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.

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

6. This process is called metamorphism

7. 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

8. Weather affects snow at and near the surface

9. Wind • Temperature • Solar • Radiation • Rain

11. Blowing Snow Definitions

12. Wind Crust

13. Wind Affected

14. Wind Slab Photo: B. Pritchett

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

16. Temperature Affected

17. Solar Affected Photo by E. Wengli

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

19. Rain Affected

20. Surface Hoar

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

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

23. Metamorphism Within the Snowpack Faceting Rounding

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

25. Corn Snow

26. Corn Snow

27. Slush

28. Melt-freeze Crust

29. 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

30. 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

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

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

33. 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

34. 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

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

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

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

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

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

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

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

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

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

45. Beginning Stage Rounding

46. Early Stage Rounding

47. Advanced Stage Rounding

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