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on southerlyaspectsthattheyter med"radiation
recrystallization",whereby solarradiation penetrates the
snowpackandmeltsthesnowafew centimetersbelow
thesurface,whilethesnowatthesurfaceiscooledby
longwave radiation losses. The end result is a strong tem-
perature gradient in the upper couple of centimeters and
the formation of faceted crystals over a melt freeze crust -
aperfect recipe forslabavalancheswhensubsequently
buried.Little mention is made of the effect of the diurnal
temperature changes they measured on the snowpack on
more northerly aspects,orhow those changes might re-
late to avalanche conditions in those areas.
More recently,Colbeck (1989) mathematically described
wide diurnal temperature swings in the upper snowpack
and subsequent temperature gradients.He mentions that
theseprocessesformfaceted snowlayersmostoftenin
polar snow, or in seasonal snow at high altitudes, although
he also states that growth rates nearthe snow surface in
the seasonal snowpack could be much greater than at the
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In spite of the recognition of the processes that form near-
surfacefacetedcrystals,andtheroleofthoselayersin
avalanche formation, little comprehensivefield work has
been done onthistopic.Mostprevious research has fo-
cused on southerly facing slopes andthe melt-freezecrusts
thatoftenaccompanytheselayersonsouthaspects.In
Montana, we have found that northerly facing slopes also
have weak layers ofnear-surface faceted crystals.Of the
30 backcountry avalanches that failed on near-surface fac-
eted snownoted inTable1, 73%of them facedeither north-
west, north, northeast, or eastand werenot associated with
crust layers, while 27% did have a more southerly aspect
andwere associatedwithcrusts(Table2).Ourobserva-
tions,andthepreceding literaturereview,indicatethat
research is lacking in two main areas:1) the formation of
near-surface faceted crystals in a variety of snow climates
and on northerly aspects, and 2) the role of large tempera-
ture gradient swings in forming these layers.In addition,
there is no research documenting the formation ofthese
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soil-snow interface.Perhaps the most comprehensive re-
searchonnear-surfacefaceted snowwasconductedby
Fukuzawa and Akitaya (1993),who observed the forma-
tion of near-surface faceted crystals on a southerly facing
slope in northern Hokkaido, Japan. Largetemperaturegra-
dients(100to300 0C/m)ledtoextremelyhighcrystal
growth rates resulting in the formation of1mmfaceted
crystals in one night. They followed up their observations
with laboratory studies that looked at the lower snow den-
sitiesand higher temperature gradients generally associ-
ated with the formation of near-surface faceted crystals.
Up until that time,all laboratory studies had focused on
the smaller temperaturegradients and higher snow densi-
ties found in the basal layers where depthhoar forms. Still,
theirresearch was primarily focused on southerly facing
slopes, and thehigh temperaturegradientsthat form above
melt crusts on subsequent cold,clear nights.
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layers in theMontana snowpack. Thepurpose of this study
wastodocumenttheformationofnear-surfacefaceted
crystalsin Montana,assessthe conditions necessaryfor
their growth, and observe their contribution to avalanche
formation.
TERMINOLOGY
Near-surface facetedsnow has been known by a variety of
names. Skiers andavalancheworkers have calledthese lay-
ers"recrystallizedsnow","spaghettisnow","recycled
powder", or"loud powder". Although we use these terms
in our avalancheforecasts, they are not particularly useful
since all snow thatis facetedhas been"recrystallized", and
the other terms do not adequatelydescribe thesnow grains.
Stratton(1977)called these layers"upper leveltempera-
ture gradientsnow", whichdescribes the location in which
they are found andthe process that forms them, but which
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