OBSERVATIONS OF SNOW CRYSTALS USING LOW TEMPERATURE
SCANNING ELECTRON MICROSCOPY

Running Title: Low Temperature SEM of Snow Crystals


William P. Wergin1, Albert Rango2 and Eric F. Erbe1


Electron Microscopy Laboratory1 and Hydrology Laboratory2, 
Agricultural Research Service, U.S. Department of Agriculture, 
Beltsville, MD 20705 USA


Address for Reprints:
	William P. Wergin
	Electron Microscopy Laboratory
	USDA-ARS, BARC-E, BLDG 177B
	Beltsville, MD 20705



Key Words: Low temperature, SEM, snowflake, snow crystal


Summary: Low temperature scanning electron microscopy (SEM) was 
used to observe precipitation particles commonly known as 
"snowflakes".   The snowflakes were collected in Beltsville, MD 
at temperatures ranging from -6 C to +1 C, mounted on SEM 
stubs, frozen in liquid nitrogen (LN2) and then transferred to 
a cryosystem mounted on a field emission SEM.  Neither sputter 
coating with platinum nor irradiation by the electron beam 
affected their delicate fine structure.  SEM observations 
revealed that snowflakes consisted of aggregations of snow 
crystals that occurred as hexagonal plates, prismatic columns, 
needles and dendrites.  In some cases the snow crystals 
contained minute surface structures that consisted of rime, 
microdroplets, short prismatic columns and amorphous films.  
Snow crystals from wet snow, which were collected at 
temperatures of 0 and +1 C, exhibited varying degrees of 
metamorphism or melting.  The discrete crystalline faces and 
their sharp intersecting angles were gradually replaced by 
sinuous surfaces that tended to exhibit more spherical shapes. 
 This study indicates that low temperature SEM is a valuable 
technique for studying the formation and metamorphosis of snow 
crystals.  The results suggest that combining low temperature 
SEM and x-ray analysis could also provide qualitative elemental 
information on the nucleation particles of snow crystals as 
well as on the composition of acid snow.


Introduction

	Snow, which may cover up to 53% of the land surface in the 
Northern Hemisphere (Foster and Rango, 1982) and up to 44% of 
the world's land areas at any one time, supplies at least one-
third of the water that is used for irrigation and the growth 
of crops (Gray and Male, 1981).  For this reason, estimating 
the quantity of water that is present in the winter snowpack is 
an extremely important forecast activity that attempts to 
predict the amount of moisture that will be available for the 
following growing season.  Remote sensing approaches, using 
microwave data, have been successfully tested in certain 
situations to calculate areal water equivalent of the snowpack 
prior to melting (Rango et al., 1989; Goodison et al., 1990).  
Unfortunately these estimates can be easily confounded by the 
sizes and shapes of the snow crystals or grains in the 
snowpack.
	A snow crystal is a single frozen ice grain that generally 
results from a process known as ice nucleation in which 
atmospheric water vapor condenses or freezes on a solid 
particle or "nucleus" at temperatures below 0 C.  When 
nucleation occurs, the water molecules form a hexagonal crystal 
lattice resulting from the specific orientation and binding 
that occurs between the oxygen and hydrogen atoms.  Depending 
on the temperature and moisture that prevails during formation 
and descent of snow crystals, the crystal shape may take the 
form of plates, stellar crystals, columns, needles or dendrites 
- all of which are based on the hexagonal lattice structure.  
An individual snow crystal may range in size from 50mm to 5 mm 
(Gray and Male, 1981); aggregations of two or more of these 
crystals form a snowflake, which may range in size from 0.1 mm 
to several cm (Hobbs, 1974).
	The shapes of snow crystals have been extensively studied 
and photographed with the light microscope (Bentley, 1904; 
1923; Bentley and Humphreys, 1931; Nakaya, 1954).  Although 
these studies have resulted in several classification systems 
that recognize as many as 9 distinct classes of snow crystals 
and over 30 subclasses (Hobbs, 1974), detailed examinations 
have been hampered by the difficulty of working with a frozen 
specimen, which is susceptible to sublimation and melting, and 
by the limiting resolution of the light microscope.  In our 
laboratory, preliminary studies indicated that low temperature 
scanning electron microscopy (SEM) could be used to examine 
snow crystals (Wergin and Erbe, 1994a; 1994b).  Encouraged by 
these investigations, this report describes the technique for 
sampling snow, the feasibility of storing snow samples, and the 
use of low temperature SEM to observe a full range of snow 
crystals.

