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Snow Metamorphism

Snow Metamorphism. “Change of the snowpack over time”. Metamorphism at and near the surface. Metamorphism within the snowpack. Photo: Scott Schell. As it falls, and piles up and sits on the ground, it changes. When snow falls from the sky, it looks like a “snow flake.”.

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Snow Metamorphism

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  1. Snow Metamorphism “Change of the snowpack over time” Metamorphism at and near the surface Metamorphism within the snowpack Photo: Scott Schell

  2. As it falls, and piles up and sits on the ground, it changes. When snow falls from the sky, it looks like a “snow flake.” How it changes affects the avalanche conditions. E. Greene slide

  3. Once snow is on the ground, the grains that make up the layers change over time.

  4. Layers undergo continual change Photo: Scott Schell At the surface and below the surface

  5. This process is called metamorphism

  6. 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

  7. Weather affects snow at and near the surface

  8. Wind • Temperature • Solar • Radiation • Rain

  9. Blowing Snow Definitions

  10. Wind Crust

  11. Wind Affected

  12. Wind Slab Photo: B. Pritchett

  13. What do the Grains Look Like in a Wind Slab?

  14. Temperature Affected

  15. Solar Affected Photo by E. Wengli

  16. Solar and Temperature Affected HEATS UP DURING DAY – REFREEZES AT NIGHT MELT FREEZE METAMORPHISM Photo: T.Carter

  17. Rain Affected

  18. Surface Hoar

  19. MetamorphismWithin the Snowpack Vapor pressure gradients  Vapor movement • Created and affected by: • Temperature gradients • Grain size • Radius and curvature

  20. Factors that Drive Change within the Snowpack • Air temp • Height of snow • Ground temp Photo: Gallatin NF

  21. Metamorphism Within the Snowpack Faceting Rounding

  22. Wet Metamorphism • Liquid water present • Temperature near 0° C

  23. Corn Snow

  24. Corn Snow

  25. Slush

  26. Melt-freeze Crust

  27. 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

  28. 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

  29. Why temps are important Temperature is only important because vapor pressure decreases with ice temperature !!!

  30. Temperature gradient “The change in temperature over height” Primary factors: ·Air temperature ·Ground temperature ·Snow height

  31. Air Colder < 0 °C } Snowpack height Snowpack Ground Warmer ~0 °C Amount of change in temperature between the ground and snow surface influences metamorphism Big Change = High Gradient Small Change= Low Gradient

  32. 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

  33. Calculate the temperature gradient: 2 Tsurf = -20, Tgnd = 0, HS = 100, TG = ______ T10 – Tgnd-------------------- = cTG HS

  34. Calculate the temperature gradient: .5 Tsurf = -5, Tgnd = 0, HS = 100, TG = ______ T10 – Tgnd -------------------- = cTG HS

  35. Calculate the temperature gradient: 2 Tsurf = -10, Tgnd = 0, HS = 50, TG = ______ Tsurf – Tgnd -------------------- = cTG HS

  36. Calculate the temperature gradient: .5 Tsurf = -10, Tgnd = 0, HS = 200, TG = ______ Tsuff – Tgnd -------------------- = cTG HS

  37. Temperature Gradient TG < 1oC per 10 cm = LOW (rounding) TG > 1oC per 10 cm = HIGH (faceting)

  38. Cool Low Gradient = Vapor Stagnates In Pore Spaces Warm

  39. Rounding Rounding is common when: The snowpack is deep and the Air temperatures are warm

  40. Rounding- common when: • Warm climate • Deep snowpack • Low temperature gradient (1°C or less/10cm) • Warm temperature regime • High density snow LOW TG

  41. high low Low TG  Rounding • Vaporis moved at a “micro-scale” • Vapor gradient from convex to concave areas

  42. Beginning Stage Rounding

  43. Early Stage Rounding

  44. Advanced Stage Rounding

  45. Rounds • Reduce surface-to-volume ratio, increase density (by filling pore space) • Increase structural strength (by building bonds)

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