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

the profiles of depth hoarin the substratumfromone of
theclassifiedsectionimages.Nearlyallofthesection
cuts showed evidence of increased bonding between the
bases of the surface hoar crystals and the snow underneath
as time progressed.This was observed in the snow pit as
well.Figure 7b shows a classified image as an example,
where chains of grains attached tothe bases ofthe hoar
crystals were exposed on the cut.We propose increased
bondingbetweenthe hoarcrystalsandthe layerunder-
neathasoneofthemechanismscontributingtogreater
strength overtime.
Weshow that thinning of layers of buried surface hoar,
and densification and grain growth in the layers immedi-
atelyaboveandbelowaccompanystrengthening.The
trend in thickness of the buried surface hoar layerseems
toshowthemostsensitivityofthesethreeprocessesto
the change in shear strength, particularly early in the test
periods. Langet al. (1985) reportedthe increasein rounded
grains in gapsbetween buried surface hoar crystals, which
we also saw on section cuts.While the mechanism, such
asdifferentialcompaction orinsituformation,remains
unclear,thiscouldcontributetolayerstrengthand was
accounted forin ourmeasurements of thickness.

CONCLUSIONS -SUMMARY

Layersofsurfacehoarcrystalsshowmorebondsatthe
topofcrystalsthanatthebaseduetowedgeor
feather-shaped crystalsgrowingfromafew roots.Also,
wedge andfeather-shaped surfacehoar crystals grow with
geometries that can prevent precipitationparticles from
the subsequent snow fall from reachingandforming bonds
to the layerbelow ("umbrella effect"). These factors tend
to isolate failure to the base of buried surface hoar.
Disaggregatedsurfacehoar crystals observed in the field
mayshowlittlechangeinsizeduringperiodsofmany
weeksduringwhichstrengthincreasessubstantially.
Hence, thechangein size of disaggregatedcrystals is prob-
ably a poorindicatorof strength.
The same cold, clear weatherthat forms surface hoar of-
ten forms facetsor depthhoar near the snowsurface;conse-
quently thebottom of surfacehoar crystals areoften weakly
bonded torelatively weaksnow. Strengthening ofburied
surfacehoar appearsdependenton bondingatthe bottom of
the crystals andthinningof the layer. If the surfacehoar has
formed on depth hoar orfacets, roundingand sintering of
this layer should accompanystrengthening.
We concurwithpreviousworkontheroleofburied
surfacehoarinavalanche formation:carefulidentifica-
tionandobservationofburiedsurfacehoarlayersover
time are critical tasks forpractitioners in areas suscepti-
ble to avalanche release on these layers.

ACKNOWLEDGEMENTS

Our thanks to Ken Black and Adrian Wilson for their field
work, to numerous heli-skiing guides and Jürg Schweizer
for stimulating discussions about surface hoar, andto Mike
Wiegele HelicopterSkiingforlogisticalsupportanda
stimulating environment forfieldstudies.Plane section
preparation andanalyses werefundedby US Army Project
4A762784AT42.Field studies and analysesoffield data
werefunded bytheBCHelicopterandSnowcat Skiing
Operators Association (BCHSSOA) and Canada's Natural
Sciences and Engineering Research Council.

REFERENCES

Breyfogle, S. R., 1987, Growth characteristics of hoarfrost with
respect to avalanche occurrence, Proceedings of the International
SnowScienceWorkshop 1986, Homewood, CA,ISSW Work-
shop Committee, pp. 216-222.

Canadian Avalanche Association. 1995. Observation Guidelines
andRecordingStandardsforWeather,SnowpackandAva-
lanches, Canadian Avalanche Centre, P.O. Box 2759, Revelstoke,
BC, V0E 2S0, Canada.

Colbeck, S.C., 1988, On themicrometeorologyof surfacehoar
growth on snow in mountainous area, Boundary-Layer Meteor-
ology, Vol. 44, p. 1-12.

Colbeck,S.C.,1996,Thebasicideasbehindsnowmetamor-
phism,Proceedings of the InternationalSnowScience Work-
shop 1996, (in press).

Colbeck,S;Akitaya,E;Armstrong,R;Gubler, H;Lafeuille, J;
Lied, K; McClung, D; and Morris, E. 1990. International Classi-
fication for Seasonal Snow on the Ground. International Com-
missionforSnowandIce(IAHS),WorldDataCenterAfor
Glaciology, University of Colorado, Boulder, CO, USA.

Figure 7. Classified section images from start, 17 January, and finish,
8 March, of first test period. Snow grainprofiles are shown as white,
pore space as black.

Predicting the strengthening trend of buried surface hoar
layers requires the integrationand synthesis of several ob-
servations.The growth of bonds between the buried sur-
face hoar crystals and the lower layer, and the thinning of
the hoarlayer,appear to be the mostsensitive variables.
Both of these require careful observation. Bonding between
the surface hoar crystals and the layer below may be more
important than shape changesin size of crystals. Bonding
at base can be observedin situas described in Schweizer
etal.(inpreparation).Roundingand sinteringofdepth
hoarin the substratum likelyaccompanies basal bonding
of the buried surface hoar.The densification of the layer
aboveandlayerbelowprobablyplaysarole.We show
thatthemaximumand minimumthicknessofthehoar
layeroverlapped forsubstantialportionsofthetestpe-
riod,sothataverages bestrepresentthe trends.Several
measurements areneededto approach suitable confidence
in the averages.

78