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

SurfaceHoar

GrowingforSeveral

Days

Akihiro

HachikuboandEiziAkitaya

Key words:surface hoar,weak layer, radiative cooling_{OBSERVATION RESULTS}

Thefollowingtwocasesofsurfacehoargrowtharede-
ABSTRACT_scribed.

Surfacehoar growingfor several clearandhumiddays were
observed. In thedaytime, the air and thesnow surfacetem-^{Case one on December 26 to 28,1994}
peratureincreased and therelative humiditydecreased,^{It was almost clear during this period except for the morn-}
henceevaporation(sublimation)occurredatthesnow^{ing Dec. 27 and the midnight Dec. 27-28,and the surface}
surface. The amount of evaporation calculated with a bulk^{hoarwas formedin thetwonights.Figure 1showsthe}
methodwasenoughtoevaporateoutthesurfacehoar^{time variations ofthe airtemperatureT} _a^{at 1mhigh and}
crystals whichgrew previous night, but theywereobserved^{the snow surface temperature T} _s^{.Twas alwayslower than}
to survive on the snow surface even in the daytime. In the^T_a^{andthedifferencebetweenTs}
andTbecamelarger
following nights new hoar crystals formed on the old ones^{during two nights.T}_s^{increased inathe cloudys}
conditions
and developed largely.This result suggestedthat the sur-^{due to thedecreasein theradiativecooling.Figure 2 shows}
face hoar crystals were cooled by outgoing radiation even^{the time variation of the latent heat flux.From 18:30, Dec.}
in the daytime and kept their size, while snow grains un-^{26 to 3:00,Dec.27 the total condensation of surface hoar}
derneath the surface were warmed by solarradiation and^{was 74 g/m2 , while thetotal evaporation from 3:30 to 15:00}
evaporated. The layercomposed ofthe surface hoarand^{on Dec. 27 was 62 g/m2 .Therefore, the surface hoar crys-}
the depthhoar crystals showed a very weak shear strength.^{talsformedinthe firstnightwere expected tobeelimi-}
nated mostly.However,they were survived on the snow
INTRODUCTION^{surface and the new hoarcrystals developed on themin}
Surface hoar crystals are formed by the deposition of wa-^{the second night.Figure 3a shows a microscopic photo-}
ter vapor onto the snowsurfaceduringthe clearnight. They^{graphof surface hoar crystals formed on a snow particle at}
havelongbeen ofinterest toavalanche researchers(e.g.^{the surfacein the first night.Their size wasless than 1mm}
Perla and Martinelli,1976),because afterbeing buried by^{in diameter.Figure 3b shows the "survived" crystal even}
a subsequent snowfall, they often form a weak layer dueto^{in the daytime though it was rounded by the evaporation.}
their lack of intercrystalline bonding and weakattachment^{New crystals in thesecondnight developedonthe old ones}
to the original snow surface.Besides, a layer composed of^{and reachedto 3mmin diameter (Fig. 3c- e).}
larger hoarcrystals ishard to metamorphose.Lang et al.
(1984) reported that the shear strength remained too low^{Case two on February 23 to 26,1995}
to measure for extended periods of time.In this paper we^{The surface hoar growth was observed forthree clear and}
reportthesurfacehoarcrystalssurvivedonthesnow^{humid nightsof Feb. 23 to 26.The time variationsofTaand}
surface
eveninthedaytimeanddeveloped largelyfor^{Tsare shown in Fig. 4.T swas 5 to 10[!]C lowerthan T}
shows^aat
several days.^{eachmidnight dueto the radiativecooling.Figure5}
the time variationof the latent heatflux. Largeamount of

OBSERVATION SITE AND OBSERVED ITEMS

Observations were carried out in the winter of 1994- 95 in
the TeshioExperimental Forest of Hokkaido University lo-
cated in Toikanbetsu, northern Hokkaido,Japan.The sta-
tion (45[!]N,142[!]E) is close to the Japan Sea (about 20km
in distance),and the southwest is the predominant wind
directionin winter. Air temperature, humidity,windspeed,
snowsurface temperatureand vaporcondensationwere
measured. Observation methods and the instrumentation
used were same as Hachikuboet al.(1995) and described
in detail by Hachikubo and Akitaya (1997).The latentheat
flux atthe surface,which corresponds to the vaporcon-
densation rate orevaporation rate,was obtained with the
weighting-type evaporimeterinnighttime,while itwas
estimated with a bulk method (Takeuchiand Kondo, 1981)
indaytimeusingthedataofairtemperature,surface
temperature, humidityand wind speed. In the calculation,
thevalue of2.9¥10^-3obtainedfromfieldobservations
(HachikuboandAkitaya,1997)wasusedasthebulk
transfer coefficient of water vapor.

Fig. 3 Microscopic photographsof surface hoar crystals formed on
December 26 to 28, 1994.(a) surface hoar crystals formed in the first
night.(b) a surface hoar crystal survived in the daytime androundedby
the evaporation.(c)- (e) new crystals in the second nightdevelopedon
the old ones.

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