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

isoutdated due the term"temperature gradient". Others
have called these layers "depth hoar growth in the surface
layer"(FukuzawaandAkitaya, 1993), but that confuses the
processes associated with depth hoarformationwith the
unique processes involved in forming faceted grains near
thesurface."Radiationrecrystallized"(LaChapelleand
Armstrong,1977) snow isdescriptive, but refers to a spe-
cial caseof near-surface facetingthat takes placeon south-
erly aspects in response to a delicate balance between in-
comingsolarradiation and outgoing longwave radiation.
Colbeck (1989),while never naming the crystals,refers to
"near-surface growth" of faceted crystals.In this study, we
expand onColbeck'sterminology,defining "near-surface
facetedcrystals"assnowformedbynear-surfacevapor
pressuregradientsresulting fromtemperature gradients
caused by diurnal snow surface temperature swings.

INFORMALOBSERVATIONS

We have noted the conditions associated with the forma-
tionofnear-surfacefaceted crystalsoverthepastseven
seasons. Since the temperature of the snow 0.25 m below
thesurface isrelativelyconstantovera24hourperiod
(Armstrong, 1985), it is the magnitudeof the daily change
inthesnowsurfacetemperature thatcreatestheupper
level temperature gradients (and resulting vapor pressure
gradients)driving the development of near-surface faceted
crystals. Ideal conditions for this process in our snow cli-
mateappeartobesunny,relativelywarm,subfreezing
(around -1⁰C) days,followed by clear,cold (around -15 ⁰
C) nights. Clear nights allow for maximum radiation cool-
ing of the snow surface and canlead to snow surface tem-
peratures farbelow the ambient airtemperature. During
the day, snow surfacetemperatures increasedue to air tem-
peratureand, on some aspects, solar radiation inputs. The
growth of near-surfacefacetedcrystals is further facilitated
by the presence of low density new snow at the snow sur-
face,an observation previously noted by Stratton (1977).
We have found layers of near-surface faceted crystals
formingonallslopeaspectsatelevationsfrom5500to
11,000 feet.Layers formed on south facing slopes,orlate
in the season, are often associated with crusts,while lay-
ersformedonnortherlyaspectsusuallyarenotassoci-
atedwithcrusts.Typically,near-surfacefaceted crystal
formation is limited to the upper 0.15 m of the snowpack,
with the weakestsnow commonly near the top of the layer.
Thefaceted crystalsmaydevelop anumberofdifferent
forms,depending on the starting grain form,the density
of the surface layer, the intensity of the snow surface tem-
perature swings,and the length oftime the near-surface
layerundergoes those temperature swings.Crystal types
that we have observed include small grained (< 0.5 mm)
beginning faceted grains,large grained (1.5 mm ormore)
advanced facets(sometimes with striations), stringy snow
that looks like needles with facets, and perfectly preserved
stellars, or parts of stellars, with facets. Like other faceted
crystals, layers of near-surface faceted snow are not easily
compressible; well developedlayers will commonly main-
tain a "fist" hand hardness fordays orweeks afterbeing
buried.In addition,variationsinthe snowsurface tem-
peratureswingsatdifferentlocationsleadtodifferent
amounts of faceted crystal growth at different elevations
andaspects.Thus,theprocessofnear-surfacefaceted

growth,like allprocesses in the mountain snowpack,is
spatially heterogeneous.
Once deposited, near-surface faceted snow is a signifi-
cant and surprisingly persistent weak layer in our region.
As discussed earlier,nearly 60% of the large avalanches
we investigated failed on weak layers of near-surface fac-
eted crystals. These layers often produce avalanchesmore
than a week afterbeing buried,and in one extreme case
during the 1991-92 season we observed an avalanche on a
layerthat had been buried forapproximately 90days.

METHODS

In order to measuretemperaturegradients, we constructed
a thermocouple array using six thermocouples mounted
on a PVC plastic pipe.The top twothermocouples were
mounted 0.01 m apart and the remaining four thermocou-
ples were mounted at 0.05 mintervals below them.This
arrangementallowed us to measure the temperature right
aboveandbelowthesnowsurface,aswellasthetem-
peratures in the upper 0.20 mof the snowpack at 0.05 m
intervals.Ideallywewouldhave likedtomeasure tem-
perature changes inthe upper0.30mofthesnowpack,
but only six thermocouples were available.
The thermocouples, calibratedin theMontanaStateUni-
versity cold lab, were attachedto an older Campbell CR21
datalogger. Since we were not able to remotely access the
datalogger(whichhadonlylimited memory), weconducted
our measurementsin an easily accessiblelocation. The flat,
open studyareawas at6000 feet in elevation, andhadsome
trees and a house nearby. We excavated a smallhole and
inserted the thermocouple apparatus intothe snow. After
new snowfall, melting, orsettling, we reset thermocouples
in the snow. Wecollecteddata from early February to April
1996 during periods which appeared favorable to near-sur-
facefacetedcrystal growth. Finally,we analyzedthe hourly
data in a spreadsheetprogram.
Wedug snow pits throughout southwest Montana dur-
ing the 1995-96winterin order to identify layers of near-
surfacefacetedcrystals. Snowcrystals were identifiedwith
the aid of a 20 power Pentax monocular. After identifying
alayerofnear-surfacefacetedsnow,welookedatthe
snowpack temperature patterns that led to its formation,
andthenfollowedthelayerwhenitwassubsequently
buried to observe its contribution to avalanche formation.

RESULTS/DISCUSSION

Formation of a layerof near-surface faceted crystals and
associated temperature conditions
In late December 1995, clear cold nights and relatively
warmdays combined to create a weak layerof near-sur-
face faceted crystals and surface hoarthroughout south-
west Montana.Unfortunately,we did not have ourther-
mocouples working,and therefore we missed the condi-
tionsthat formed that layer.In late March 1996,we had
twotofournewinches ofsnowfallthroughourregion
before an unusual and short cold spell. The cold and clear
spell lasted two nights and a day, with a low temperature
on the first night of about-20 ⁰C,followed by a day with
temperatures up to -3 ⁰C and another night of -15 ⁰C. Sub-
sequent observations revealed that these conditions were
sufficient to create a layer of near-surface faceted crystals
(up to 1 mm in size) on a wide variety of aspects and el

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