Snow: Low Temperature Scanning Electron Microscope

Snow crystals have been studied with the light microscope for more than 100 years. However, because of their small size, delicate structure, and topography, the light microscope has failed to produce many of the structural details of the crystals. Snow is also hard to transport, store, image, and photograph without subjecting the samples to structural changes from sublimation, melting, and recrystalization.^[1]Armstrong, R.L. (1992) The mountain snowpack: pp. 47-83 in: The Avalanche Book, ed. by B.R. Armstrong and K. Williams.Colorado Geological Survey, Denver.^[2]Bentley, W.A. and Humphreys, W.J. (1931) Snow crystals. McGraw-Hill Book Co. Inc., New York, 227 pp.

Scanning Electron Microscope (SEM)

The scanning electron microscope (SEM) has been widely used by biologists for over fifty years to study living organisms. The high resolution, which is less than 10nm in modern instruments, and it’s depth of focus, which exceeds that of the common microscope by nearly 1,000 fold, provides detailed information and surface topography that is not obtainable with a light microscope.^[3]Blanchard, D.C. (1970) Wilson Bentley, the snowflake man, Weatherwise, 23, 260-69.^[4]Wergin, W.P., Yaklich, R. W. and Erbe, E.F. (1996) Advantages and applications of low temperature scanning electron microscopy. In: Focus on Modern Microscopy. World Scientific Publishing Co, Ltd., New Jersey. In Press.

Low Temperature Scanning Microscopy (LT-SEM)

The SEM was first modified to image samples of snow and ice roughly 20 years ago. The conventional SEM, which operates at an ambient temperature, was replaced with a stage that could be maintained at or near liquid nitrogen temperatures. As a result, high resolution images of the surface of frozen specimens could be observed and photographed with a technique known as Low Temperature Scanning Microscopy (LT-SEM).

The studies under LT-SEM include observations and imagery of columns, needles, plates, stellar dendrites, crystals, graupel, and artificial snow, and metamorphosed snow.

The techniques that have developed observing snow and ice crystals with low-temperature scanning electron microscopy have been found to be successful in a wide range of snow and ice environments. Samples of snow, ice are collected by dislodging the crystals from the face of a snow pit or the surface of the snow onto copper metal sample plates containing precooled methyl cellulose solution. Within fractions of a second these plates are plunged into a reservoir of liquid nitrogen which rapidly cools them to -196°C and attaches these prefrozen materials to the plates. Due to the low surface tension of liquid nitrogen and the extreme hardness of materials cooled to these temperatures, very fragile samples can be shipped by aircraft, in dry shipping dewars (cryogenic containers) from snow study sites throughout the country.^[5]Brownscombe, J.L. and Hallet, J. (1967) Experimental and field studies of precipitation particles formed by the freezing of supercooled water, Quarterly J. Roy. Met. Soc., 93, 455-73. ^[6]Castruccio, P.A., Loats, H.L., Lloyd, D. and Newman, P.A.B.^[7](1980) Cost/benefit analysis for the operational applications of satellite snowcover observations (OASSO): pp. 239-254 in: NASA Conf Publication 2116, ed. by R. Rango and R. Peterson. Sci. Tech. Info. Off.^[8]Dobrowolski, A. (1903) La niege et le givre Expédition Antarctique Belge: pp. 1-78 in: Résultats du voyage du S.Y. Belgica en 1897- 1898-1899, ed by J.E. Buschmann, Antwerp.^[9]Foster, J.L. and Rango, A. (1982) Snow cover conditions in the northern hemisphere during the winter of 1981, J. Climatology, 20, 171-183.^[10]Czygan, F.C. (1970) Blutregen und Blutschnee: Stickstoffmangel- Zellen von Haematoccus pluvialis und Chlamydomonas nivalis, Archiv Mikrobiolgie, 74, 69-76.

After arrival at a microscopy facility, the copper plates can be stored at -196°C in storage dewars. Selected samples are transferred to the preparation chamber for sputter coating with platinum. This renders them electrically conductive and they are placed on the precooled (-170°C) stage of a field emission Scanning Electron Microscope where they are imaged and photographed. Hydrologists study photographs of the grain sizes, shapes in an effort to determine the water content of the winter snow pack. This information is critical to the determination of the nation’s water supply as well as protection from flooding.^[11]Nordenskiold, G. (1893) The inner structure of snow crystals. Nature Land., 48, 592-94.^[12]Pérez-Vicente, R., Burón, M.I., González-Reyes, J.A., Cárdenas, J. and Pineda, M. (1995) Xanthine accumulation and vacuolization in Chlamydomonas reinhardtii cells, Protoplasma, 186, 93-98.^[13]Rango, A., Martinec, J., Chang, A.T.C. and Foster, J.L. (1989) Average areal water equivalent of snow in a mountain basin using microwave and visible satellite data, I.E.E.E. Trans. Geoscience and Remote Sensing, 27, 740-745.^[14]Robards, A.W. and Sleytr, U.B. (1985) Low temperature methods in biological electron microscopy. Elsevier, Oxford, 551 pp.^[15]Stoyanova, V ., Genadiyev, N. and Nenow, D. (1987) An application of the replica method for SEM study of the ice crystal instability, J. Phys. (Paris), 48, 375-381.^[16]Takahashi, T. and Fukuta, N. (1987) J. Phys. (Paris), 48, 405- 411.

