Transcript CREATION OF THE MOUNTAIN SNOWPACK

Snow Metamorphism
“Change of the snowpack over time”
Metamorphism
at and near the
surface
Metamorphism
within the
snowpack
Photo: Scott Schell
When snow falls
from the sky, it
looks like a
“snow flake.”
E. Greene slide
As it falls,
and piles up
and sits on
the ground, it
changes.
How it changes
affects the
avalanche
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
dif f usion
suspension
more than 100
more than 15
Blowing Snow Definitions
This graph should change to:
Type of
Moveme nt
Snow particle
Me chan is
m
Creep
Near surf ace rolling
Saltation
B ouncing <1m height
Turbulent
dif f usion
Airborn suspension
W ind
spe e d
(Kph)
Light: less
than 18
Moderate:
20-40
Strong: >50
Re-distribution
of snow
Little:
Less than 10%
Most: about 80%
Little:
less than 10%
less t
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
Warmer ~0 °C
}
Snowpack
height
Ground
Amount of change in
temperature between the
ground and snow surface
influences metamorphism
Big Change = High Gradient
Small Change= Low Gradient
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
low
• Vapor is moved at a
“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°
HS
Temperature Regime
-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?