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


Tsurf = -5, Tgnd = 0, HS = 100, TG = ______

T10 – Tgnd-------------------- = cTG HS

.5


Tsurf = -10, Tgnd = 0, HS = 50, TG = ______

Tsurf – Tgnd-------------------- = cTG HS

2


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?


HS

T°C 0

-1050


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?

Snow Metamorphism

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