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Near Surface Faceted Crystals: Conditions Necessary for Growth and
Contribution to Avalanche Formation, Southwest Montana, U.S.A.
KarlBirkeland1,RonJohnson2,andScottSchmidt3
1 Gallatin NationalForest Avalanche Center,P.O. Box130, Bozeman,MT 59771
and Departmentof Geography, Arizona State University, Tempe, AZ 85287-0104
2 Gallatin NationalForest Avalanche Center,P.O. Box130, Bozeman,MT 59771
3 Department of Civiland AgriculturalEngineering,Montana State University, Bozeman, MT 59717
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Keywords: Avalanche, snow metamorphism, faceted crystals,
temperaturegradient
ABSTRACT
In the winter of 1995-96 we investigated the formation of
facetedcrystalsthatdevelopintheupperlevelsofthe
snowpack.We used anarrayofsixthermocouplescon-
nected to a datalogger to measure hourly diurnal tempera-
ture changes in the region from 0.005 mabove the snow
surface to 0.20 m below the snow surface. Measurements
during clear sky conditions in March showed temperature
gradients in excess of 200 0C/m at night in the top 0.05 m
ofthesnowpack,withthe temperature gradientshifting
directionandexceeding1000C/m throughthis layer during
the day. A significant weak layerof faceted snow formed
within36hourswitha grain sizeofabout1mminthe
upper snowpack. Widespreadavalancheactivity occurred
for up to nine days after this layer was buried by 0.50 m of
snow.
INTRODUCTION
During seven seasons of backcountry avalancheforecasting
in southwest Montana, we have observed the formation of
layers of facetedcrystals near thesnow surface(Class 4b in
theInternationalClassification forSeasonalSnow on the
Ground(Colbecketal.,1990)).These layers,which com-
monly form between0 and0.15 m below the snow surface,
consistentlycreatesignificantsnowpackweaknessesin our
snowpack when they are subsequently buried. Often near-
surfacefacetedsnow is toppedoff with surface hoar, a well
documentedweak layer.However,near-surfacefaceted
crystals withoutsurfacehoar may also createdangerous and
lastingweak layers.We investigated 51backcountry ava-
lanches, typicallyClass3orlarger(Perla and Martinelli,
1978),that usually involved backcountry skiers, snowmo-
bilers, snowboarders and snowshoers (Table 1).The weak
layer in nearly two-thirds (59%) of those slides was a layer
of small-grained(mostly up to 1 mm, but sometimes as large
as 1.5 mm) faceted crystals formed near the surfacebefore
beingsubsequentlyburied.Nearlyathird(31%)ofthe
slides failed on surface hoar, which was often sitting over
the topof a layer of near-surfacefacetedcrystals. Only 6%of
theslides failed onbasal depth hoar,while 4% failed on
other weak layers.
In spite of their role in avalancheformation in ourarea,
and reports that they form a significant weaknessin other
regions, near-surfacefacetedcrystals have receivedfar less
attentionin the scientific andpopular literaturethan depth
hoar or surfacehoar. The processeswhichform near-surface
facetedlayers are discussedbriefly,if at all, in populartexts,
and have been minimally addressed by scientific research.
The purpose of this study was to observe the formation of
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near-surface faceted crystalsin the field, todocument the
specificconditions necessaryfortheirformation,and to
observe their contribution to avalancheformation.
LITERATURE REVIEW
Facetedcrystals have long been recognizedas a significant
weaklayerand aprimecontributortoavalanche forma-
tion(Seligman,1936).Mostresearch onfaceted crystals
hasfocusedonthe formationofthebasallayeroflarge
faceted crystals, commonly called depthhoar (some of the
many examples include: de Quervain, 1954; Giddings and
LaChapelle,1961; Bradley et al., 1977; Perla, 1978). Akitaya
(1974) made exhaustive laboratory examinations of depth
hoardevelopment and defined various crystal types and
strength changes. Surface hoar is anothertype offaceted
crystal that forms a wellrecognized weak layer.Aswith
depthhoar,there isasignificant(and growing)bodyof
research on surface hoar formation (i.e., Langet al., 1984;
Colbeck, 1988; Hachikuboandothers, 1994; Hachikuboand
Akitaya, 1996; Daviset al., 1996). In addition, generaltexts
on avalanchescommonly include detailedexplanations of
depthhoarandsurfacehoargrowthandtheirrolein
avalanche formation (Perla and Martinelli, 1978;Daffern,
1992;McClungand Shaerer,1993;Fredston andFesler,
1994). The researchandattentionon depthhoarandsurface
hoar arewell deserved, since both of these facetedcrystals
create undeniably dangerous and persistent weak layers.
Although the formation of faceted crystals near and just
beneath thesnowsurface hasnotbeen aswidely recog-
nized as the processes which form surface hoarordepth
hoar,near-surfacefaceted crystalshavepreviouslybeen
identifiedbymanyavalancheworkersandsomesnow
scientists as a significant weak layer. Stratton (1977) typed
up two short papers related tonear-surface faceted crys-
tals and their contribution to avalancheformation in Utah.
Cleardays withradiation inputsjustbelowthat needed
formelting,cold clear nights,and low density(less than
100 kg/m 3) surface snow were cited as contributors to the
formationofwhathetermed "upperleveltemperature
gradient" or U.L.T.G. snow. He also noted that the faceted
crystals were often associated with thin,overlying crusts
on southerly aspects; thiswas particularly dangerous when
those crusts were overloaded until they collapsed into the
weaker, underlying snow.
Concurrent withStratton'sobservations, LaChapelleand
Armstrong (1977) and Armstrong (1985) investigated snow
metamorphismintheSanJuan mountainsofColorado.
They measureddiurnaltemperature fluctuationsandstrong
temperaturegradients (leadingtostrongvaporpressure
gradients)in the upper0.25 m of the snowpack. Of primary
interest was a special form of near-surface faceting
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