Snow Metamorphism as Revealed by ScanningElectron MicroscopyFLORENT DOMINE,1* THOMAS LAUZIER,1 AXEL CABANES,1 LOIC LEGAGNEUX,1 WERNER F. KUHS,2

KIRSTEN TECHMER,2 AND TILL HEINRICHS3

1CNRS, Laboratoire de Glaciologie et Geophysique de l’Environnement, 38402 Saint Martin d’Heres, cedex, France2GZG, Abt. Kristallographie, Universitat Gottingen, 37077 Gottingen, Germany3GZG, Abt. Allg. und Angew. Geologie, Universitat Gottingen, 37077 Gottingen, Germany

KEY WORDS crystal growth; temperature gradient; isothermal metamorphism

ABSTRACT Current theories of snow metamorphism indicate that sublimating snow crystalshave rounded shapes, while growing crystals have shapes that depend on growth rates. At slowgrowth rates, crystals are rounded. At moderate rates, they have flat faces with rounded edges. Atfast growth rates, crystals have flat faces with sharp edges, and they have hollow faces at very fastgrowth rates. The main growth/sublimation mechanism is thought to be by the homogeneousnucleation of new layers at or near crystal edges. It was also suggested that the equilibrium shapeof snow crystals would be temperature dependent: rounded above �10.5°C, and faceted below. Totest these paradigms, we have performed SEM investigations of snow samples having undergonemetamorphism under natural conditions, and of snow samples subjected to isothermal metamor-phism at �4° and �15°C in the laboratory. In general, current theories predicting crystal shapesas a function of growth rates, and of whether crystals are growing or sublimating, are verified.However, the transition in equilibrium shapes from rounded to faceted at �10.5°C is not observedin our isothermal experiments that reveal a predominance of rounded shapes after more than amonth of metamorphism at �4 and �15°C. Some small crystals with flat faces that also have sharpangles at �15°C, are observed in our isothermal experiments. These faces are newly formed, andcontradict current theory. Several hypotheses are proposed to explain their occurrence. One is that theyare due to sublimation at emerging dislocations. Microsc. Res. Tech. 62:33–48, 2003. © 2003 Wiley-Liss, Inc.

INTRODUCTIONThe interest of studying the physics and microphys-

ics of the snowpack has recently increased with thegrowing awareness that exchanges of reactive tracegases between the snow cover and the atmospherecould considerably modify the composition and chem-istry of the lower atmosphere (Domine and Shepson,2002, and references therein). These air-snow interac-tions not only affect atmospheric composition, but theyalso modify the composition of the snow, which even-tually forms glaciers and ice caps where cores are drilledand analyzed to reconstruct past variations of atmo-spheric composition and climate (Dibb and Jaffrezo, 1997;Legrand and Mayewski, 1997; Steffensen et al., 1997).Therefore, the study of snowpack processes is one amongseveral fields where efforts are needed to understandpresent and past atmospheric chemistry, and model itsvariations in relation to variables such as climate forcingby greenhouse gases and anthropogenic emissions.

Numerous chemical processes have been identified inthe snowpack, that lead to the release of reactive gasesto the atmosphere. These include the release of NO andNO2 from the photolysis of the nitrate ion, NO3

�, ad-sorbed on the surface of snow grains (Beine et al.,2002a,b; Honrath et al., 2000a,b), and the release offormaldehyde, HCHO, by solid phase diffusion out ofsnow grains (Perrier et al., 2003).

Modeling HCHO fluxes requires a detailed knowl-edge of physical parameters that characterize the snow

and transport through the snowpack. These include (1)physical parameters such as density and permeability(Albert and Shultz, 2002) and (2) microphysical param-eters such as the shapes and size distribution of snowgrains, and the specific surface area (SSA) of the snow,i.e., the surface area accessible to gases (Domine et al.,2002; Legagneux et al., 2002), and the curvature dis-tribution of snow grains (Brzoska et al., 1999). Forexample, modeling the diffusion time of HCHO out ofsnow grains requires the knowledge of snow grain sizes(Perrier et al., 2002), and modeling diffusion fluxesinvolves snow SSA.

In the case of NO and NO2 emissions by snow, ourcurrent understanding is that the production will be afunction of the amount of NO3

� adsorbed on the surfaceof snow grains (Beine et al., 2002a,b, 2003). Photolysisof NO3

� will lead to the depletion of the snow grainsurfaces in this species, which can be resupplied bysolid state diffusion of dissolved NO3

�. However, in thecase of alkaline snow, NO3

� diffusion will be very slow(Beine et al., 2003; Thibert and Domine, 1998) and

*Correspondence to: Florent Domine, CNRS, Laboratoire de Glaciologie etGeophysique de l’Environnement, B.P. 96, 54 Rue Moliere, 38402 Saint Martind’Heres, cedex, France. E-mail: florent@lgge.obs.ujf-grenoble.fr

Received 7 February 2003; accepted in revised form 3 April 2003Grant sponsor: CNRS.DOI 10.1002/jemt.10384Published online in Wiley InterScience (www.interscience.wiley.com).

MICROSCOPY RESEARCH AND TECHNIQUE 62:33–48 (2003)

© 2003 WILEY-LISS, INC.

other processes such as snow metamorphism will haveto come into play to replenish surface NO3

�.Metamorphism of snow is a set of processes that lead

to changes in snow grain sizes and shapes. In the caseof dry metamorphism (i.e., in the absence of liquidwater), the main processes involved are sublimationand condensation of water molecules, which are trans-ported through the gas phase (Colbeck, 1983a) withinthe snowpack interstitial air. Metamorphism thus re-mobilizes water molecules and solutes, and releasedsolutes then have the potential to adsorb onto the snowgrain surfaces. If metamorphism is rapid, solutes canbe released much faster than by solid state diffusion.For example, the data of Nelson (1998) indicate that afew hundred microns of ice can sublimate in a day,while the diffusion distance of HCHO or HNO3 mole-cules over that time range is about 10 �m (Perrier etal., 2003; Thibert and Domine, 1998).

Thus, understanding snow metamorphism quantita-tively is crucial for the parameterization of snowpackphysical processes in coupled air-snow models. Snowmetamorphism is also a crucial field in avalanche re-search, as different types of metamorphism lead todifferent snowpack mechanical properties (Durand etal., 1999). Yet numerous other scientific fields are in-terested in snow metamorphism. Snow albedo, and,therefore, its radiative effect, depend on snow crystalsize and shape (Schwander et al., 1999), which aredetermined by metamorphism. Quantifying interac-tions between the snow cover and soils and vegetationrequire a description of snow metamorphism (Booneand Etchevers, 2001), and predicting the duration ofsnow cover as well (Tappeiner et al., 2001). A detaileddescription of snow metamorphism that will be usefulat the microscopic scales needed to predict solute re-lease and the evolution of crystal shapes must take intoaccount the physics of snow crystal growth. Numeroustheories have been developed to that end (e.g., Colbeck,1983a,b, 1989; Kobayashi and Kuroda, 1987), and ob-servations have been made using optical microscopy(Colbeck, 1986). However, because ice is transparent tovisible light and opaque to electron beams, scanningelectron microscopy (SEM) has an enormous potentialto reveal details in the changes in surface morphologyof snow crystals. Yet SEM has only been used a fewtimes to study snow (Domine et al, 2001; Iliescu andBaker, 2002; Wergin et al., 1995, 1996, 1998, 1999,2002) and only once to observe specifically changes incrystal morphology during metamorphism (Legagneuxet al., 2003).

