SNOW CRYSTALS: CAPTURING SNOW FLAKES FOR OBSERVATION WITH THE 
LOW TEMPERATURE SCANNING ELECTRON MICROSCOPE

William P. Wergin and Eric F. Erbe

Electron Microscopy Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 
Beltsville, MD 20705 USA

	Snow, which may occasionally cover up to 23% of the earth's land, supplies about one-third 
of the water that is used for irrigation and the growth of crops (Gray, 1981).  For this reason 
estimating the amount of water in winter snow pack is an extremely important forecast activity that 
attempts to predict the amount of water that may be available for the following growing season.  
Unfortunately these estimates can be easily confounded by the sizes and shapes of the snow crystals 
that comprise the snowpack.
	A snow crystal is a single frozen ice grain that generally results from a process know as 
nucleation in which atmospheric water vapor condenses 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, nucleation may result in 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, 1981); aggregations of two or more of these crystals 
form a snowflake.
	The shapes of snow crystals have been extensively studied and photographed with the light 
microscope (Bentley and Humphreys, 1931; Nakaya, 1954).  Although these studies have resulted 
in a classification system that currently recognizes 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.  For these reasons, an attempt was made to determine whether 
snow crystals could be collected/stored and prepared for observation and recording in the low 
temperature SEM.
	Attempts to merely allow snowflakes to settle on 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 on a holder and precooling it to the prevailing outside temperature 
during a snow fall.  Next, snowflakes were allowed to settle on the surface of the methyl cellulose 
solution.  After a few minutes, the holder was plunged into liquid nitrogen and transferred to the 
laboratory where it was retrieved from liquid nitrogen, mounted on the transfer rod of an Oxford 
CT-1500 HR Cryosystem, moved into the prechamber for sputter coating with Au/Pd and then 
inserted into a Hitachi S-4100 field emission SEM equipped with a cold stage that was maintained 
at -1850 C.
	These procedures allowed us to observe several forms of the individual snow crystals as 
well as their nucleation centers.  At low magnification, the specimens, which did not appear to be 
altered by the sputter coating, resembled those that had been previously photographed with the light 
microscope.  The snow crystals were stable in the beam, did not sublime and could be observed at 
magnifications of 20,000x or more to reveal microcrystalline water deposits or rime on the surface 
of some the snow crystals.  This procedure, which was used to collect specimens during several 
snow falls in Beltsville, MD during the 1993-94 winter season, was also capable of preserving sleet, 
graupel and hail.  Furthermore, storage holders were devised that allowed capture of the snowflakes 
and their storage in liquid nitrogen until the specimens could be processed for examination in the 
SEM.  Finally, the specimen stage of the SEM allowed specimen tilt so that stereo images of the 
snow crystals could be recorded (Fig 1).
	In conclusion, low temperature SEM is 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 samples enables investigators to accumulate samples from numerous locations or 
different time intervals so that detailed observations and comparisons can be done in a convenient 
and orderly manner.

References

1. Bentley WA, Humphreys WJ: In Snow Crystals. McGraw Hill, New York (1931) 1-227.
2. Gray DM, Male DH: In Handbook of Snow: Principles, processes, management and use. 
Pergamon Press, Ontario (1981) 60-152.
3. Hobbs PV: In Ice Physics. Clarendon Press, Oxford (1974) pp 653-656.
4. Nakaya, U. In Snow Crystals: Natural and artificial. Harvard University Press, Cambridge 
(1954) pp 7-77.























Figure 1.  Stereo pair of micrographs illustrating single snow crystal consisting of a crystalline plate 
with broad branches.  Nucleation center appears as dark hole in the middle of the crystal.  
Horizontal field width = 1.4 mm.