Snow Metamorphism
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Transcript of Snow Metamorphism
Snow MetamorphismSnow MetamorphismSnow MetamorphismSnow Metamorphism“Change of the snowpack over time”“Change of the snowpack over time”
Photo: Scott Schell
Metamorphism at and near the surface
Metamorphism within the snowpack
When snow falls from the sky, it looks like a “snow flake.”
As it falls, and piles up and sits on the ground, it changes.
How it changes affects the avalanche conditions.E. Greene slide
Once snow is on the ground,the grains that make up the layers change over time.
Once snow is on the ground,the grains that make up the layers change over time.
Layers undergo continual change
At the surface and below the surface
Photo: Scott Schell
This process is called metamorphism
Snow MetamorphismSnow 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• Wind
• Temperature• Temperature
• Solar Radiation• Solar Radiation
• Rain• Rain
Blowing Snow DefinitionsBlowing Snow Definitions
Wind Crust
Wind Affected
Wind Slab
Photo: B. Pritchett
What do the Grains Look Like in a Wind Slab?
Temperature Affected
Photo by E. Wengli
Solar AffectedSolar Affected
Solar and Temperature AffectedSolar and Temperature Affected
HEATS UP DURING DAY – REFREEZES AT NIGHTHEATS UP DURING DAY – REFREEZES AT NIGHT
MELT FREEZE METAMORPHISM MELT FREEZE METAMORPHISM
Photo: T.Carter
Rain Affected
Surface Hoar
MetamorphismWithin the Snowpack
Vapor pressure gradients Vapor movement
Created and affected by:
•Temperature gradients
•Grain size
•Radius and curvature
Photo: Gallatin NF
Factors that Drive Change within the Snowpack
• Air temp
•Height of snow
•Ground temp
Metamorphism Within the Metamorphism Within the SnowpackSnowpack
Faceting Rounding
Wet MetamorphismWet Metamorphism
• Liquid water present
• Temperature near 0° C
Corn Snow
Corn SnowCorn Snow
SlushSlush
Melt-freeze CrustMelt-freeze Crust
Conditions that Promote Conditions that Promote
Melt-freeze: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
Result of vapor movement Vapor movement is driven
by vapor pressure gradient,
controlled by:– Temperature– Grain size– Radius and curvature
Dry MetamorphismDry Metamorphism• No liquid water present
• Temperatures less than 0° C
Why temps are importantWhy temps are important
Temperature is only important because vapor pressure decreases with ice temperature !!!
Temperature gradientTemperature gradient
“The change in temperature over height”
Air temperature Ground temperature Snow height
Primary factors:
Snowpack
Ground
Air
Colder < 0 Colder < 0 °C°C
Warmer ~0 Warmer ~0 °C°C
Amount of change in temperature between the ground and snow surface influences metamorphism
Snowpack height}
Big Change = High Gradient
Small Change= Low Gradient
Calculated Temperature GradientCalculated 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:Calculate the temperature gradient:
Tsurf = -20, Tgnd = 0, HS = 100, TG = ______
T10 – Tgnd-------------------- = cTG HS
2
Calculate the temperature gradient:Calculate the temperature gradient:
Tsurf = -5, Tgnd = 0, HS = 100, TG = ______
T10 – Tgnd-------------------- = cTG HS
.5
Calculate the temperature gradient:Calculate the temperature gradient:
Tsurf = -10, Tgnd = 0, HS = 50, TG = ______
Tsurf – Tgnd-------------------- = cTG HS
2
Calculate the temperature gradient:Calculate the temperature gradient:
Tsurf = -10, Tgnd = 0, HS = 200, TG = ______
Tsuff – Tgnd-------------------- = cTG HS
.5
Temperature GradientTemperature Gradient
TG < 1oC per 10 cm = LOW (rounding)
TG > 1oC per 10 cm = HIGH (faceting)
WarmWarm
CoolCool
Low GradientLow Gradient
==
Vapor Vapor StagnatesStagnates
InInPore SpacesPore Spaces
RoundingRounding RoundingRounding
Rounding is common when: Rounding is common when:
The snowpack is deepand the
Air temperatures are warm
Rounding- common when:Rounding- common when:
LOW TG
•Warm climate•Deep snowpack•Low temperature gradient (1°C or less/10cm)•Warm temperature regime•High density snow
Low TG Low TG Rounding Rounding
• Vapor is moved at a “micro-scale”
• Vapor gradient from convex to concave areas
low
high
Beginning Stage RoundingBeginning Stage Rounding
Early Stage RoundingEarly Stage Rounding
Advanced Stage RoundingAdvanced Stage Rounding
RoundsRounds
• Reduce surface-to-volume ratio, increase density (by filling pore space)
• Increase structural strength (by building bonds)
Deep Snowpack, Warm Temps Deep Snowpack, Warm Temps Rounding Likely Rounding Likely
What type of layer is likely to be created?
SinteringSintering
M. Schneebeli
E. Adams, R. Brown and D. Miller
Warm
ColdCold
High GradientHigh Gradient
==Vapor Vapor MovesMovesfromfrom
WarmWarmto to
ColdCold(macro scale)(macro scale)
High Gradient High Gradient Large Growth Large Growth Rate Rate Rapid Edge Growth Rapid Edge Growth
Faceted GrainsFaceted Grains
Faceting Faceting Faceting Faceting Faceting is common when: Faceting is common when:
The snowpack is shallow and theair temperatures are cold
Time
Faceting- Common When:Faceting- Common When:
High TG
•Cold climate•Shallow snowpack•High temperature gradient (1°C or more/10cm)•Warm temperature regime•Low density snow
High TG- FacetingHigh TG- Faceting
• Vapor is moved at a “macro-scale”• Fast vapor transport • Vapor gradient from warm to cold areas
Must have a density of less than 350kg/m³
Rounds, beginning stage facetingRounds, beginning stage faceting
Early Stage FacetingEarly Stage Faceting
K. Elder
Advanced Stage FacetingAdvanced Stage Faceting
Depth HoarDepth Hoar
• Increases surface-to-volume ratio. Density decreases as larger, angular grains form• Structural strength decreases (poor bonding)
Faceted GrainFaceted Grain Faceted GrainFaceted Grain
Advanced Facet - “Depth Hoar”Advanced Facet - “Depth Hoar”
NEAR SURFACEFACETS
NEAR SURFACEFACETS
Photo: Karl Birkeland
What type of a layer would this create?What type of a layer would this create?
Shallow Snowpack – Cold TempsFaceting LikelyShallow Snowpack – Cold TempsFaceting Likely
TG variations within the TG variations within the SnowpackSnowpack
HS
ToC
Weak TG
Strong TG
Weak TG
Temperature RegimesTemperature Regimes (Warm or Cold) (Warm or Cold)
HS
200
T°C 0
-10
Temperature RegimeTemperature Regime
HS
200
T°C 0
-10 -20
-10
Weak TG- which regime promotes rounding faster?
Temperature RegimeTemperature Regime
HS
T°C 0
-1050
Temperature RegimeTemperature Regime
HS
T°C 0
-10 -2050
Strong TG- which regime promotes rounding faster?
SettlementSettlement… is the slow deformation of the snow as it
sags under the influence of gravity becoming denser
SummarySummaryIn a LOW Temperature Gradient:In a LOW Temperature Gradient: Ice sublimates, vapor pressure moves vapor from:
Vapor condenses as ice in:
As a result, grains break down into:
Eventually, the grains become:
If this goes on long enough:
The result is:
Convex regions to concave regions
Concave regions
Smaller pieces
Rounds, growing in size
Sintering takes place
Stronger snow
SummarySummaryIn a High Temperature Gradient:In a High Temperature Gradient:
Ice sublimates into water vapor and the heat flux moves water vapor from:
Vapor condenses as ice on:
As a result, grains:
Eventually, the grains become:
The result is:
Warmer regions to cooler regions
Convexities
Increase in size and become angular
Faceted, growing in size
Weaker snow (persistent)
HOW IS AVALANCHE DANGERAFFECTED BY WEATHER?
HOW IS AVALANCHE DANGERAFFECTED BY WEATHER?
HOW DO ROUNDING AND FACETINGDEEPER IN THE SNOWPACK
INFLUENCE AVALANCHE DANGER?
HOW DO ROUNDING AND FACETINGDEEPER IN THE SNOWPACK
INFLUENCE AVALANCHE DANGER?