Inside the Belly of the Beast
Wendy Wagner, Leigh Jones and C. David Whiteman
Atmos 3200/Geog 3280
Mountain Weather and Climate
Mountain Snowpack
Beauty
Ripp’n it in the Wasatch
or
Beast?
© Tim Lane
Recap - Snow Formation and Accumulation
How do we grow snow?
1 - Vapor Deposition
2 - Accretion
3 - Aggregation
What does crystal
type depend on?
1- Temperature
2- Supersaturation
© Kenneth G. Libbrecht
SnowCrystals.com
New Snow - SLR (Snow Liquid Ratio)
Volume water 10 cm^3
X 100 = 10%
Volume snow 100 cm^3
Baxter et al. 2005
Mountain Snowpack - Diversity
Creep and glide - Snow Deformation
© F. Baker
McClung and Schaerer (1993)
Accumulating a Snowpack
 Snow pack layering
– Character of fallen snow
initially defines the
layering in the snowpack
– The snowpack can be
transformed by wind
action even after snowfall
has stopped. Location of
wind deposited snow
depends on terrain.
Jim Steenburgh in 15’ snowpit
Ben Lomond
Snowpack Densities
SWE/depth
ratio
Percent
density
Snow density
Cold new
snow
<.04
<4%
<40kg/m3
Average
snowpack
~0.20
~20%
~200kg/m3
Very dense
snowpack
0.40
40%
400kg/m3
Slalom race
course
0.60
60%
600kg/m3
Pure ice
0.917
91.7%
917kg/m3
Water
1.0
100%
1000kg/m3
Snowpack Physical Characteristics
 Snowpack density (can vary for different
layers)
 Albedo – Solar reflectivity
– Fresh snow >0.9
– Wet snow 0.6
 Snowpack temperature profile
– In temperate zones, the snowpack-ground interface
is maintained throughout the winter very close to
the melting point of 0°C.
– The snowpack temperature gradient is determined
by the thickness of the snowpack and snow surface
temperature
What’s Inside the Belly of the Beast?
 Snowpack Stratigraphy is a
record of
meteorology/climate.
 Sequence of weak/strong
layers and binding between
layers
– Wind strength and direction
– Number and intensity of
storms – rain
– Solar and longwave radiation
– Temperature, melting,
humidity, pressure
Center for Snow and Avalanche Studies
Snowpack Temperature Profile
 Isothermal?
 Weak temperature gradient?
 Strong temperature gradient?
Snowpack Metamorphism
 Changes in the snowpack due to heat flow and pressure
 Three types of snowpack metamorphism:
– Equitemperature – rounded grains (strong snow)
– Temperature-gradient – faceted grains (weak snow)
– Melt-freeze – rain water or melt water, percolation, undergoes
diurnal melt and freeze cycles. (weak in melt phase, strong in
frozen phase)
 ***The temperature gradient of a snow layer determines
the type of metamorphism while the temperature
determines the rate of metamorphism
Snow Temperature Profile and Snow Metamorphism
Barchet 1978
Equitemperature Metamorphism
*** Weak temperature gradient < 10ْ C / m
Sintering - formation of bonds between snow grains
There is a tendency for water vapor to evaporate
from convex surfaces and condense at concave
surfaces where grains touch to form necks.
Perla & Martinelli (1975)
Equitemperature Metamorphism
 AKA:
– Sintering
– Rounding
– Destructive metamorphism
Settlement cones
 Grain is decreasing its surface area to
volume ratio => surface area/volume
Snow grain metamorphism (at constant
temperature) by curvature effects.
The time in days in given in the lower right.
Equitemperature Metamorphism
Growth rates of snow grains within a snowpack
 Low grain growth rates
– The colder the temperature
the slower the growth rate
– The weaker the temperature
gradient the slower the
growth rate
 Produces more bonds per
unit volume and greater
strength in the snowpack
USDA Electron Microscopy Snow Unit
Temperature Gradient Metamorphism
*** Strong temperature gradient
 The critical temperature gradient to produce faceted forms in
alpine snowpacks is > 10°C / m.
