Snow I: Formation, distribution,
measurement, metamorphism,
avalanches
Why is snow important?
• Snow is a critical water resource in
many parts of the world including
Colorado
• It provides recreational
opportunities, which equates to $
• It is an excellent insulator
• Through its high albedo and latent
heat exchanges, it modulates
climate from local to global scales
• It can present hazard (avalanches,
transportation)
Historic NWS collection
Photo R. Armstrong
http://www.its.caltech.edu/~atomic/snowcrystals/photos/photos.htm
Snow formation
Snow formation requires: (1) relative humidity ≥ 100%, (2) T < 0 oC, (3)
condensation nuclei (CCN), (4) supercooled droplets
The basic steps:
H20 Vapor + CCN + Supercooled droplets + T < 0oC
Nucleation (spontaneous below about -40oC)
Ice crystals
-----------------------------------Snow-----------------------------------Riming
(graupel)
vapor diffusion
(flakes)
aggregation
(flakes)
1. Vapor diffusion
Bergeron-Findeisen theory applies: If ice and
supercooled water droplets exist together in a
cloud, the ice crystals
grow
at the–expense
of
Arapahoe
Basin
May 2005
water droplets, the reason being that if the cloud
is saturated with respect to water, it is
supersaturated with respect to ice, the net result
being a transfer of water vapor from the
supercooled (liquid) droplets to the ice crystals.
http://www.geminibv.nl/tidbits/dauwpunt-druken-temperatuur?set_language=en
The process of vapor diffusion is most
efficient at around -12 to -15oC ,
corresponding to the temperature of
largest difference between water and
ice saturation vapor pressures.
Different combinations of temperature and
supersaturation determine the type of
snowflake that forms
http://www.gi.alaska.edu/alison/ALISON_Science_Snow.html
2. Aggregation
•
Growth by aggregation of snowflakes (snowflakes clump together to make
bigger aggregations)
3. Riming
•
•
•
Growth by collision and accretion of existing ice crystals with supercooled
droplets. Contact between the supercooled droplets and the ice crystals causes
the supercooled water to freeze on the crystal. Ice crystals that exhibit frozen
droplets on their surface are said to be rimed.
Riming forms graupel (ice pellets) and hail
Most snowflakes are partially rimed
Graupel pellets
Hail
http://en.wikipedia.org/wiki/Graupe
http://en.wikipedia.org/wiki/Hail
Average seasonal snow cover duration
Average number of weeks of snow cover over the Northern Hemisphere, based
on the NSIDC blended weekly product [from Serreze and Barry, 2005]. Snow
covers a large part of the Northern Hemisphere in winter; snow cover duration
shows a general increase to the north reflecting a primary control by
temperature.
Snow cover formation and disappearance
Dingman 2002 Figure 5-12
Average dates of snow cover formation (left) and disappearance
(right) over North America are controlled by a combination of
latitude, elevation and precipitation.
Snow depth
Maximum snow depth (mm) over Eurasia compiled from Russian
sources [courtesy of H. Ye, California State University, Los Angeles,
CA, from Serreze and Barry, 2005].
Snow measurement
•
•
•
Satellite: Optical and passive microwave instruments can measure snow cover
extent (a yes/no designation - a pixel is either snow covered or snow free), but
measurements are difficult under forest cover. Satellite retrievals of snowpack
depth/water equivalent don’t work very well.
Gauges and snowboards: Used to measure the depth of snowfall and (with
knowledge of density), snow water equivalent. The Wyoming snow gauge was
described earlier. A snowboard is generally a piece of painted plywood, depth is
measured using a ruler.
SNOTEL and Snowcourses: These networks were already described under the
topic of precipitation measurement. There are over 600 SNOTEL and 1000
snowcourse sites across the western U.S. These provide measurements of
snow water equivalent of the snowpack, which is key for seasonal streamflow
forecasting. Snow courses are about 1000 feet long in meadows protected
from the wind, tube samples are taken of depth and mass of the sample, from
which snow water equivalent is determined. SNOTEL sites are automated, and
directly measure water equivalent by the weight of snow atop snow pillows.
