm) • cTG is the calculated temperature gradient Calculate the temperature gradient: 2 Tsurf = -20, Tgnd = 0, HS = 100, TG = ______ T10 – Tgnd -------------------- = cTG HS Calculate the temperature gradient: .5 Tsurf = -5, Tgnd = 0, HS = 100, TG = ______ T10 – Tgnd -------------------- = cTG HS Calculate the temperature gradient: 2 Tsurf = -10, Tgnd = 0, HS = 50, TG = ______ Tsurf – Tgnd -------------------- = cTG HS Calculate the temperature gradient: .5 Tsurf = -10, Tgnd = 0, HS = 200, TG = ______ Tsuff – Tgnd -------------------- = cTG HS Temperature Gradient TG < 1oC per 10 cm = LOW (rounding) TG > 1oC per 10 cm = HIGH (faceting) Cool Low Gradient = Vapor Stagnates In Pore Spaces Warm Rounding Rounding is common when: The snowpack is deep and the Air temperatures are warm Rounding- common when: •Warm climate •Deep snowpack •Low temperature gradient (1°C or less/10cm) •Warm temperature regime •High density snow LOW TG Low TG Rounding high low • Vapor is moved at a “micro-scale” • Vapor gradient from convex to concave areas Beginning Stage Rounding Early Stage Rounding Advanced Stage Rounding Rounds • Reduce surface-to-volume ratio, increase density (by filling pore space) • Increase structural strength (by building bonds) Deep Snowpack, Warm Temps Rounding Likely What type of layer is likely to be created? Sintering M. Schneebeli E. Adams, R. Brown and D. Miller Cold High Gradient = Vapor Moves from Warm to Cold (macro scale) Warm High Gradient Large Growth Rate Rapid Edge Growth Faceted Grains Faceting Faceting is common when: The snowpack is shallow and the air temperatures are cold Time Faceting- Common When: •Cold climate •Shallow snowpack •High temperature gradient (1°C or more/10cm) •Warm temperature regime •Low density snow High TG High TG- Faceting Must have a density of less than 350kg/m³ • Vapor is moved at a “macro-scale” • Fast vapor transport • Vapor gradient from warm to cold areas Rounds, beginning stage faceting Early Stage Faceting K. Elder Advanced Stage Faceting Depth Hoar • Increases surface-to-volume ratio. Density decreases as larger, angular grains form • Structural strength decreases (poor bonding) Faceted Grain Advanced Facet - “Depth Hoar” NEAR SURFACE FACETS Photo: Karl Birkeland Shallow Snowpack – Cold Temps Faceting Likely What type of a layer would this create? TG variations within the Snowpack Weak TG HS Strong TG Weak TG o T C Temperature Regimes (Warm or Cold) HS -10 200 0 T° Temperature Regime HS -10 -20 200 Weak TG- which regime promotes rounding faster? 0 -10 T° HS Temperature Regime -10 50 0 T° Temperature Regime HS Strong TG- which regime promotes rounding faster? -10 -20 50 0 T° Settlement … is the slow deformation of the snow as it sags under the influence of gravity becoming denser Summary In a LOW Temperature Gradient: Ice sublimates, vapor pressure moves vapor from: Convex regions to concave regions Vapor condenses as ice in: Concave regions As a result, grains break down into: Smaller pieces Eventually, the grains become: Rounds, growing in size If this goes on long enough: Sintering takes place The result is: Stronger snow Summary In a High Temperature Gradient: Ice sublimates into water vapor and the heat flux moves water vapor from: Warmer regions to cooler regions Vapor condenses as ice on: Convexities As a result, grains: Increase in size and become angular Eventually, the grains become: Faceted, growing in size The result is: Weaker snow (persistent) HOW IS AVALANCHE DANGER AFFECTED BY WEATHER? HOW DO ROUNDING AND FACETING DEEPER IN THE SNOWPACK INFLUENCE AVALANCHE DANGER?