The purpose of this article is to use SEM to test ourcurrent understanding of the dry metamorphism ofsnow, and possibly to reveal new aspects that requireclarification. We will first briefly recall the currentstate of knowledge in the field, and then report ob-servations on snow samples having evolved in natu-ral environments, where many variables can affectsnow metamorphism. To constrain some of thesevariables, we have also performed laboratory exper-iments under isothermal conditions and made SEMobservations on several snow samples evolved at �4and �15°C.

DRY METAMORPHISM OF SNOW:THE MAIN CONCEPTS

It is now recognized that transport of water mole-cules in dry metamorphism is mostly via the gas phase(Colbeck, 1983a). The driving force for transport is thegradient in water vapor partial pressure, �PH2O (Col-beck 1983a), which is caused essentially by tempera-ture gradients in the snowpack, but also by differencesin the curvature of snow grains, according to the Kelvinequation:

PH2O�r� � P0exp�2�Vm/rRT� (1)

where PH2O(r) is the saturating vapor pressure of wa-ter over an ice surface of radius of curvature r attemperature T, P0 is the saturating vapor pressureover a flat ice surface, � is the surface tension of ice, Vmis the ice molar volume, and R is the gas constant. Ofcourse, because ice is anisotropic (Hobbs, 1974), thesurface tension depends on the crystallographic faceand eq. (1) is written here in a simplified form.

The rate of snow metamorphism is obviously depen-dent on �PH2O, and this rate has been found to affectthe shapes and sizes of crystals, and the cohesivenessof the snowpack. Numerous experiments and observa-tions come to the conclusion that fast crystal growthrates lead to the formation of facets, or even concave(hollow) structures at very fast growth rates (Nelsonand Baker, 1996), while slow growth rates formrounded structures (Colbeck, 1983b). Under atmo-spheric pressure, sublimation of snow crystals alsoleads to rounded structures (Nelson, 1998). Rapidgrowth takes place under high temperature gradientsthat induce high �PH2O, and this can occur in the fall orearly winter, when a shallow snowpack insulates arelatively warm ground from a cold atmosphere, orwhen nighttime radiative cooling leads to a snow sur-face temperature a few degrees colder than the under-lying snow (Alley et al., 1990; Domine et al., 2002;Sturm and Benson, 1997). The first situation leads tothe formation of hexagonal crystals, often hollow andcup-shaped, called depth hoar crystals. The second sit-uation leads to the formation of faceted surface hoarcrystals, but their shapes show more variation andinclude hollow cups and feather-shaped crystals (Do-mine et al., 2002; Hachikubo and Akitaya, 1997; Hanotand Domine, 1999; Legagneux et al., 2002). Both depthhoar and surface hoar crystals grow to large sizes,sometimes several cm, that have little cohesion andform snow layers of high permeability. Cold room ex-periments (Marbouty, 1980) have shown that a temper-ature gradient of 25°C/m was needed for the formationof depth hoar crystals. Moreover, depth hoar formationalso required snow densities lower than about0.35 g/cm3. Above that limit, Marbouty (1980) inferredthat insufficient free space prevented crystal growth.

Slow growth takes place under low temperature gra-dients and forms rounded crystals that develop bondsbetween them, leading to snow layers of much strongercohesion than hoar layers. Because of the reducedgrowth rates, crystal sizes often remain sub-millimet-ric under low-temperature gradient metamorphism(Domine et al., 2002; Marbouty, 1980). Under all typesof gradients, the crystals that supply the water vapor

34 F. DOMINE ET AL.

undergo sublimation and show rounded shapes. This istrue even in depth and surface hoar crystals, whereparts of the crystals exhibit rounded shapes (Colbeck,1983a).

Numerous studies have been devoted to the explana-tion of crystal shapes as a function of growth rate, andof whether they are growing or sublimating. Reviewingthese studies is clearly beyond the scope of this report,but the main conclusions and remaining uncertaintieswill be briefly mentioned here, to underline the poten-tial of SEM in testing current theories and in contrib-uting to resolving uncertainties.

It is now widely believed that the most importantmechanism responsible for snow crystal growth is thehomogeneous nucleation of new ice layers at or nearcrystal edges (Beckmann and Lacmann, 1982; Frank,1982; Nelson and Knight, 1998). Likewise, sublimationof crystals is initiated at edges, and sublimation oflayers then propagate towards the center of the faces(Nelson, 1998). The growth/sublimation from spiralsteps at emerging dislocations is not totally ruled out(Beckmann and Lacmann, 1982; Sei and Gonda, 1989)but should be negligible once the threshold for layernucleation is reached (Nelson and Knight, 1998), whichis easily the case in clouds and in the snowpack evenunder fairly low temperature gradients. This mecha-nism of layer nucleation seems to explain crystalshapes during sublimation and during growth at dif-ferent rates as follows (Nelson and Baker, 1996; Nel-son, 1998).

Similarly to crystal growth, but in the opposite sense,the sublimation of ice crystals requires the nucleationof new sublimating ice layers, and this nucleation alsotakes place at crystal edges, where molecules are lessstrongly bonded. As the sublimating layer propagatestowards the center of the face, the propagation ratedecreases because the PH2O gradient is lower towardsthe center of the face and diffusion becomes more rate-limiting (Nelson, 1998). Thus, newly nucleated layersprogress faster than overlying layers, and this leads tocrystal rounding. This is valid for isothermal crystals(Nelson, 1998) but has also been observed for crystalsublimating in a temperature gradient (Colbeck, 1986).

During growth, layers newly nucleated at edges ini-tially progress rapidly towards the center of the crystalface, but again because of the diffusion limitation abovethe center of faces of sufficient size, layers tend to slowdown. This is compensated by an increase in the ac-commodation coefficient of water molecules on the crys-tal face, resulting in essentially flat faces. However,this compensation can only work up to a point. Typi-cally, the accommodation coefficient cannot be greaterthan unity, and at very rapid growth rates, this com-pensation effect becomes insufficient (Nelson andBaker, 1996). The growth rate at the center is thenslower than at the edges and the crystal becomes con-cave and eventually hollow. At moderate growth rates,Colbeck (1983b) observed with optical microscopy thatcrystals had flat faces with rounded edges. At evenslower growth rates, he observed totally rounded crys-tals above �4°C. These last features have been con-firmed by Nelson and Knight (1998) who studied smallcrystals, but no detailed physical explanation was pro-posed. The preferential sublimation of sharp edges, asexpected from eq. (1), is certainly involved in the for-

mation of these structures, however. Here, the adjec-tives very fast, fast, moderate, and slow, are used toqualify growth rates in a relative sense, because thelimits between two of these adjectives vary with tem-perature (Colbeck, 1983b).

