Metamorphism of Snowflakes as Observed with Low Temperature SEM William P. Wergin1, Albert Rango2 and Eric F. Erbe1 Nematology Laboratory1 and Hydrology Laboratory2 Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705 USA A recent study has used low temperature scanning electron microscopy (SEM) to document the natural types of snow crystals that can be found in newly precipitated snow (Wergin et al., 1995). However, from the moment snowflakes begin to form, they are subjected to fluctuations in temperature and vapor pressure that cause a variety of distinct morphological changes. This general process is referred to as metamorphism (Armstrong, 1992). The present study concentrates on three types of changes, which are known as equi-temperature metamorphism, temperature-gradient metamorphism and freeze-melt metamorphism. These changes affect all snowflakes that have fallen and accumulated in the winter snowpack. Snow was collected during 1994-95 from sites near Loveland Pass, Colorado and Fairbanks, Alaska. The samples were obtained when the air temperatures ranged from -110 C to +180 C. A precooled scalpel was used to gently dislodge a snow sample from the walls of snowpits that were excavated in undisturbed winter snowpack measuring up to 1m in depth. This sample was collected on a flat copper plate (15mm x 27mm) that contained a thin layer of methyl cellulose solution that was precooled to the ambient outdoor temperature and then either rapidly plunged into a styrofoam container containing LN2 or placed on a brass block that had been precooled with LN2. After a few minutes the plates were inserted diagonally into 20 cm segments of square brass channelling and lowered into a dry shipping dewar that had been previously cooled with LN2. The dewar was shipped by air to the laboratory in Beltsville, Maryland. Upon reaching the laboratory the samples were transferred under LN2 to a LN2 storage dewar where they remained for as long as nine months before being coated for observation with low temperature SEM. Snow crystals that were sampled several cm below the surface of snowpacks, which were not subjected to significant temperature gradients between the ground surface and the air, underwent changes that are referred to as equi-temperature metamorphism. These types of changes were characterized by sublimation of the fine delicate structures on the edges and surfaces of the snow crystals. In many cases the general shapes of the original crystals could be identified, e. g. dendritic or plate-like, but their surfaces were smooth and sinuous. As this process proceeded, the ends of adjacent snow crystals became joined or sintered with one another (Fig. 1). This process, which resulted in bonding between adjacent snow crystals, resulted in compaction of the snowpack and somewhat reduced the air spaces that were present between the interconnected crystals. Temperature-gradient metamorphism occurred in the snowpack when the temperature of the ground was significantly greater than the air temperature at the surface of the snowpack, i. e., a temperature gradient of at least 100C/m had been present. This situation resulted in sublimation of the lower surfaces of a snow crystal followed by recrystallization of the water vapor on upper lying surfaces. With time, this process resulted in the formation and growth of large crystals, which are known as depth hoar, near the base of the snowpack. The crystals of the depth hoar frequently had parallel arrays of facets or steps, which resulted from the refreezing of successive molecular layers of ascending water vapor. Depth hoar crystals were not significantly sintered or bonded to adjacent crystals (Fig. 2). In this layer of the snowpack, large air spaces generally occurred between these loosely connected crystals. In spring, the snowpack in Colorado is frequently subjected to a process of successive melting and freezing cycles that produced changes referred to as melt-freeze metamorphism. This process resulted in the formation of clusters of snow grains that were well bonded to adjacent crystals when the temperature was below freezing and the water content was low; however, the grains were only weakly bonded when the temperature was above freezing and the snowpack was saturated with melting water. In the advanced stages of this process the individual grains tended to become spherical. Cells believed to represent a green alga were occasionally observed in the upper layer of this type of snowpack. Our results indicate that low temperature SEM can be used to illustrate the sizes and shapes of snow crystals and to follow their subsequent metamorphisms that are influenced by temperature, and vapor pressure. Equi-temperature metamorphism, which occurs when snowpacks are not subjected to significant temperature gradients, leads to snow crystals that are rounded or sinuous and highly sintered; the snowpack becomes compacted as a result of the smaller air spaces that exist between adjacent crystals. The extensive bonding between snow grains creates a very stable layer of snow. Temperature-gradient metamorphism results in a layer of large unsintered crystals know as depth hoar. Unlike the condition described above, the presence of this layer creates an unstable snowpack whose failure could lead to an avalanche. Finally, melt-freeze metamorphism which normally occurs in spring, results in spherical snow crystals that are bonded in clusters; the strength of the bonds is affected by the amount and status of water that is present in the snowpack. Conditions that favor melt-freeze metamorphism will apparently support the growth of algae on the surface of the snowpack. Armstrong RL: The mountain snowpack. In: (Eds. Armstrong BR, Williams, K) The Avalanche Book. Denver, Colorado Geological Survey, Denver: 47-83 (1992) Wergin WP, Rango A, Erbe EF: Observations of snow crystals using low-temperature scanning electron microscopy. Scanning 17, 41-49 (1995) This research was partially funded by the NASA Goddard Space Flight Center.