Materials and Methods

	Samples of snowflakes and snow crystals were collected in 
Beltsville, Maryland on six different occasions.  The samples, 
which were obtained when the air temperatures ranged from -6 C 
to +1 C, consisted of freshly fallen snowflakes as well as 
snow that had fallen up to 30 hours earlier.
	The initial attempts to allow snowflakes to settle onto a 
precooled specimen holder were unsuccessful; the snowflakes 
tended to "bounce" off the holder and those that did alight did 
not remain attached during subsequent handling.  A successful 
procedure consisted of placing a thin layer of methyl cellulose 
solution (Tissue Tek) on a holder.  The holder was then placed 
in a Petri dish, taken outside and precooled to the ambient 
outdoor temperature.  Next, the lid of the Petri dish was 
removed and snowflakes were either allowed to settle on the 
surface of the methyl cellulose solution or lightly brushed 
onto its surface.  After a visible sample of snowflakes was 
obtained in this manner, the holder was plunged into a 
styrofoam cup containing liquid nitrogen (LN2) at -196 C, 
transferred to the laboratory and placed into the slush chamber 
of an Oxford CT 1500 HF Cryotrans system.  The holder was then 
attached to the transfer rod of the Oxford cryosystem, moved 
under vacuum into the prechamber for sputter coating with 
platinum (Pt) and then inserted into a Hitachi S-4100 field 
emission SEM equipped with a cold stage that was maintained at 
-185 C.
	Several samples of snowflakes were stored in LN2 for 
future study.  To store samples of snow, a flat metal plate 
(15mm x 27mm) was substituted for the Oxford holder.  After the 
samples had been collected and frozen they were stored in a LN2 
for periods up to one month before being attached to a modified 
Oxford holder and processed for examination as described above.
	Accelerating voltages of either 2 kV or 5 kV were used to 
observe and record images onto Polaroid Type 55 P/N film.  To 
obtain stereo pairs, the first image was recorded, the stage 
was tilted 5 to 10 degrees, the specimen was recentered and a second 
image was recorded.


Results

Samples obtained at subfreezing temperatures.