When crystals are neither growing nor sublimating,they can be thought of as being in equilibrium. Ini-tially, parts of the crystal may sublimate and othersmay grow, but eventually an equilibrium must bereached, as would be the case in an isothermal snow-pack. Colbeck (1983b) performed growth experimentsat different growth rates and postulated (Colbeck,1983b, 1986) that the equilibrium shape should betemperature dependent: rounded above about �10.5°C,and faceted below. However, this conclusion is basedrather on an extrapolation to very low growth ratesthan on actual equilibrium observations. Colbeck(1986) himself acknowledged that he was “aware of noreport of faceted crystals at low temperature and lowgrowth rates in seasonal snow but such crystals mayexist.” Uncertainties, therefore, exist on the actualequilibrium shapes of snow crystals as a function oftemperature. Moreover, the abrupt shape transition at�10.5°C has not been explained in terms of the mech-anism of crystal growth and sublimation.

In summary, layer nucleation during growth or sub-limation can explain the flat faces of crystals at fastgrowth rates, their hollowing at very fast growth rates,and their rounding during sublimation. However, crys-tal shapes at slow growth rates, and in particular thepostulated transition from rounded to faceted shapesat �10.5°C has not been fully explained.

EXPERIMENTAL TECHNIQUESSnow Sampling

The snow samples whose natural metamorphismwas studied were collected in the French Alps in Feb-ruary 2001 at Col du Lautaret (45°02’10”N, 6°24’26”E),55 km E-SE of Grenoble. The sampling site was locatedin a small south-facing sheltered basin at an elevationof 2,058 m. This elevated site was used despite theabsence of a nearby meteorological station, becauseduring winter 2000–2001, there was no snow at thelower elevations where stations are located.

The sampling method was similar to that detailed inprevious publications (Domine et al., 2002; Legagneuxet al., 2002): vertical faces were dug to observe thestratigraphy of the snowpack and to locate the differentsnow layers to sample. Snow and air temperatureswere measured at different heights with a thermocou-ple. Density was measured at different depths using a500-ml plexiglas sampler. The snow was sampled inglass vials of about 150 cm3 that were immediatelyimmersed in liquid nitrogen (N2(liq)) to stop metamor-phism. Snow was kept in N2(liq) until transfer into theSEM.

The snow samples used for cold room experimentswere collected on 6 February 2002 at Bachad-Bouloud,near the Chamrousse ski area (45°7’10”N, 5°52’35”E),15 km E-SE of Grenoble, at an altitude of 1,750 m.Sampling was done while it was still snowing. Thefresh snow layer was 30 cm thick, and the top 3 cmwere sampled into cubic boxes made of 3-cm-thick in-sulating foam, and whose volume was 1 liter. Glassvials were also filled with snow and immediately im-

35SNOW METAMORPHISM AS REVEALED BY SEM

mersed in N2(liq) to observe the morphology of the freshsnow. The snow temperature was �2.3°C. The insulat-ing boxes were placed inside a thermally insulatedtrunk, which was driven to the laboratory in30 minutes. One set of boxes was placed inside a coldroom at �4°C, and the other set in another cold room at�15°C. The cold rooms are regulated within �0.3°C,but the insulating material of the boxes resulted inundetectable temperature fluctuations inside theboxes. The evolution of these snow samples was thenunder the simplest conditions: no temperature gradi-ent, no wind, and no exchange of water vapor or radi-ation with the atmosphere. Boxes were then sampledat various time intervals, and the sampled snow wasplaced inside a glass vial that was immersed in N2(liq)until transfer into the SEM. A given box was onlysampled once.

The measurements of the specific surface areas(SSA) of the snow samples studied here were also per-formed, to provided extra information on microphysicalchanges. However, most of these have already beenpublished and will not be detailed in this paper devotedto SEM observations. The method used, which has areproducibility better than 6%, was CH4 adsorption at77 K, followed by B.E.T. analysis, as detailed in Lega-gneux et al. (2002). The SSA of some of the samplesfrom Col du Lautaret are reported in Cabanes et al.(2003), and those of samples used in cold room experi-ments are detailed in Legagneux et al. (2003).

Scanning Electron Microscopy ObservationsObservations of snow crystals were made with a

field-emission scanning electron microscope (FE-SEM)LEO 1530 at GZG/ University of Gottingen. The glassvials containing the snow were immersed and openedin N2(liq) and the snow was transferred onto the cup-shaped, 15 mm2 sample holder in the N2(liq). The sam-ple holder was then rapidly inserted into a mobiletransfer chamber, so that the snow was in contact withthe atmosphere for only a fraction of a second. Themobile transfer chamber was evacuated and connectedto an Oxford CT1500HF cryo system, in which thesample holder was cooled to 90 K or colder. Windowsallowed the inspection of the sample prior to its intro-duction in the main chamber of the SEM, where thesample holder was in thermal contact with a N2(liq)-cooled stage. The snow was protected from vapor dep-osition by an anti-contaminator maintained around80 K, i.e., about 10 K colder than the sample. TheFE-SEM allowed operation at an acceleration voltageof 1 to 1.5 kV, with a current of 10 pA, with excellentresolution. Metal-coating the snow surfaces was, there-fore, not necessary and was never done. One disadvan-tage of not using metal coating was occasional chargingof the snow crystal surfaces that perturbed the imagesby generating bright lines or spots. After prolongedexposure to the electron beam, surface roughening bysublimation etching was also sometimes observed. Pro-longed means 1 to 10 minutes, depending on the exactlocation of the snow sample in the receptacle. Some-times, sublimation of snow crystals was observed un-der the beam. Figure 1 illustrates this problem. Figure1a shows a column with capping plates that hadstarted to grow at both ends. Figure 1b shows that aftera few minutes under the beam, the plates had started

to disintegrate. These problems were minimized byreducing beam exposure as consistent with observa-tions, and by discarding pictures with obvious artifactscaused by the electron beam. Magnifications greaterthan 25,000 were obtained, with a resolution betterthan 0.1 �m, and 30 to 100 pictures were taken for eachsample.

Observation of Snow Metamorphism UnderNatural Conditions

Snow that fell during the night of 8–9 February andin the early morning of 9 February 2001 was sampledthree times at Col de Lautaret on 9, 13, and15 February. Meteorological conditions during sam-pling and the stratigraphy of the snowpack aresummed up in Table 1 and Figure 2. On 9 February,the snowpack had a total thickness of 135 cm. Freshdry snow with a significant proportion of small stellarcrystals formed a 24-cm-thick layer (hereafter: layer A)on the surface. Its density increased with depth from0.10 to 0.20 g/cm3. Sample A1 was taken on the verysurface, sample A2 was taken 2 cm below the surface,and sample A3 was taken in the bottom half of thelayer. This fresh snow overlaid a 16-cm-thick refrozensnow layer “B,” that fell on 8 February in temperaturesnear 0°C, where sample B1 was taken. Underneath,older snow layers were present.

On 13 February, the fresh snow layer “A” had shrunkdown to 13 cm in thickness. The previous day waswarm, with air temperature slightly exceeding 0°C,which caused slight melting of surface snow. The nightof 12–13 February was clear and radiative cooling wasefficient, so that 1 cm of surface hoar had formed, fromwhich sample A4 was taken. The melt/freeze layer atthe top of layer A was 2 cm thick (sample A5). Snowgrains were very rounded and about 0.5 mm in size. Avisual examination with a magnifying glass indicatedthat the bottom of layer A (sample A6) was unaffectedby melting and was made up mostly of small roundedgrains, whose initial form could not be recognized, witha few crystals whose original shapes (mostly stellar ordendritic) could still be recognized. The refrozen snowlayer “B” had also slightly shrunk down to 13 cm, and

Fig. 1. SEM pictures illustrating the sublimation of a snow crystalunder the electron beam. a: After a few seconds of exposure. b: Afterseveral minutes of exposure.

36 F. DOMINE ET AL.

sample B2 was collected. No modification was observedfor lower layers.

On 15 February, surface hoar had been wind-blownfrom the top of the snowpack, and had accumulated inhollows. It was sampled in a shallow pit that we haddug on 13 February, and where only layer A had beenremoved. There, it formed a soft 2-cm-thick layer con-sisting of recognizable surface hoar crystals (Fig. 2)from which sample A7 was taken. Around the pit, thetop of the snowpack consisted of a melt-freeze sub-layeras observed under the surface hoar on 13 February.Sample A8 was taken from this melt-freeze layer. Un-derneath, the rest of layer A had visibly metamor-phosed to a mixture of rounded and faceted crystals,where sample A9 was taken. Sample B3 was taken fromlayer B, which showed no visible change since theprevious sampling.

This report focuses on dry metamorphism, and onlythe samples that were not affected by melting will bediscussed. Samples subjected to melting (A5, A8, B1 toB3) will be discussed in a later publication, and arementioned and numbered here to keep the numberingconsistent with future work. We will successively in-vestigate the sequence A1-A4-A7 to discuss surface hoarformation, the sequence A1-A2-A3 to discuss the evolu-tion of the snow shortly after precipitation, and thesequence A3-A6-A9 to discuss the metamorphism of thelower part of layer A.

Surface Hoar Formation: Sequence A1-A4-A7

Figure 3 shows SEM pictures of sample A1. Twotypes of crystals can be seen. Precipitated crystals,most of them dendritic, that have rounded edges (Fig.3b), coexist with a majority of crystals with flat facesand very sharp angles. These latter features are indic-ative of rapid growth. Some faces are even hollow (Fig.3c), indicating even faster growth. Pictures clearlyshow that these crystals grew onto rounded crystals(Fig. 3a,d), by the rapid deposition of water vapor. Thisis consistent with our temperature measurements,which indicate that the snow surface was colder thanboth the overlying air and the lower part of the snow-pack (Table 1). Hence, water vapor fluxes all convergedtowards the snow surface, resulting in rapid crystalgrowth. The average linear growth rates can be esti-mated: precipitation had stopped about 2 hours beforesampling. Crystals thus grew to 50–150 �m in 2 hours,so that the average growth rate was 25–75 �m/h. Suchgrowth continued during the following days. Eventhough partial melting took place between 9 and13 February, radiative cooling at night allowed furtheror new growth and the formation of well-developedsurface hoar crystals, as shown in Figure 4. Not allcrystals were formed the preceding night, however, assome larger crystals (Fig. 4e) show signs of melting,and after having survived the warm day, appeared tohave resumed growth the following night. The fastgrowth is evidenced by the development of very well-marked hollow structures. One crystal shows stepsthat we interpret as growth steps (Fig. 4c, magnified onFig. 4d). These steps clearly do not originate from anemerging screw dislocation, and it appears reasonableto suggest that they are nucleated at a crystal edge.The step height cannot be evaluated precisely, but itcould be of the order of a micron. The average lineargrowth rate of crystals sampled on 13 February canagain be estimated. The growth of crystals showing nosign of melting took place during the night precedingsampling. Crystals grew to 200–1,000 �m in about14 hours, and the average growth rate was then 15–70 �m/h. Obviously, in the absence of meteorologicalmonitoring, these rate values are only estimates. It ispossible that growth rates reached significantly highervalues over a limited time range at optimal growthconditions of T gradient and relative humidity. Thepresence of larger crystals in A4 relative to A1 mani-

Fig. 2. Stratigraphy of the snowpack observed at col du Lautaretin February 2001. Surface hoar on 15 February (A

7) was only observed

in wind-sheltered areas.

TABLE 1. Sampling conditions and some measurements performed at Col du Lautaret, where natural metamorphism was studied

Dateand time (GMT) Weather conditions

Tair(°C) Layer/sample Depth (cm) Tsnow (°C) SSA (cm2/g)

9-Feb 11:00 Cloudy �2.0 A1 0 �2.5 690Light west wind A2 2 �2.5 666

A3 20 �2.0 627B1 27 �2.0 240

13-Feb 11:00 Sunny, no wind �1.2 A4 0 0.0 316A5 2 0.0 134A6 8 �1.5 329B2 20 �3.5 225

15-Feb 11:00 Sunny �1.0 A7 1 0.0 473Light east wind A8 3 0.0 118

A9 8 �0.5 257B3 20 �2.7 207

37SNOW METAMORPHISM AS REVEALED BY SEM

fests itself in a large reduction of SSA: 690 to 316 cm2/g(Table 1).

Pictures of crystals from sample A7 (15 February) areshown in Figure 5. This sample consists of a mixture ofcrystals with flat or hollow faces (Figs. 5a,d), and ofcrystals showing rounded edges (Figs. 5b,c). Since thissample was wind-blown, edge rounding such as seen inFigure 5c may be assigned to sublimation during windtransport. This is not the only possibility, however, asthe morphology of the crystals in Figure 5b may also bereasonably interpreted by melting prior to wind trans-port. SSA of sample A7 (473 cm2/g) shows a significantincrease relative to A4. This can be explained by thebreaking of grains by wind, that produced new sur-faces, and by sublimation during wind blowing, if itactually took place, that would have reduced grain size.A similar wind blowing event observed in the Arcticwas also reported to increase snow SSA (Cabanes et al.,2002).

In summary, the sequence, A1-A4-A7 illustratesthat the rapid growth of surface hoar leads initiallyto flat faceted crystals, even though a few hollow

faces were observed. Hollow faces become moreabundant as crystal sizes increase. These observa-tions are consistent with the present understandingof snow crystal growth during metamorphism, dis-cussed earlier. We can also suggest that wind trans-port may have induced sublimation, resulting inrounded edges, again in agreement with current the-ory. The sequences A1-A2-A3 and A3-A6-A9 will nowbe discussed to test other aspects of snow metamor-phism.

Snow Evolution Just After Precipitation:Sequence A1-A2-A3

Sample A2 (not shown) consisted of a mixture ofprecipitated crystals similar to that shown in Figure 3band of crystals grown onto those crystals by watervapor deposition. This sample was essentially similarto A1, except that the proportion of precipitated crys-tals was higher and they were slightly more rounded.

Sample A3 was made up of precipitated crystals (Fig.6) and no crystal resembling surface hoar was ob-served. Stellar crystals and dendritic fragments can

Fig. 3. SEM pictures of sampleA1, collected 2 hours after the endof the precipitation event, on9 February 2001. Newly formedsurface hoar crystals with sharpedges are seen together withfreshly precipitated crystals thathave slightly rounded edges (seetext). Amorphous ice deposits,formed by the condensation of at-mospheric water vapor duringtransfer to the SEM chamber, canbe seen on a, c, e.

38 F. DOMINE ET AL.

clearly be seen (Fig. 6a,b), but the initial shapes ofnumerous crystals cannot be recognized, as extensiverounding caused by metamorphism has already takenplace. Table 1 shows that the temperature gradientaround sample A3 was low: a few °C/m, and fairly slowgrowth rates are then expected, leading to roundedshapes. A few small flat faces were seen, however (Fig.6d), and 8 small crystals (20 to 50 �m) with sharpangles were seen in 51 pictures taken on this sample(Fig. 6c). Observing sharp angles on such small crystalswould be very difficult with an optical microscope.

If we assume that the snow fallen during the night of8–9 February was homogeneous in time, the maintrend in the sequence A1-A2-A3 is crystal rounding. Thepresent observations confirm that under a low temper-ature gradient, metamorphism leads to roundedshapes. Shapes become rounded even though metamor-phism was rapid, because around �2°C, very high wa-ter vapor supersaturations (and therefore growthrates) must be reached to observe facets and sharpangles (Colbeck, 1983b). The main observed trend inthis sequence thus appears qualitatively consistentwith the present understanding of crystal growth dur-ing metamorphism.

However, the sharp angles observed on small crys-tals suggest that locally, crystal growth may have been

rapid, at least according to current theory. Heteroge-neous growth rates within the snowpack is indeed apossibility. The light wind present on 9 February in-duced air circulation in this surface snow layer. Thisfresh snow layer that formed a tortuous network doubt-less had a low permeability (Albert and Schultz, 2002),

Fig. 4. SEM pictures of sample A4, collected on 13 February 2001,which consisted of large surface hoar crystals. d: A blow up of c thatreveals structures interpreted as bunched growth steps (see text).

Fig. 5. SEM pictures of surface hoar crystals from sample A7,collected on 15 February 2001. a,d: Crystals with sharp angles. c: Arounded edge possibly produced by sublimation during wind trans-port. b: Rounded edges that may have been caused by either meltingor sublimation during wind transport.

Fig. 6. SEM pictures of sample A3, sampled on 9 February 2001, atthe bottom of layer A. a,b: The predominance of rounded shapes.c,d: Flat faces and very sharp angles, which were observed only onsmall structures.

39SNOW METAMORPHISM AS REVEALED BY SEM

and it is certain that local variations in this permeabil-ity existed in this snow layer. Entrainment of air withhigh humidity coming from deeper layers or from theatmosphere then took place, and the distribution ofthis humid air was also heterogeneous. We, therefore,suggest that, although most of this layer underwentsublimation and slow crystal growth leading to round-ing, parts of the layer were subjected to large flows ofhumid air that led to crystal growth at a rate sufficientto form sharp angles. At this stage, we then come to thetemporary conclusion that even in low temperaturegradient metamorphism, local conditions can lead torapid crystal growth and to the formation of facets andsharp angles. This is an addition to our present under-standing, that is made possible by the use of SEM.

Metamorphism of the Base of Layer A:Sequence A3-A6-A9

The metamorphism of the base of snow layer A cannow be observed with images from the sequence A3-A6-A9. Figure 7 shows pictures of sample A6. In general,crystal size has increased relative to sample A3 (Fig. 6)collected 4 days earlier. Grain boundaries are moreabundant and have larger cross sections. Original crys-tal shapes cannot be recognized, except in rare in-stances such as the stellar crystal in Figure 7d. Al-though rounded shapes still dominate, there are nownumerous newly formed flat faces with rounded edges.A few sharp angles were also observed, as on Figure 7d,where the tip of the stellar crystal shows a flat facewith sharp angles.

Figure 8 shows pictures of sample A9, collected on15 February. Growth has continued, and crystal sizesfrequently exceed 500 �m. Flat faces now dominateover rounded structures. Grain boundaries have con-tinued their growth. Very distinct flat faces, with edgeshaving radii of curvature 10 to 20 �m, are frequent.

Sharp angles, although rare, were observed (Fig. 8g).Some of these facets are even hollow, indicating veryrapid growth at some stage. The predominance of flatfaces in sample A9 indicates moderately rapid growthbetween 13 and 15 February. Crystal growth was prob-ably driven by the warm days and cold clear nights thatprevailed at that time, resulting in thermal cycling andthe establishment of transient thermal gradients nearthe surface, although these gradients were certainlydamped by about 10 cm of overlying snow. This cyclingmost likely produced alternatively sublimation andrapid growth stages, that lead to the disappearance ofthe smaller structures, the growth of the larger ones,and the formation of flat faces (Nelson, 1998). Crystalgrowth also led to the decrease of the tortuosity of thesnowpack, which results in an enhanced permeability(Albert and Schultz, 2002), faster flows, larger watervapor fluxes, and faster crystal growth. Both the in-tense thermal cycling and the increase in snow perme-ability may then have contributed to the faster growthobserved between 13 and 15 February. Average lineargrowth rates between 13 and 15 February can be esti-mated. A crystal size, as shown in Figure 7, is difficultto define considering the complexity of shapes. We pro-pose to consider the size of a convex unit, and this is100–200 �m in Figure 7. In Figure 8, the size hasincreased to 150–500 �m in 2 days, leading to an

Fig. 7. SEM pictures of sample A6, sampled at the bottom of layerA on 13 February 2001. Crystal sizes have increased relative to Figure6. Some newly formed flat faces with sharp angles can be seen in d,while rounded shapes dominate.

Fig. 8. SEM pictures of sample A9, collected on 15 February2001 at the bottom of layer A. Relative to Figure 7, crystal size hassignificantly increased, and flat faces with rounded edges predomi-nate. Grain boundaries with large cross sections are common. f: Flatfaces with sharp boundaries. g: Flat faces with sharp angles, and alsohollow faces, suggesting very rapid growth.

40 F. DOMINE ET AL.

average linear growth rate of 1–6 �m/h. Although thisestimate is a lower limit because growth occurredmostly when sufficiently elevated gradients werepresent, this value appears significantly lower thanthat estimated for surface hoar: 15–75 �m/h. The sharpedges and hollow faces observed in surface hoar andthe rounded edges observed in this deeper sublayer arethen in agreement with the estimated growth rates andwith the current understanding of snow metamor-phism.

The presence of sharp angles again suggest that aircirculation and water vapor fluxes were heterogeneous.Again according to current theory, we propose thatlocally high water vapor fluxes were probably respon-sible for the formation of sharp angles and even hollowfaces, as in Figure 8f and g.

A summary of the evolution of the sequence A3-A6-A9is that thermal cycling caused the number of crystals todecrease due to the sublimation of the smaller ones,average grain size became larger, grain boundariesincreased in cross section, and flat crystalline faceslarger than 500 �m appeared in large numbers. Thisincrease in crystal size is also well detected by thedecrease in SSA of his sequence (Table 1): 627 cm2/g forsample A3, decreasing to 329 cm2/g for A6, and furtherto 257 cm2/g for A9. In contrast, the SSA of layer Bremained fairly stable. Even though it is less than aday older than layer A, its SSA on 9 February was only240 cm2/g, decreasing little to 207 cm2/g on15 February (Table 1). We suggest that the initialmelt/freeze cycling that layer B on 8 February under-went considerably decreased its SSA, and producedfairly stable snow that evolved slowly. This is con-firmed by the sequence A2-A5-A8, where an initiallyfast SSA decrease due to melting (666 to 134 cm2/g)was observed, followed by a much slower decrease, to118 cm2/g.

Summary of Observations in aNatural Environment

As a conclusion to this section, it can be stated thatthe general paradigms of snow metamorphism appearto be confirmed. Fast growth rates, as observed duringsurface hoar formation, lead to flat faces with sharpangles, and even to hollow (concave) faces. Moderategrowth rates lead to flat faces with rounded edges. Thenovel observation is that in snow undergoing metamor-phism at a slow to moderate rate, and where roundededges largely predominate, flat faces with sharp an-gles, or even hollow faces, are observed. According tocurrent theory, this indicates rapid growth, whichleads us to conclude that growth rates can be veryheterogeneous, probably because of heterogeneoussnow permeability that leads to spatially variable wa-ter vapor fluxes. We have also made one observationthat we interpret as crystal growth by ledge propaga-tion from a crystal edge. We propose that this excep-tional feature was observable because step bunchingtook place, leading to very thick ledges. The reason whysuch important step bunching took place is not under-stood.

A more quantitative understanding of snow meta-morphism would require modeling, which is beyond thescope of this report. Furthermore, modeling the naturalenvironment is complex, especially considering that

high-resolution 3-D modeling would be required to un-derstand crucial details such as the formation of sharpangles during metamorphism at a moderate rate. Tofacilitate the interpretation of SEM observations, andin particular to test the hypothesis that the equilib-rium shapes of snow crystals would change at �10.5°C,we have performed laboratory experiments under sim-ple controlled conditions.

OBSERVATION OF SNOW METAMORPHISMDURING LABORATORY EXPERIMENTS

The snow used for these experiments had an initialdensity of 0.12 and an initial SSA of 763 cm2/g. Nosettling was observed in the boxes, even after a month,from which we infer that the density remained con-stant. Under a magnifying glass, this snow appeared tobe made up of a wide variety of crystal types: columns,plates, capped columns, stellar and dendritic crystals,column rosettes, various irregular crystals, and combi-nations of all of the above types were observed, sug-gesting that a thick cloud with a strong vertical tem-perature gradient was causing this snow fall. Theshapes of falling crystals varied rapidly during the fall:at a given moment, numerous dendritic crystals wouldbe observed, but 3 minutes later, this type of crystalwould be totally absent, and a predominance of platesand irregular crystals could be seen. These rapidlyrepeated variations appeared, however, to give a fairlyhomogeneous snow layer. For SEM examination, only alimited number of crystals can be loaded onto the sup-port, and variations of shape distributions betweenloaded samples is possible. Rime (i.e., supercooled wa-ter droplets that rapidly froze upon impacting the crys-tal) could not be visually detected.

Figure 9 shows SEM pictures of the snow immersedin N2(liq) during field sampling. The wide variety ofshapes is confirmed. A small number of rimed dropletswas observed only on the column shown in Figure 9a.Most crystal edges are very sharp. The edges of theplates of Figure 9b and c appear rounded, but a carefulexamination reveals the presence of pyramidal facesthat intersect the basal and prismatic faces with angleswhose radii of curvature are less than 2 �m. Someedges are slightly rounded, however (Fig. 9h), and sub-limation during the fall of snow crystals probably tookplace.

Figure 10 shows pictures of the snow after 1 day at�4°C. The shapes of almost all crystals can still bereadily recognized. The most visible effect is the round-ing of edges. Some fairly sharp edges do persist, as seenon Figure 10d, even though the radius of curvature isgreater than 2 �m. On most crystals, radii of curvatureof edges can be estimated to be within the range 10 to20 �m. Although morphological changes appear mod-est, the SSA shows a significant decrease, to 516 cm2/g.

After 5 days at �4°C, crystal shapes are significantlyaltered, as shown in Figure 11, and many new grainboundaries have started to form. Original shapes suchas hollow columns and dendritic fragments can never-theless be easily recognized. Although radii of curva-ture in the 50–100 �m range seem to dominate, somevery sharp angles are present, that may be newlyformed. Figure 11d shows an angle between a prismand a pyramidal face that has a radius of curvature ofabout 1 �m. Here and in Figure 11e, the roughening of

41SNOW METAMORPHISM AS REVEALED BY SEM

the surface is due to beam abrasion, but this cannot beinvoked to explain the sharp angle, as Figure 11f alsoshows a similar radius of curvature, while no abrasioncan be detected. These extensive morphologicalchanges manifest themselves in a large SSA decrease,to 381 cm2/g.

After 14 days at �4°C, extensive changes in crystalshapes have taken place (Fig. 12) and original shapescan rarely be recognized (Fig. 12d). The SSA is now324 cm2/g, rounding is widespread, and grain bound-aries with thick cross sections have formed (Fig. 12c).While no sharp edges between two flat faces could befound in 35 pictures taken, several flat faces were ob-served on rounded crystals (Fig. 12f and g). From ob-servations of metamorphism under natural conditions,we concluded that, in a general context of growth at amoderate rate, local conditions could lead to the forma-tion of sharp edges, or even hollow faces indicative ofvery rapid growth (Fig. 8). We are led to a similarconclusion here: Figure 12e–g shows three essentiallyrounded crystals of fairly similar sizes but with clearmorphological differences: The crystal in Figure 12e iscompletely rounded, the crystal in Figure 12f has dis-tinct flat faces with rounded edges, while the one inFigure 12g shows flat faces with very sharp bound-aries.

The sharp angles observed locally during naturalmetamorphism led us to suggest heterogeneous flow inthe snow pack. This process cannot be invoked here:under the present isothermal conditions, PH2O gradi-ents can only be caused by differences in curvature,and supersaturations remain quite low (Colbeck,1983a). For example, eq. (1) indicates that a radius ofcurvature of 1 �m would only lead to a supersaturationof 0.8 Pa at �4°C. As a comparison, a temperaturedifference of only 0.027°C would produce the same su-persaturation, illustrating that curvature effects can-not in principle produce the water vapor fluxes causedby high temperature gradient metamorphism usuallyconsidered as required to form sharp angles (e.g., Col-beck, 1983b). Thus, the presence of those facets is trou-bling. A locally high concentration of structures withvery small curvature appears very unlikely, as no suchfeatures were observed. Other possibilities will be re-viewed in the Discussion.

After 28 days at �4°C, the snow crystals have com-pletely lost their original shapes (Fig. 13) and the SSAhas further decreased to 272 cm2/g. Grain boundariesalmost all have a thick cross section. Very roundedshapes largely dominate. As in Figure 12, a few flatfaces, most of them with rounded edges, could be ob-served (Fig. 13d,e). Some of these faces had fairly sharp

Fig. 9. SEM pictures of thesnow collected near Chamrousseon 6 February 2002, and used inthe isothermal experiments. Thissample was collected within min-utes of its fall and immediately im-mersed in liquid nitrogen. A widevariety of crystal shapes and hab-its were observed, almost all ofthem with sharp angles. Rimedroplets can be seen in a.

42 F. DOMINE ET AL.

boundaries, such as one of the faces in Figure 13d.Between Figures 12 and 13 (14 and 28 days after sam-pling), the rate of change appears significantly slowerthan in the first part of the evolution. This is confirmedby the SSA measurements, that showed that SSAevolved as Ln(t) (Legagneux et al., 2003).

In summary, the main features observed during iso-thermal metamorphism at �4°C are crystal roundingand the growth of grain boundaries. This is in excellentagreement with the current understanding of snowmetamorphism. Figures 9 to 13 strongly suggest thatcrystal shapes tend asymptotically towards veryrounded shapes, and this is consistent with therounded equilibrium shape postulated by Colbeck(1986) for T �10.5°C. However, small flat faces, thatare probably newly formed, were observed. Althoughno sharp angles were seen between two adjacent flatfaces, except possibly in Figure 13d, the intersectionbetween a flat face and the rounded part of the crystalwas sometimes quite sharp. These structures are notexplained by the current paradigms of snow metamor-phism.

A further test of the theory of snow metamorphism isprovided by our cold room study at �15°C. Only the

sample that had evolved for 34 days was observed bySEM, and pictures are shown in Figure 14. Some orig-inal shapes such as plates, stellar crystals, hollow col-umns, and dendritic branches can still be recognized,demonstrating that isothermal metamorphism is muchslower at �15°C than at �4°C. The degree of transfor-mation at �15°C after 34 days is about similar to thatafter 5 days at �4°C, although here the SSA has al-ready decreased to 309 cm2/g. Crystal rounding isclearly visible in Figure 14, and grain boundaries havestarted to form (Fig. 14e), although their cross sectionsare not very thick. These observations are similar tothose already made by SEM on another snow samplesubjected to isothermal metamorphism in a cold roomat �15°C (Legagneux et al., 2003). That previous studyshowed pictures after 49 days of evolution, which dem-onstrated that rounding was then even more pro-nounced. Our SEM investigations at �15°C, therefore,do not support the suggestion of Colbeck (1986) thatthe equilibrium shapes of snow crystals at T �10.5°Care faceted with sharp angles. The observations of Do-mine et al. (2002) on the Arctic snowpack also showedno indication of such faceted equilibrium shapes. In-deed for snow having evolved at T �35°C for several

Fig. 10. SEM pictures of Chamrousse snow af-ter 1 day of isothermal metamorphism at �4°C.Original crystal shapes can still readily be recog-nized, but edges have started to round off.

43SNOW METAMORPHISM AS REVEALED BY SEM

weeks, and perhaps months, even under some temper-ature gradient, rounded shapes were found to be dom-inant from photomacrographs. We are, therefore, obli-gated to question the suggestion that the equilibriumshape of snow crystal is faceted below �10.5°C. Col-beck (1983b) showed by laboratory experiments thatthe supersaturation needed for faceted crystal growthdecreased exponentially with temperature. It is thenpossible that the field observations and laboratory ex-periments on which this conclusion is based were notmade under sufficiently low temperature gradients.Indeed, in natural environments, perfectly isothermalconditions never exist for long durations.

Figure 14 does show, however, some facets that in-tersect with very sharp angles having radii of curva-ture of 1 �m or less. These facets have no resemblancewith those of fresh snow and are clearly newly formed.The coexistence, after such long evolution times, ofsuch sharp features with a predominance of roundedshapes is again puzzling. We also note that the anglesbetween faces on the sample studied by Legagneux etal. (2003) during isothermal experiments at �15°Cwere not as sharp, although the metamorphic condi-tions were identical. Tentative hypotheses to explainthese observations will be detailed in the followingsection.

DISCUSSIONThe main aspect addressed here is the formation of

flat faces during isothermal metamorphism at �4°C,and the formation of flat faces with sharp angles undersimilar conditions at �15°C and during metamorphismunder natural conditions. At this point, we do not claimto explain fully these observations, but we wish topropose hypotheses that can be tested in future stud-ies.

First of all, it is noteworthy that in both laboratoryexperiments and in field observations, the structuresshowing flat or sharp features are small, typically60 �m in diameter. We will first explore the possibilitythat peculiar geometries can generate locally largefluxes, with enhanced water vapor sinks on small par-ticles. We, therefore, compare flux equations onto largestructures with those obtained onto small structures.

We consider a large structure with an equilibriumPH2O value resulting in a water vapor concentrationCin. This structure is fed by a neighboring structurehaving an equilibrium concentration Cout, a distance daway. Assuming steady state, i.e., �PH2O�0, where � isthe Laplacian operator, the flux of water molecules,Jlarge, on the large structure is:

Jlarge � D�Cout � Cin�/d (2)

where D is the diffusion coefficient of water moleculesin air.

In the case of a small structure, we now consider aspherical grain of radius Rin, with a water vapor con-

Fig. 11. SEM pictures of Chamrousse snow after 5 days of isother-mal metamorphism at �4°C. Original crystal shapes can still some-times be recognized, and rounding is widespread. Sharp angles in d–gare probably newly formed. The rough surface structure on d and e isdue to beam abrasion.

Fig. 12. SEM pictures of Chamrousse snow after 14 days of iso-thermal metamorphism at �4°C. Original crystal shapes can onlyrarely be recognized, as in d. New grain boundaries with thick crosssections are very common, as in c. Small rounded crystals havedifferent shapes: completely rounded in e, with flat faces havingrounded edges in f, and with flat faces having sharp boundaries in g.

44 F. DOMINE ET AL.

centration Cin in its immediate vicinity, interactingwith other structures located symmetrically around it,a distance d away, and with a concentration Cout > Cinin its immediate vicinity. The flux Jsmall on the smallstructure will then be:

Jsmall � �D�Cout � Cin�/d��d � Rin�/Rin (3)

The ratio Jsmall/Jlarge is then (d-Rin)/Rin, and this canreach significant values. With Rin�30 �m, as typicallyobserved for the small structures with flat faces, andd�150 �m, as compatible with a snow density of 0.12,we get Jsmall/Jlarge � 4. In some cases, this can besufficient to change the growth mode from rounded tofaceted. Since the flux threshold for faceted growth ismuch lower at �15°C than at �4°C (Colbeck, 1983b),this is consistent with more numerous and sharperstructures at �15°C.

This suggestion of local geometrical effects can ex-plain fairly convincingly the observations under natu-ral conditions, where even small temperature gradi-ents can generate fluxes onto structures with small

radii of curvature. For example, we believe that thefacets and sharp angles of Figure 8g, which we sug-gested could be caused by locally enhanced air flow, canalso be explained by the local geometry: a small struc-ture with larger structures located relatively far awayacting as an H2O source will experience fast growth.Under controlled isothermal conditions, however, eq.(1) implies that small structures will act as sources ofwater vapor, not sinks. To reconcile our hypothesiswith the experimental conditions, we have to suggestthat small temperature gradients may actually exist inour isothermal experiments. This is not impossible.The PH2O difference between surfaces having radii ofcurvature of 30 and 150 �m is only 0.021 Pa at �4°C,and this can be compensated by a temperature differ-ence of only 6 10-4°C. Since our cold room is regulatedwithin �0.3°C, and even with the insulation providedby the boxes containing the snow, it is clear that suchsmall temperature differences would be undetectableand cannot be entirely ruled out. This is not our favor-ite explanation, however.

