Snow Avalanches Flashcards
Snow Avalanches
Masses of snow that separate from snow pack and slide or flow downslope.
Slide
Movement as a coherent mass of snow
Flow
Coherent mass of snow that rapidly disintegrates into small particles moving independently of one another
Snowfall and accumulation influenced by:
latitude – seasonal/annual net radiation/energy balance
altitude – atmospheric temperature decreases with altitude (i.e., lapse rate = -10oC/1000 m)
proximity to moisture source – moist air masses generate more precipitation
slope angle – snow accumulates on slopes less than 45o; sloughs off on steeper slopes
wind redistributes snow to form cornices and slabs
Avalanche initiation
point release avalanches
point-release avalanches
failure of small
volumes of loose snow (i.e., flows)
often occur after heavy snowfall events
failing snow initiates failure in adjacent snow pack; produces a distinct V-shaped, downslope-widening trough
loose snow is unstable because:
1) increased mass on slope;
2) snow crystals have had little time to bond to one
another
slab avalanches
failure of coherent mass of snow (i.e., slide)
initiated by fracturing of snow pack along a weak layer at depth
failure propagates along weak layer
slab slips downslope with top of slab moving more rapidly than bottom of slab
slab is bounded by crown and flank fractures
strength of snow pack influenced by:
grain size and grain type
degree of bonding between ice crystals; compaction (affected
by snowfall amounts)
presence of anchors (e.g. rocks, vegetation)
temperature: heating by solar radiation (affected by slope aspect); snow pack thermal gradient
Weak layers
avalanches require a buried weak layer and an overlying stronger layer
weak layers develop in several ways:
changes in air temperature during snowfall events
hoar frost formation within the snow pack
hoar frost formation at surface of snow pack
“Right Side Up” storm
air temperatures are warm when the snow starts falling, and then become colder
snow is light and fluffy on top and becomes more dense with depth; results in a strong layer at depth
“Upside Down” storm
ncreasing air temperatures during a snowfall event
heavy, denser snow lies on top of lighter snow; results in a slab of more dense snow lying over a weak layer of less dense snow, providing the necessary ingredients for slab avalanches
Surface Hoar
develops during clear and calm conditions in the evening; promotes radiative cooling of snow surface
humid air lies over cold snow surface; promotes frost formation
forms a thin, fragile and persistent weak layer in the snowpack
Depth Hoar
heat moves from warm to cold, and moisture follows the same gradient
moisture in the form of water molecules is constantly moving upward from the relatively warm ground surface below the snow through the porous snow pack
moisture condenses on lower sides of crystals, causing them to grow (and have razor-sharp edges), and sublimating from the tops of the grains, making the tops rounded
Avalanche Motion - Slide
sliding motion occurs in avalanches moving up to 40 km/hr
after the snow fails and has overcome initial friction, it can accelerate rapidly downslope
slabs break into smaller fragments and snow glides along the surface with little mixing and turbulence
Avalanche Motion – Dry Snow
avalanches become turbulent (i.e., flows) when velocities exceed about 40 km/hr
dry flowing avalanche contains a dense core of snow particles (10-30 cm in diameter)
finer particles mix with air at the front and along the upper surface of moving snow forming a powder cloud
velocity tends to be greater in the center of the flow
avalanche generally moves along the surface of the terrain, uninfluenced by small irregularities
Avalanche Motion – Wet Snow
wet snow avalanches develop in the same manner as dry snow avalanches but have no powder cloud
moving snow is dense and composed of rounded particles with a diameter of 10 cm to rounded lumps up to several metres; contains intergranular water
wet snow avalanches tend to flow in channels and are easily deflected by irregularities in the terrain
Start zone
location on hillslope where snowpack fails
Track
path along which avalanche travels and accelerates to achieve its highest velocity
Run-out zone
location where avalanche decelerates and snow is deposited
Increase in mass of materials on slope
addition of snow during and soon after snow storms
weight of people traversing slopes
Slope angle
most important terrain factor for avalanche initiation
slope angles >60o – frequent sluffs
slope angles 45o-60o – frequent point-release avalanches
slope angles 30o-45o – frequent large slab avalanches
slope angles <30o – wet snow avalanches only
Avalanche Triggers
wind erodes snow from the windward (upwind) side of obstacles, such as a ridge, and deposits the same snow on the leeward (downwind) terrain
wind-drifted snow is often much denser than non-wind loaded snow; adds significant weight on top of buried weak layers
wind loading
is a common denominator in most avalanche accidents; wind can deposit snow 10 times more rapidly than snow falling from the sky