	At -5 C air temperature the samples consist of snowflakes 
that are composed of several types of snow crystals.  In 
Figures 1 and 2 the majority of the snow crystals, which are 
irregularly shaped, fall into a class known as simple flat 
hexagonal plates.  The largest plates, which are frequently 
associated in complex assemblages, measure 0.7 to 0.8 mm in 
diameter and 0.07 to 0.08 mm thick.  The edges of the thicker 
plates frequently reveal an indentation between the two broad 
narrow surfaces (Fig. 1, arrow).  This type of plate appears to 
correspond to the double sheet structure described by Nakaya 
(1954).  Single flat hexagonal sheets occasionally appear as 
caps on short columns that measure 0.4 to 0.5 mm in length 
(Figure 2, arrow).
	Other types of individual snow crystals consist of 
variations of the hexagonal plates.  A single hexagonal snow 
crystal having six broad branches is illustrated in Figure 3.  
The hole that appears in the center of this crystal probably 
represents the nucleation center from which crystallization was 
initiated.  Although the nucleation particle is not apparent, 
the hole measures about 30 x 50 mm and the crystal has an edge 
to edge span of 1.5 mm.
	Irregularly shaped snow crystals are by far more common 
than the symmetrical forms.  However, even the irregular forms 
of the crystalline plates have a basic hexagonal structure 
(Fig. 4).  In Figure 4, the center of the crystal probably 
began as a hexagonal plate; however further growth resulted in 
six dissimilar branches that also exhibit hexagonal patterns.
	A sample collected at -6 C reveals snowflakes whose 
surface consists of aggregations of numerous short prismatic 
crystals (Fig. 5).  These individual crystals, which are 0.5 to 
0.7 mm in diameter and 0.3 to 0.8 mm in height are well defined 
on the surface of the snowflake but the aggregate seems to be 
held together by an amorphous, non crystalline matrix (Figs. 5 
and 6).  This matrix, which appears continuous with the outer 
surface of the crystal (Fig. 6), may represent a water droplet 
that collided with the prismatic crystals either during 
formation or while descending.   No obvious secondary surface 
structures could be observed on the surface of the prismatic 
snow crystals (Fig. 6).
	Other examples of snowflakes are composed of snow crystals 
that are aggregated as an assemblage of hexagonal plates and as 
"bullets" capped with these plates (Fig. 7).  The assemblage of 
plates is a non symmetrical aggregation in which two or three 
interconnected hexagonal plates appears to alternate at 
approximate right angles with other interconnected plates.  The 
bullets consist of short columns that are tapered at one end.  
Three or four bullets are frequently joined at their tapered 
ends.  The opposite end of the bullet, which is flat, is capped 
with an hexagonal plate that is centered on the top of the 
bullet and whose planer axis is perpendicular to that of the 
bullet.  In the hexagonal plates, small holes, which may 
correspond to the nucleating particle, suggest that a single 
nucleation center may have given rise to the hexagonal plate as 
well as to its associated bullet.
	Four distinct types of surface structures can be observed 
on snow crystals.  At low magnification, the most common type 
of surface structure, which generally occurs along an edge of a 
snow crystal, appears as a fuzzy coating (Fig. 8).  When this 
coating is resolved at higher magnifications it appears as a 
fine, non hexagonal crystalline covering, heterogeneous in 
structure and distribution, predominantly attached to the edge 
of a snow crystal (Figs. 9).  This coating appears to 
correspond to "rime".  Rime is a formation that results when a 
growing crystal falls through a cloud composed of supercooled 
water droplets that freeze on its surface.  Rime, which is not 
restricted to any one type of snow crystal, can be observed on 
the edges of the hexagonal plates (Figs. 8 and 9) as well as on 
the edges of the columnar crystals, branched arms (Fig. 10) and 
needles (Fig. 11).  The needles represent another subclass of 
snow crystals that consist of long slender columns, frequently 
hollow and generally associated in irregular bundles (Fig. 11).
	The dominant snow crystal illustrated in Figure 12 is a 
hexagonal plate with six radiating arms.  However, associated 
with the surface of the central plate and extending into the 
arms are numerous spherical "droplets".  These particles, which 
are about 0.03 to 0.05 mm in diameter, are consistent with the 
descriptions of cloud particles or supercooled water droplets 
that have been described in light microscopic examinations of 
snow crystals (Bentley and Humphreys, 1931; Nakaya, 1954).  The 
radiating arms contain another type of surface structure, 
namely small microcrystals that consist of short hexagonal 
columns most of which are only 0.05 to 0.10 mm in diameter.  
Some of these structures appear to be transitional stages that 
arise from the droplets.
	The fourth type of surface structure that is observed on 
snow crystals appears as a thin amorphous coating (Fig. 13, 
arrows).  This type of coating is observed on snow crystals 
that appear to have undergone some degree of "melting" because 
they do not exhibit the sharply delineated crystalline edges 
that are generally associated with snow crystals that were 
collected at subzero temperatures.

Snow samples obtained at or above freezing temperatures.

