M o u n t a i n

We a t h e r

a n d

S n o w p a c k

evations throughout southwest Montana. Surface hoar was
not observed. On southerly and easterly facing slopes this
layerwas located overa hard melt/freeze crust,while on
northerlyaspectsitsimplyformed a loosecohesionless
layernear the top of the snowpack.
Afterthe formation of the layerof near-surface faceted
crystals in March, we analyzedthe temperature conditions
found at ourstudy plot in the upperlayers of the snow.
There was a total of0.65 mof snow on the ground at the
study plot.The diurnal changes in the near-surface tem-
per atur eprofileweresimilartothatobservedby
LaChapelle and Armstrong (1977) in the San Juan moun-
tainsinColorado(Figure1).Atmidnightthe snowsur-
facewasquitecold,resulting inastrongnegativetem-
perature gradient through the uppersnowpack.By noon
the snow surface was warming, while the previousnight's
coldtemperaturesresultedincoolertemperaturesfrom
0.05 to 0.20 m below the snow surface. At 1400 hours the
snow surface had continued towarmand now the tem-
perature gradient wasagain strong,butthedirectionof
the gradient had reversed.By 1800 hours the pattern was
starting to return to the same conditions observed during
the previous night,with the uppersurface cooling faster
than the snow beneath it.Clearly,the snow surface went
through wide swings in temperature, while the tempera-
ture 0.20 m below the surface was relatively constant.
Wide variations in surface temperature,combined with
therelativelyconsistent temperatures atdepth, served to
create large temperature gradients in the upper snowpack
(Figure 2). The temperaturegradientin theupper 0.05 m of
the snowpackwas greater than-200 ⁰C/m during the night.
By 1300 hours the temperature gradienthadchangeddirec-
tionsinresponsetothewarmingsnowsurface,butthe
magnitude ofthe gradient was stillhigh (100⁰C/m).The

temperaturegradientfrom 0.05 to 0.10 m below the surface
was also high, exceeding-50⁰C/m at night andswitchingto
greater than 50⁰C/m at mid day. In general, the magnitude
of the temperature gradientthrougha givenlayer decreased
with increasingdistance from the snow surface.
It is important to point out that it is the vapor pressure
gradientresultingfromthetemperaturegradient which
causesfacetedcrystaldevelopment(Armstrong,1985).
Vapor pressure gradientsare a product of the temperature
gradient and the mean snow layertemperature, and val-
ues greater than 5 mb/m are sufficient forfaceted crystal
growth(LaChapelleandArmstrong,1977).However,
since vaporpressure gradients are difficult tomeasure,
temperaturegradientsareoftenusedasasurrogate.
Since we observed widespread faceting of the near-sur-
face snow layers during this time, the magnitude of these
temperature gradientsled tosufficientvaporpressure
gradientstorapidlyformnear-surface faceted snow.

Contribution of near-surface faceted crystals to avalanche
formation
Aftertheformationofthislayerofnear-surfacefac-
etedcr ystals,wecarefullyfolloweditssubsequent
burialtosee how the layerwould react to anew snow
load.IntheBridgerRange(locatedjustnor thof
Bozeman,Montana,and immediately westofourlevel
studyplot) and in the Madison Range (located south of
Bozeman) the faceted crystalswere immediately buried
by 8to 10 inches ofnew snow overthe next three days.
BridgerBowlandBigSkySkiPatrolsreportedwide-
spread avalanching on allaspectsduring control work.
There were also several natural backcountry avalanches
which failed on the layeroffaceted crystals.

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