Snow water equivalent (SWE)
For water management, SWE is the key snow-related hydrologic variable
SWE= h(ρs/ρw),
h = snow depth (m)
ρs = snow density (approximately 1000 kg m-3)
ρs = liquid water density (varies from typically 100-500 kg m-3)
• Snow depth, density and SWE depend many factors, including
snowfall, temperature history, and wind redistribution.
• Variation in h are generally more important than variations ρs in
determining SWE
• SNOTEL and snowcourse sites are not randomly sited, they are
location at sites where SWE and streamflow are strongly correlated!
Examples: March 1 SWE time series from SNOTEL
sites near Trinidad, CO
Courtesy Drew Slater
April 1 SWE for the U.S. West
The figure at right shows 1 April SWE as
recorded at SNOTEL sites based on
data through the 1995/1996 snow
season. SWE on 1 April typically
represents peak seasonal SWE and is
hence valuable for seasonal streamflow
forecasting. SWE values tend to be
highest in the Pacific NW due to high
precipitation rates (orogragphic uplift and
cooling of moisture-laden Pacific
airmasses). Values tend to be lowest
over the montane areas of the
Southwest, in large part due to high
temperatures. Typically SWE increases
with elevation but this pattern tends to
break down above the treeline. There
has been much work to predict the
spatial distribution of SWE in relation to
elevation, slope and the surface energy
balance.
Serreze et al., 1999, WRR
Example SNOTEL Records: Lake Eldora, CO
The records at left, for the 1982/1983
and 1988/1989 snow seasons, show
cumulate daily SWE (top panels) and
daily changes in SWE at Lake Eldora,
west of Nederland. Key point from the
bottom panels: Seasonal SWE
accumulation is dominated by a relatively
small number of large events. For
SNOTEL sites in the Colorado region,
on average the single largest snowfall
event accounts for about 10% of the total
annual snowfall (as SWE). In the
Arizona/New Mexico region, the
corresponding figure is 23%. Note also
how rapid the snow melt season is
compared to the accumulation season.
Serreze et al., 2000
Winter snowpack evolution
•
•
•
The density of fresh snowfall can be quite low (<50 kg m-3) although 100 kg m-3 is
often assumed. Winds break up snowflakes, and pack them into denser layers; the
lowest snowfall densities tend to occur under cold, calm conditions.
As soon as snow accumulates, it begins a process a metamorphism that continues
until melting is complete. The bulk density of the snowpack increases from typical
mid-winter values of 200-300 kg m-3 to spring values of 400-500 kg m-3.
Snowpack loss through sublimation (solid directly to vapor) can be considerable in
dry areas with a strong solar radiation flux. At Niwot Ridge, Colorado, it is estimated
that 15% of the snowpack is lost due to sublimation.
Metamorphism processes:
1) Gravitational settling
2) Destructive (equi-temperature) metamorphism;
3) Constructive metamorphism (sintering and temperature gradient metamorphism)
4) Melt metamorphism
1. Gravitational settling
The settling rate, which acts to increase snowpack density, takes place at rates that
increase with the weight of the snow layer and the temperature of the layer but decrease
with increasing density of the layer. Typical rates of density increase from gravitational
settling are 2-50 kg m-3 per day.
2. Equi-temperature (destructive) metamorphism
Vapor pressure is higher over sharp points of a snowflake and lower over the hollows of
the flake. Water vapor preferentially sublimates from the spikes and is deposited in the
hollows, making larger, more spherical snow grains with time. The highest rate of equitemperature metamorphosis occurs as temperatures approach 0oC with a small vertical
temperature gradient in the snowpack. The process does not occur below -40oC. The
process is effective until densities reach about 250 kg m-3.