	Snow that is sampled when the air temperature close to the 
ground is at or above freezing exhibits snow crystals that 
appear to be undergoing melt-freeze metamorphism.  In general, 
all of the crystals, e. g. plates, needles, columns and 
dendrites, in a snowflake exhibit this phenomenon (Fig. 14).  
Melt-freeze metamorphism is characterized by a general lack of 
the sharply defined faces and angles that characterize a snow 
crystal (Fig. 14 and 15).  Alternatively the angular corners of 
the crystal become rounded and the surface flows from one face 
to the next (Fig. 15).  Occasionally a snow crystal is captured 
when melt metamorphism appears to be actively occurring in a 
unidirectional manner (Fig. 16).  In this case, the same snow 
crystal will exhibit an area where the crystalline faces and 
their angular intersects have melted as well as adjacent areas 
that have not been affected and continue to exhibit the 
crystalline features.  As the melting process continues, the 
snow crystal, which is gradually engulfed in an amorphous 
liquid matrix, losses its identity (Fig. 17).
	Snow crystals illustrated in Figures 14 through 17 were 
examined after the samples had been collected, frozen and 
stored in liquid nitrogen for one month.  No indication of 
secondary formation of ice crystals any other destructive 
structural changes could be associated with the storage 
procedure.
	At 0 C, "wet snow" consists of snowflakes that are 
composed of aggregations of partly melted snow crystals or 
decomposed precipitation particles that are held together in 
what appears to be an amorphous liquid matrix (Fig. 18).  At 
more advanced stages, the individual snow crystals are no 
longer recognizable, the "snowflake' consists of an amorphous 
mass in which the angular intersects of individual crystalline 
faces are replaced by rounded protrusions of melting crystals 
(Fig. 19).  The most advanced stage of this phenomenon consists 
of "wet snow" that was collected at 1 C and shows continuous 
spherical masses with no crystalline characteristics (Fig. 20).

Discussion

	Drawings of snowflakes based on light microscopic 
observations, were first published by Robert Hooke in his book 
Micrographia in 1665; however, the first comprehensive and 
systematic light microscopic observations of snow crystals were 
made by Bentley who secured more than 4000 photomicrographs of 
snow crystals over a 37 year period (Bentley, 1904; Bentley, 
1923; Bentley and Humphreys, 1931).  Bentley obtained his 
photomicrographs with transmitted illumination whereby light 
passes through the snow crystals.  This procedure provided 
information about the shapes and internal structure of the 
crystal, however details about the surface structure were 
limited.  To reveal the surface structure of snow crystals, 
Nakaya (1954) used reflected illumination with a light 
microscope.  The light microscopic studies by Bentley and 
Nakaya have provided valuable information about snow crystals, 
however their techniques required rapid collection and handling 
of snow crystals that could quickly metamorphose.  For example, 
evaporation is a problem when the air is dry, humidity can 
cause riming and subtle changes in temperature may result in 
melting or recrystallization.  The approach that we have taken 
with low temperature scanning electron microscopy appears to 
solve many of these limitations.  The procedure not only 
allowed us to observe the same forms of snow crystals that had 
been previously described in the light microscopic studies but 
also easily enabled us to resolve the nucleation centers and 
other minute surface features.  The specimens did not appear to 
be altered by the sputter coating.  The snow crystals were 
stable in the electron beam, did not sublime and could be 
magnified several thousand times to reveal microcrystalline 
water deposits and rime on the surface of the snow crystals.  
The procedure, which was used to collect specimens during 
several snowfalls in Beltsville, MD during the 1993-94 winter 
season, was also capable of preserving melting snow or graupel. 
 Furthermore, alternative holders allowed capture of the 
snowflakes and their storage in LN2 until the specimens could 
be processed for examination in the SEM.  Storage of samples in 
LN2 had no obvious affects on the structure of the crystals.   
This observation is consistent with that of Wergin and Erbe 
(1991) who had used this procedure to store and subsequently 
observe frozen biological specimens.  Finally, the specimen 
stage of the SEM allowed specimen tilt so that stereo images of 
the snow crystals could be recorded.  This procedure has 
apparently not been demonstrated with the light microscope.
	Through the years several classification systems have been 
proposed to categorize snow crystals (Shedd, 1919; Nakaya, 
1954; Magano and Lee, 1966; National Research Council, 1954; 
Colbeck et al., 1990).  Of these, the system that was first 
proposed by the International Association of Hydrology, 
Commission on Snow and Ice in 1951 and more recently revised 
(Colbeck et al., 1990) is most widely accepted.  In this 
system, snow crystals or precipitation particles are divided 
into eight major categories: columns, needles, plates, stellar 
dendrites, irregular crystals, graupel, hail and ice pellets.  
Each category can be further subdivided on the basis of shape. 
 Although neither hail nor ice pellets were observed in the 
present study, examples of all the other shapes were observed 
and appeared to be consistent with the descriptions that were 
based on light microscopic observations.  Therefore, sample 
preparation and observation with low temperature SEM do not 
appear to alter the basic descriptions of snow crystals.  The 
higher resolution that is possible with SEM will allow further 
detailed structural observations on nucleation, on surface 
features such as riming and on changes associated with 
metamorphism.
	The formation of snow crystals most commonly occurs by a 
process known as heterogeneous nucleation.  This process, which 
occurs when temperatures are less than -40 C, results when 
aerosol particles (0.01 to 0.1 mm) cause the formation of ice 
through either the direct freezing of cloud droplets or the 
freezing of water deposited as a vapor onto the surface of a 
particle (LaChapelle, 1969).  Clay-silicate particles are one 
of the most common aerosols or ice-nucleating agents.  However 
other particles can arise from industrial plants, forest fires, 
other organic matter or extraterrestrial material.  Artificial 
nucleation has also been accomplished by using a seeding agent 
such as dry ice or silver iodide.
	In addition to the nucleation particles, snow can capture 
and transfer large quantities of other particulate matter from 
the atmosphere (Gray and Male, 1981).  Two important processes 
involved in this phenomenon are washout, in which atmospheric 
particles are "scavenged" by snow, and snowout in which 
atmospheric particles become attached or incorporated into the 
falling/developing snow crystal.  Therefore, snow like rain can 
be used to sample the concentration of particulate matter and 
aerosols that are present in the atmosphere.  Gray and Male 
(1981) indicate that because a snowflake falls at a slower rate 
than a rain drop and sweeps out a larger area, the snowflake 
will have had a greater exposure to pollutants and therefore 
would be a better indicator of their presence.  Such particles 
have not been described with the light microscope.  However, if 
low temperature SEM were used in conjunction with an x-ray 
detector, analysis of foreign particles or nucleation centers, 
which range in size from 0.01mm to 0.01mm, could probably be 
mapped with the SEM; the x-ray data would contribute 
qualitative elemental composition of these particles, thereby 
providing an important new tool for studying nucleation and 
acid snow.
	In conclusion, low temperature SEM has considerable 
potential as a viable technique for examining snow crystals at 
magnifications that far exceed the resolution of the light 
microscope.  Furthermore, the ability to collect and store snow 
will enable investigators to accumulate samples from numerous 
locations or at different time intervals so that detailed 
observations and comparisons can be done in a convenient and 
orderly manner.  In addition this technique will allow 
investigators to compare core samples at different depths as 
well as to compare and contrast artificially made snow.  The 
ability to add x-ray microanalysis to the low temperature SEM 
would further allow one to gain information on the elemental 
composition of the nucleation particles and any pollutants that 
were incorporated into the developing snow crystal.  Future 
studies in our laboratory will focus on the shape, size and 
structure of snow crystals in a snowpack that has experienced 
equi-temperature and temperature gradient metamorphism - 
factors that have a direct effect on the microwave remote 
sensing of snow.

Acknowledgements:  The authors thank Christopher Pooley for 
converting the SEM negatives to the digital images that were 
used to illustrate this study.


References

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Bentley WA: The magic beauty of snow and dew. National 
Geographic 43, 103-112 (1923)

Bentley WA, Humphreys WJ: In Snow Crystals. McGraw Hill, New 
York (1931) 1-227

Colbeck S, Akitaya E, Armstrong R, Gubler H, Lafeuille J, Lied 
K, McClung D, Morris E: The international classification for 
seasonal snow on the ground. Int. Comm. on Snow and Ice (IAHS), 
World Data Center for Glaciology, Univ. Colorado, Boulder, CO. 
(1990) 1-23

Foster JL, Rango A:  Snow cover conditions in the northern 
hemisphere during the winter of 1981. J Climatology (1982) 20, 
171-183

Goodison B, Walker AE, Thirkettle F: Determination of snow 
water equivalent on the Canadian prairies using near real-time 
passive microwave data. In Proc. Workshop on Application of 
Remote Sensing in Hydrology.  National Hydrology Research 
Institute, Saskatoon, Saskatchewan, Canada (1990) 297-316 

Gray DM, Male DH: In Handbook of Snow: Principles, processes, 
management and use. Pergamon Press, Ontario (1981) 1-776

Hobbs PV: In Ice Physics. Clarendon Press, Oxford (1974) 653-
656

LaChapelle ER: In Field guide to snow crystals. University of 
Washington Press. Seattle 1-101 (1969) 1-101

Magano C, Lee CW: Meteorological classification of natural snow 
crystals.  J. Faculty of Science, Hokkaido University, Ser. VII 
(Geophysics), II 4, 321-35 (1966)

Nakaya U: In Snow Crystals: Natural and artificial.  Harvard 
University Press, Cambridge (1954) 7-77

National Research Council: The international classification for 
snow.  In Int. Assoc. Sci. Hydrol., Tech. Memo. 31, National 
Research Council Canada., Ottawa, Ont. (1954)

Rango A, Martinec J, Chang ATC, Foster JL: Average areal water 
equivalent of snow in a mountain basin using microwave and 
visible satellite data. IEEE Trans Geoscience and Remote 
Sensing 27, 740-745 (1989)

Shedd JC: The evolution of the snow crystal. Monthly Weather 
Rev. 47, 691-94 (1919)

Wergin WP, Erbe EF: Increasing resolution and versatility in 
low temperature conventional and field emission scanning 
electron microscopy. Scanning Microscopy 5, 927-936 (1991)

Wergin WP, Erbe EF: Snow crystals: Capturing snow flakes for 
observation with the low temperature scanning electron 
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microscopy to examine snow crystals.  Proc. 13th Internat. 
Cong. Electron Microsc. 3B, 993-994 (1994b)


Figure Legends

Figure 1.  Stereo pair illustrating a snowflake composed of 
numerous snow crystals, which largely consist of hexagonal 
plates.  The thicker plates have two apposed thin hexagonal 
sheets and generally exhibit an indentation between the two 
layers (arrow).  (Collected at -5 C, photographed at 5 kV).  
Horizontal field width = 2.0 mm.

Figure 2.  Snowflake composed of numerous snow crystals.  
Crystals consisting of short columns are occasionally capped 
with the hexagonal plates (arrow).  (Collected at -5 C, 
photographed at 5 kV).  Horizontal field width = 2.3 mm.

Figure 3.  A snow crystal with broad hexagonal branches is a 
variation of the hexagonal plate.  In the center of the plate 
is a hole that may have contained the nucleation particle at 
the time of formation.   (Collected at -5 C, photographed at 5 
kV).  Horizontal field width = 2.0 mm.

Figure 4.  Irregularly shaped snow crystals are far more common 
than the symmetrical forms.  However even though the branches 
are non symmetrical they do exhibit variations of the hexagonal 
plates.  (Collected at -5 C, photographed at 5 kV).  
Horizontal field width = 1.8 mm.

Figure 5.  Snowflake composed of short prismatic snow crystals. 
 This aggregate of crystals appears to be held together by an 
amorphous non crystalline matrix.  (Collected at -6 C, 
photographed at 5 kV).  Horizontal field width = 600 mm.

Figure 6.  Single short prismatic snow crystal and its 
continuity with the non crystalline matrix illustrated in Fig. 
5.  This figure illustrates that crystals can be observed and 
photographed at magnifications of several hundred times.  
(Collected at -6 C, photographed at 5 kV).  Horizontal field 
width = 90 mm.

Figure 7.  Snowflakes consisting of an assemblage of hexagonal 
plates and several "bullets" capped with plates.  The bullets 
appear to be joined at their tapered ends.  (Collected at -5
C, photographed at 5 kV).  Horizontal field width = 1.6 mm.

Figure 8.  Single hexagonal plate containing random 
accumulation of "rime" along its edge.  (Collected at -6 C, 
photographed at 5 kV).  Horizontal field width = 850 mm.

Figure 9.  Edge of the hexagonal plate illustrated in Fig. 8.  
Rime appears as a fine, non hexagonal, heterogeneous 
crystalline accumulation that generally collects along one edge 
of a snow crystal.  (Collected at -6 C, photographed at 5 kV). 
 Horizontal field width = 280 mm.

Figure 10.  Snowflakes consisting of needles, short columns and 
branched hexagonal plates.  Area showing rime is indicated by 
arrow.  (Collected at -6 C, photographed at 5 kV).  Horizontal 
field width = 3.0 mm.

Figure 11.  Needles consist of long slender columns that 
frequently have a hollow core.  The needles are generally 
associated in irregular bundles.  (Collected at -6 C, 
photographed at 5 kV).  Horizontal field width = 1.1 mm.

Figure 12.  Snow crystal consisting of a hexagonal plate with 
branches.  The central portion of the crystal exhibits surface 
structures consisting of spherical "droplets" whereas the 
branches contain microcrystals that consist of short hexagonal 
columns, which may arise from the droplets.  (Collected at -6 
C, photographed at 5 kV).  Horizontal field width = 2.3 mm.

Figure 13.  Rimed edge of hexagonal plate that also exhibits 
areas that contain a thin amorphous coating (arrows).  Rounded 
edges of this snow crystal suggest that some melting may have 
occurred at some time during formation or descent.  (Collected 
at -6 C, photographed at 5 kV).  Horizontal field width = 420 
mm.

Figure 14.  Dendritic snow crystal showing effects of melt-
freeze metamorphism.  The apices of the crystalline faces do 
not form sharp angles.  Other types of snow crystals are 
apparent in the background.  (Collected at 0 C, stored for 1 
month in LN2, photographed at 2 kV ).  Horizontal field width = 
2.9 mm.

Figure 15.  Branched end of metamorphosed dendritic crystal 
shown in Figure 14.  The surface of the "crystal" appears to 
flow from one face to the next rather than forming sharp 
angular intersections.  (Collected at 0 C, stored for 1 month 
in LN2, photographed at 5 kV).  Horizontal field width = 1.0 
mm.

Figure 16.  Stereo pair illustrating the end of a hexagonal 
column.  The column has round cylindrical core that is 
asymmetrically positioned in the crystal.  Upper portion of the 
column shows the effects of unidirectional melt-freeze 
metamorphism while the end continues to exhibit the sharply 
defined crystalline characteristics of a snow crystal.  
(Collected at 0 C, stored for 1 month in LN2, photographed at 2 
kV).  Horizontal field width = 400 mm.

Figure 17.  Graupel or wet snow that consists of a column 
undergoing melt-freeze metamorphism.  (Collected at 0 C, 
stored for 1 month in LN2, photographed at 2 kV).  Horizontal 
field width = 650 mm.

Figure 18.  Snowflakes consisting of aggregations of partly 
melted snow crystals held together in an amorphous matrix.  
Several microspheres (arrows) are associated with the surface 
of the melting snowflake.  (Collected at 0 C, photographed at 
5 kV).  Horizontal field width = 4.0 mm.

Figure 19.  At more advanced stages of melting, the individual 
snow crystals are no longer distinguished.  The particle 
consists of a highly amorphous mass without any crystalline 
faces.  (Collected at 0 C, photographed at 5 kV).  Horizontal 
field width = 1.6 mm.

Figure 20.  At the most advanced stage of melt-freeze, the wet 
snow consists of an aggregation of spherical masses that do not 
have any crystalline characteristics.  (Collected at 1 C, 
photographed at 5 kV).  Horizontal field width = 1.4 mm.