 AKA:
– Faceting
– Kinetic growth
– Constructive
metamorphism
 Growth by vapor diffusion
from areas of relatively
high vapor pressures
(temperatures) to low
vapor pressures
(temperatures).
Perla & Martinelli (1975)
 Minimal grain bonding
Temperature Gradient Metamorphism
www.avalanche.org
Depth Hoar
USDA Electron Microscopy Snow Unit
Radiation Recrystalization
Also called:
Near surface faceting
 Faceting near or on the
surface of the snowpack
due to a temperature
gradient induced by
absorbed solar radiation
 Peak in snow temperature
occurs 2-5cm below the
snow surface
(Morstad, Adams and McKittrick 2004)
Temperature Gradient Metamorphism
Growth rates of snow grains within a snowpack
 High grain growth rates
– The stronger the temperature gradient the higher the
growth rate
– The warmer the temperature the higher the growth rate
– The larger the pore space size the higher the growth rate
 Grains produced under high growth rates (surface
hoar, depth hoar, faceted snow, radiation
recrystallization) form weak, unstable snow that is
often responsible for serious avalanche conditions.
Metamorphic Forms
LaChapelle (1962)
Temperature Gradients – Different Climates
 Maritime climate: usually, temperatures are mild and
snowpacks are deep so that there are weak temperature
gradients and warm temperatures in the snowpack.
 Continental climate: thinner snowpacks and colder air
temperatures produce large temperature gradients. This
leads, more often, to faceted snow crystals in the
snowpack and buried layers of instability.
 Intermountain climate: deeper snowpack than continental
climates so temperature gradients are lower, yet they still
can exist and lead to snowpack instability.
Surface Hoar
 Faceted crystals formed by deposition onto
the snowpack surface when water vapor
pressure in the air exceeds the equilibrium
vapor pressure over ice at the surface.
 The crystals usually form when
– a sufficient supply of water vapor is present
in the air
– a high temperature gradient (inversion) is
present above the snow surface.
***Thus surface hoar usually forms on cold, clear
nights with calm or nearly calm conditions
(common in continental climates).
USFS (1968)
 Surface hoar may be inhibited in concave
areas of the snow surface and under trees
Eastern Sierra Avalanche Center
Surface Hoar
 Surface hoar is extremely
fragile and easily destroyed by
sublimation, wind, melt-freeze
cycles, and freezing rain.
 When buried in the snowpack, a
surface hoar layer is extremely
efficient in propagating shear
instabilities (fractures).
 Surface hoar may gain strength
by bond formation with
adjacent layers, but thick layers
may persist for months within
the snowpack.
www.avalanche.org
Melt-Freeze Metamorphism
 Warm snowpacks have variable amounts of liquid water
– Water-saturated snow (slush; water content > 15% by volume)
– Very wet snow (8-15% by volume)
– Wet snow (3-8%, water cannot be pressed out by gentle hand
squeezing, but a meniscus of water occurs between grains)
– Moist (<3%; snow sticks together to make a snowball).
 Strength of wet snow decreases with increasing water
content.
 Small particles have a lower melting temperature than
larger ones, so they melt first. The heat of melting comes
from larger particles, which undergo surface re-freezing
(release of heat) and an increase in size; corn snow!!
Melt-Freeze Metamorphism
Sun crusts
At low water contents, clusters of grains
form near the snowpack surface where
melting and freezing cycles can occur.
After night cooling this combination
produces very strong crusts composed of
frozen grain clusters that lose almost all
their strength after midday heating.
Perla &Martinelli (1975)
www.avalanche.org
Snowpack Layering
Bruce Tremper, Utah Avalanche Center
Exposing the Belly of the Beast
Avalanches…
Bill Gallagher