The LT-SEM offers a look at the miniature universe and snow and ice like we have never seen before. Snow has been collected from sites all over the U.S. to include; Beltsville, Maryland; Davis, West Virginia; Loveland Pass and Jones Pass, Colorado; and Fairbanks, Alaska. The samples, were obtained in air temperatures ranging from +18°C to -11°C, consisted of freshly fallen snowflakes, as well as, snow crystals that were collected from the walls of snowpits. The snowpits were established in pristine areas, which were located from about 50m to nearly 10km from plowed roadways. Sleds, snow shoes, backcountry AT setups, are used to transport the sampling supplies, including liquid nitrogen, to these remote sites.^[17]Wolff, E.W. and Reid, A.P. (1994) Capture and scanning electron microscopy of individual snow crystals, J. Glaciology, 40, 195-197.^[18]Wergin, W.P. and Erbe, E.F. (1994a) Can you image a snowflake with an SEM? Certainly! Proc. Royal Microsc. Soc., 29, 138-140.^[19]Wergin, W.P. and Erbe, E.F. (1994b) Snow crystals: Capturing snowflakes for observation with the low temperature scanning electron microscope, Scanning, 16, IV88-IV89.^[20]Wergin, W.P. and Erbe, E.F. (1994c) Use of low temperature scanning electron microscopy to examine snow crystals. Proc. 14th Int. Cong. Electron Microsc., 3B, 993-994.^[21]Wergin, W.P., Rango, A. and Erbe, E.F. (1995a) Observations of snow crystals using low-temperature scanning electron microscopy, Scanning, 17, 41-49.^[22]Wergin, W.P., Rango, A. and Erbe, E.F. (1995b) Three dimensional characterization of snow crystals using low temperature scanning electron microscopy, Scanning, 17, V29-V30.^[23]Wergin, W.P., Rango, A. and Erbe, E.F. (1996) Use of low temperature scanning electron microscopy to observe icicles, ice fabric, rime and frost, J. Microsc. Soc. Amer., Proc. In Press.

Snow Crystals under LE-SEM

If you’ve ever wondered what snowflakes truly look like, spend a few moments with these images from the LT-SEM. Snow and crystal structure is pretty amazing, and something we don’t often get to see, even under a 10X loupe on a snowcard. The images below are courtesy of Electron and Confocal Microscopy Laboratory, Agricultural Research Service, U. S. Department of Agriculture, obtained with permissions.

Rime under LE-SEM

The next set of LT-SEM images are of rime that develops on snow crystals as they fall. Under some atmospheric conditions, forming and descending snow crystals may encounter and pass through atmospheric supercooled cloud droplets. These droplets can exist in the unfrozen state down to temperatures near -40° C. Contact between the snow crystal and the supercooled droplets results in freezing of the liquid droplets onto the surface of the crystals. This process of crystal growth is know as accretion. Crystals that exhibit frozen droplets on their surfaces are referred to as rimed. When this process continues so that the shape of the original snow crystal is no longer identifiable, the resulting crystal is referred to as graupel.

Snow Pit using LE-SEM (Avalanche Study)

This last set of images is a snow pit from top to bottom with samples taken every 10cm in the snowpack. You can clearly see the transformation form the new snow to the dreaded depth hoar (the sleeping giant of avalanches) as you follow the images down to the bottom:

Total Pit Depth: 81 cm AG= (above ground)

70-81cm AG – New snow (dendrites and pyramid graupel)
62-70cm AG – ( 2-3 day old snow crystals)
62cm AG – weak layer
57-62cm AG – small faceted grains
57cm AG – weak layer
38-57cm AG – Faceted grains
37-38cm AG – 0.5 cm thick ice layer (old sun crust)
0-37cm AG – Faceted Depth Hoar

The snow pit images above are courtesy of Electron and Confocal Microscopy Laboratory, Agricultural Research Service, U. S. Department of Agriculture, obtained with permissions.

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Conclusion

Snow, which may cover up to 53% of the land surface in the Northern Hemisphere 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. For this reason, calculating the quantity of water that is present in the winter snowpack is an extremely important forecast activity that attempts to predict the amount of water that ultimately will reach reservoirs and estimates how much water will be available for agricultural purposes during the following growing season.^[24]McClung, D. and Schaerer, P. (1993) The avalanche handbook. The Mountaineers, Seattle, 271 pp.^[25]Mason, B.J. (1992) Snow crystals, natural and man made, Contemporary physics, 33, 227-243.^[26]LaChapelle, E.R. (1969) Field guide to snow crystals. University of Washington Press, Seattle, 101 pp.^[27]Nakaya, U. (1954) Snow Crystals. Harvard University Press, Cambridge, 510 pp.

LT-SEM provides a new technique for observing various types of newly precipitated and metamorphosed snow crystals that can be collected, stored and shipped from remote sites. The technique has resolution, depth of field and stereology that cannot be achieved with the light microscope. In addition, application of this technique can be used to observe the process of sublimation of snow crystals, and offers the potential for x-ray analysis to acquire data about the elemental composition of condensing nuclei and particulate pollutants that may become incorporated into snow and ice.^[28]Hellman, G. (1893) Schneekrystalle. J. Muckenberger, Berlin, 27 pp.^[29]Gray, D.M. and Male D.H. (1981) Handbook of Snow: Principles, processes, management and use. Pergamon Press, Ontario, 776 pp.^[30]Goodison, B., Walker, A.E. and Thirkettle, F. (1990) Determination of snow water equivalent on the Canadian prairies using near real-time passive microwave data: pp. 297-316 in Proc.^[31]Workshop on Application of Remote Sensing in Hydrology. National Hydrology Research Institute, Saskatoon, Canada.^[32]Hobbs, P.V. (1974) Ice physics. Clarendon Press, Oxford, 837 pp.^[33]Hoham, R.W. (1992) Environmental influences on snow algal microbes: pp. 78-83 in Proc. 16th Annual Western Snow Conference, ed. by B.Shafer. Jackson Hole, Wyoming.

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See also: