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CREATION OF THE MOUNTAIN SNOWPACK

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CREATION OF THE MOUNTAIN SNOWPACK Powered By Docstoc
					            Snow Metamorphism
     “Change of the snowpack over time”

Metamorphism
at and near the
surface



Metamorphism
within the
snowpack
                                     Photo: Scott Schell
When snow falls     As it falls,
from the sky, it    and piles up
looks like a        and sits on
“snow flake.”       the ground, it
                    changes.




                   How it changes
                   affects the
                   avalanche
 E. Greene slide   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
                 suspension    more than 100           more than 15          less t
           n
    diffusio



           Blowing Snow Definitions
This graph should change to:


                                               Wind
    Type of          Snow particle                         Re-distribution
                                               speed
   Movement           Mechanism                               of snow
                                               (Kph)
                                           Light: less           Little:
      Creep         Near surface rolling
                                            than 18          Less than 10%
                                           Moderate:
    Saltation      Bouncing <1m height                     Most: about 80%
                                             20-40

    Tu rbulent                                                   Little:
                    Airborn suspension     Strong: >50
            n
     diffusio                                                less than 10%
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       }   Snowpack
                                       height

    Warmer ~0 °C
                      Ground

Amount of change in
temperature between the     Big Change = High Gradient
ground and snow surface     Small Change= Low Gradient
influences metamorphism
   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
               • Vapor is moved at a
low              “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°
     Temperature Regime
HS




          -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?

				
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posted:2/13/2012
language:English
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