3. Constructive metamorphism
a. Sintering (bonding) of snow grains, in which water vapor molecules are deposited in
concavities where two snow grains touch, gradually building a “neck” between
adjacent grains.
b. Temperature gradient metamorphism - works over longer distances (next slide)
3b. Temperature gradient metamorphism
Cold atmosphere and snow surface (e.g., -20oC)
------------------------------------------------------------------snow surface
Low vapor pressure
upward
temperature
gradient
upward
vapor
gradient
High vapor pressure
------------------------------------------------------------------- ground
•If temperature decreases upwards in the snowpack, vapor
pressure also decreases upwards
Depth hoar crystals
•Water vapor migrates upward, from high to low vapor pressure
• Snow at the base sublimates, water vapor is deposited higher up
in the snowpack
•Results in larger and weaker snow crystals, and development of
depth hoar at the base of the snowpack (a weak low density layer)
•Process requires a big vertical temperature gradient and a shallow
snowpack (the deeper the snowpack, the smaller the temperature
gradient dT/dh)
http://thesnowpit.com/main/index.php?option
=com_content&task=view&id=22&Itemid=40
4. Melt Metamorphism
This occurs from two processes:
1) Liquid water formed by surface melt
or introduced by rain percolates
downward in the snowpack and refreezes. This can produce ice layers
and lenses in the snow. The latent
heat release from freezing also
warms the snowpack.
2) A rapid disappearance of small snow
grains and the growth of larger grains
that occurs in the presence of water.
From this processes, a melting
snowpack typically has an
aggregation of rounded grains of 1-2
mm. This is sometimes referred to
as corn snow.
http://dailyapple.blogspot.com/2007/11/
apple-282-kinds-of-snow.html
Snow climates
1) Maritime - Sierra Nevada, Cascades
•
Heavy snow and mild temperatures
•
Rain on snow events are relative common
•
Avalanches occur during or immediately following storms
•
Equi-temperature metamorphism dominates in winter
2) Continental – Rockies
•
Less snow and lower temperatures
•
Temperature gradient metamorphism is common in autumn and early winter
•
Structural weaknesses (promoting avalanches) are more common than in maritime
regions, these include depth hoar, hard wind-packed layers and buried surface hoar
Avalanches
An avalanche is a sudden, rapid flow of snow down a slope, triggered by either
natural or human causes. They occur when the stress on the snow exceeds the
shear, ductile, and tensile strength either within the snow pack or at the contact of
the base of the snow pack with the ground or rock surface.
While the are different types
of avalanches, they all involve
a trigger which causes the
avalanche, a start zone from
which the avalanche
originates, a slide path (or
track) along which the
avalanche flows, a runout
zone where the avalanche
comes to rest, and a debris
deposit which is the
accumulated mass of the
avalanched snow once it has
come to rest (wikipedia)
http://www.meted.ucar.edu/afwa/avalanche/medi
a/graphics/avPaths_partsLabeled.jpg
Avalanches are facilitated by snowpack weaknesses: TG metamorphism, hoar, surface melt crusts
Temperature Gradient Metamorphism : Depth Hoar
http://emu.arsusda.gov/snowsite/Light_and_LT-SEM/default.html
http://sawtoothmts.wordpress.com/2009/02/
http://www.mrablog.com/picture-of-the-week-16/
Surface hoar: Dew as ice crystals on
the snow surface. On humid nights,
long-wave radiational cooling of the
snow pack causes the temperature to
drop below the dew point at the snow
surface. This results in condensation
and the growth of ice crystals
When buried by subsequent storms it
forms a weak layer in the snowpack.
http://www.avalanche.ca/cac/library/avalanche-imagegalleries/Avalanches2009-2010
Near surface faceted crystals form bi-directionally near the snow surface as a
result of T-G metamorphism. Diurnal changes in the temperature (and thus vapor
pressure) gradients give rise to the changes in crystal growth direction.
Day
Night
Warm
Like depth and surface hoar, these faceted crystals can form
persistent weak layers in a the snowpack.
Warm
T-G
T-G
Cold
Cold
The drier “Continental” West (i.e. Colorado) is
more prone to avalanches due to a climate that
favors T-G metamorphism of the snowpack.
Basic avalanche types
Loose snow (or point) avalanches:
Occur in freshly fallen snow that has a low density and are most common on
steeper terrain. In fresh, loose snow the release is usually at a point and the
avalanche then gradually widens down the slope as more snow is entrained, usually
forming a teardrop appearance. Can move 75-100 mph.
Slab avalanche (“Wet” or “Dry” Slabs):
These are the most destructive type. Slab avalanches occur when there is a
strong, cohesive snow layer (a slab), usually formed when falling snow is deposited
by the wind on a lee slope, or when loose ground snow is transported elsewhere.
When there is a failure in a weak layer, a fracture very rapidly propagates so that a
large area, that can be hundreds of meters in extent and several meters thick, starts
moving. Can move 50-75 mph.
Wet snow avalanche:
Occurs when the snow pack becomes saturated by water. These tend to also start
and spread out from a point. If water content is high they may take the form of
slush flows. The move 25 mph.
Point Release
Avalanches:
Sierra Nevadas
http://www.sierraavalanchecenter.org/phpBB2/viewtopic.php?f=10&t=642
http://www.sierradescents.com/skiing/harwood/2006/stockton-flats.html
Anatomy of a slab avalanche
Key elements:
•A bed surface (snow, rock, ground) that the avalanche runs on
•A weak layer of snow (poorly bonded with weak shear strength)
that fails, initiating the avalanche
•A slab (a cohesive layer) that moves during the avalanche
http://cagem.bayindirlik.gov.tr/turleri-en.htm
http://library.thinkquest.org/03oct/01027/avalanches.html
30-45 deg. at
starting zone
http://www.math.utah.edu/~eyre/lectures/snow/anatomy_cross.html
The Physics of Failure
An avalanche occurs if: shear stress > shear strength
(downslope gravitational force) > (cohesion/friction)
depth
Forces on a hillslope: Gravity is the primary driver of slope failures and avalanches. A
component of the weight (or gravitational force) of the snowpack is acting parallel to the slope
proportional to the sine of the slope angle, this is the shear stress. The other component of
gravity acts perpendicular to the slope, compressing the snowpack, and increasing friction.
Fgsinθ
Fgcosθ
θ
Fg: Force
of gravity
30-45 deg. at
starting zone
http://www.math.utah.edu/~eyre/lectures/snow/anatomy_cross.html
Slab avalanches typically occur on slope angles of 30-45 degrees: at low slopes the shear stress is insufficient
to cause failure, and at higher slopes snow is constantly shed and does not accumulate to sufficient depths.
Factors such as wind loading, new snow, skiers, and explosives all increase the shear stress on
the snow pack. If weak layers are present (low shear strength), then a slab avalanche may occur.
Dry Slab Avalanche:
1) Shear failure of weak layer caused by loading (or snow metamorphism)
2) Fracture of weak layer propagates outward (putting snow in tension)
3) Tension cracks propagate vertically through the snow (eventual crown)
4) Slab releases…during an avalanche the slab may entrain deeper layers of
snow, sometimes all the way to the ground.
3
2
1
4
2
θ
θ
θ
Slide stepped to ground
in some locations
Vail Pass, 1/29/2011
Source: CAIC
This slab avalanche failed on a faceted
layer that formed two months prior.
Note a nearly two week faceting period
in November, followed by a steadily
increasing snow pack. Perfect
conditions for instability. Early season
weak layers can persist and result in
avalanches many months later.
All Information: Colorado Avalanche Information Center, CAIC
Wet Slab Avalanche: Water either increases the load (rain) or decreases
cohesion and friction due to melt.
Arapahoe Basin Ski Area (In-Bounds), May 20th, 2005, 1 skier killed
All Information: Colorado Avalanche Information Center, CAIC
Wet Slab Avalanche
Melt-water percolates through the
snowpack until it reaches a relatively
impermeable layer (an ice layer/the
ground). The water then lubricates the slide
surface causing the overlying slab to fail.
Wet debris is very dense
All Information: Colorado Avalanche Information Center, CAIC