The other possibility that can be explored is thatlocally, crystal growth would not be by layer nucle-ation. Dislocations and stacking faults are present insnow (McKnight and Hallett, 1978; Mizuno, 1978;Oguro and Igashi, 1971), although in small numbers,and they may be involved locally in growth, especiallyat low growth rates where the threshold for layer nu-cleation may not be reached (Beckmann and Lacmann,1982; Ming et al., 1988). Explaining quantitatively theformation of sharp angles by growth at dislocations orstacking faults would require mathematical develop-ments and modeling that are well beyond the purposeof this paper. These developments may show that thelimitation of water vapor fluxes by gas phase diffusion,which is thought to be responsible for rounded edges(Nelson, 1998), may not apply here. Indeed, growth bylayer nucleation starts from the edges, which results inwater vapor depletion at these very edges that favorssublimation and rounding at low supersaturation. Onthe contrary, spiral step growth starts from the centerof faces, and a smaller depletion at edges could favortheir stability during slow growth.

Equally possible is the suggestion that sublimationstarting at emerging dislocations would produce flatfaces. Nelson (1998) has shown that rounded edgesduring sublimation by layer nucleation was caused bydiffusion limitations, resulting in step propagation be-ing faster near edges than near the center. With dislo-cations, sublimation would start at the center andmove outwards, and this could lead to flat faces. Thissuggestion has the advantage of being easily reconciledwith the observation of these flat faces mostly on smallstructures, which are expected to be sublimating ac-cording to eq. (1). Why some small structures wouldshow such faces and others not, as in Figure 12e–g,may be explained by the presence or the absence ofdislocations. Another contributing factor could also bethe local geometry, as eq. (2) and (3) also apply tosublimation.

Different dislocation densities may also explain thedifferent observations reported by Legagneux et al.(2003), who studied the isothermal metamorphism ofanother snow sample at �15°C. We suggest here thatthat other snow sample may have had a lower disloca-

Fig. 13. SEM pictures of Chamrousse snow after 28 days of iso-thermal metamorphism at �4°C. Original crystal shapes cannot berecognized at all. Crystals are very rounded, and grain boundarieshave very thick cross sections. A few flat faces were neverthelessobserved, as in d and e. d appears to show a sharp edge between twoflat faces.

45SNOW METAMORPHISM AS REVEALED BY SEM

tion density than that used in the present experiments.We postulate here that the nature of the ice nucleuscan affect the dislocation density on the snow crystalgrowing in the atmosphere. Both snow samples wereformed in different air masses that probably had dif-ferent aerosol types with different ice nucleation prop-erties. This would result in different dislocation densi-ties, which could affect the sharpness of angles duringthe sublimation of small structures.

A last possibility involving sublimation is that thediffusion limitation, held responsible for edge roundingduring sublimation (Nelson, 1998), would not be oper-ative on small structures. This appears reasonable, asthe hollowing of faces during fast growth, which is alsodue to diffusion limitations, does not take place onsmall structures (Nelson and Baker, 1996). While thissuggestion may indeed contribute to the explanantionof the phenomenon, it is difficult to reconcile with Fig-ure 12e–g, which shows flat faces in some small crys-tals and none on others.

The chemical composition of the snow may also affectits growth mechanism. Indeed, snow metamorphisminvolves sublimation-condensation cycles that entrainsolutes (Domine and Shepson, 2002). Laboratory exper-iments have revealed that ice crystal growth was per-turbed by the presence of dopants in the gas and liquidphases. Odencrantz (1968) used organic and inorganicdopants in the ppm concentration range to grow snowcrystals from the gas phase. They observed habit andcrystal size changes. Neustaedter and Gallily (1987)grew snow crystals from the gas phase in the presenceof 5% (molar) of several alkohols and observed a changein habit and a reduction in growth rates. Anderson etal. (1969) grew snow crystals from the gas phase in thepresence of unspecified, but apparently large, amounts

of methyl 2-cyanoacrylate and also observed a changein crystal size and habit. Oguro and Igashi (1971) ob-served that growing ice crystals from the liquid phasein the presence of ammonia increased the density ofdislocation loops. This effect increased with increasingNH3 concentration, but no concentration value wasgiven. Cross (1971) observed by SEM the sublimationof undoped and doped ice crystals, and concluded thatdopants modified the mechanism of ice sublimation.The dopants were in the ppm range.

Dopants in the atmosphere and in the snow arepresent in much lower concentrations that those usedin the above experiments. Most atmospheric tracegases are in the low ppb to high ppt concentrationrange (Domine and Shepson, 2002). Most snow dopantsare in the low to high ppb range (Legrand andMayewski, 1997; Maupetit and Delmas, 1994). Sincethe above studies suggest that the effects of dopantsincrease with their concentration, it appears likely thattheir effect on the growth and sublimation of Alpinesnow is minimal. More detailed experimental studieswould be needed, however, to fully confirm this.

SUMMARYWe have performed SEM observations of snow crys-

tals to test current paradigms of snow crystal shapesduring metamorphism. In general, we confirm the gen-eral trends that are presently widely accepted. Theunderstanding of these trends is largely based on thehypothesis that snow crystal growth and sublimationoccurs by the homogeneous nucleation of new ice layersat or near crystal edges. Because growth and sublima-tion are diffusion limited, this mechanism leads torounded faces during sublimation and slow growth.Growth at moderate rates leads to faceted crystals with

Fig. 14. SEM pictures of Cham-rousse snow after 34 days of isother-mal metamorphism at �15°C. Orig-inal crystal shapes can fairly oftenbe recognized: a stellar crystal (a),hollow columns (b,f), and a brokendendrite (f). Metamorphism thusappears much slower than at �4°C.Rounded shapes predominate, butflat faces with sharp angles arefairly frequent (c,f, and inset).Sharp angles were observed exclu-sively on small structures.

46 F. DOMINE ET AL.

rounded edges. At fast growth rates, flat faces withsharp angles are observed, and at very fast growthrates, faces become hollow, again because of diffusionlimitations. Our observations confirm these generaltrends.

Moreover, Colbeck (1983b, 1986) observed that thetransition from rounded to faceted crystals duringgrowth occurred at a supersaturation that decreasedexponentially with decreasing temperature, and postu-lated that at slow growth rates and low temperatures(T �10.5°C), faceted crystals would be predominant.Our experiments of isothermal metamorphism at�15°C, together with those reported by Legagneux etal. (2003), indicate that this suggestion is not correct,and that after over a month at �15°C, rounded shapeslargely predominate in snow during isothermal meta-morphism.

A novel observation has been made here, as we haveseen a significant number of flat faces on small crys-tals, typically 60 �m in size during isothermal labora-tory experiments. These flat faces had rounded edgesat �4°C, and in the snow sample studied here, severalsuch faces intersected with very sharp angles at�15°C. We have proposed several hypotheses to qual-itatively explain these features. Our favorite explana-tion is that these flat faces are due to sublimationinitiated at emerging dislocations. Sublimation onthese faces, therefore, starts at the center of faces, andpropagates toward the edges, so that diffusion limita-tion cannot lead to edge rounding. The frequency ofthese flat faces can be expected to depend on the dislo-cation density, which presumably is different for eachsnow sample. It can also be affected by the arrange-ment of the snow crystals, as expected from eq. (3).Further mathematical developments and modeling arerequired to test the hypotheses presented here.

ACKNOWLEDGMENTSThis work was funded by CNRS through Programme

National de Chimie Atmospherique and through GDRRS-glace.

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