Low Temperature SEM of Precipitated and Metamorphosed Snow Crystals Collected and Transported from Remote Sites

Abstract

Procedures were developed to sample, store, ship, and process precipitated and metamorphosed snow crystals, collectively known as “snowflakes,” from remote sites to a laboratory where they could be observed and photographed using low temperature scanning electron microscopy (LTSEM). Snow samples were collected during 1994–96 from West Virginia, Colorado, and Alaska and sent to Beltsville, Maryland for observation. The samples consisted of freshly precipitated snowflakes as well as snow that was collected from pits that were excavated in winter snowfields measuring up to 1.5m in depth. The snow crystals were mounted onto copper plates, plunged into lN₂ and then transferred to a storage dewar that was shipped to the laboratory. Observations, which could be easily recorded in stereo format (three-dimension), revealed detailed surface features on the precipitated crystals consisting of rime, graupel, and skeletal features. Samples from snowpacks preserved the metamorphosed crystals, which had unique structural features and bonding patterns resulting from temperature and vapor pressure gradients. In late spring, the surface of a snowpack in an alpine region exhibited a reddish hue. Undisturbed surfaces from these snowpacks could be sampled to observe the snow crystals as well as the organisms responsible for the coloration. Etching the surface of samples from these sites exposed the presence of numerous cells believed to be algae. The results of this study indicate that LTSEM can be used to provide detailed information about the surface features of precipitated and metamorphosed snow crystals sampled at remote locations. The technique can also be used to increase our understanding about the ecology of snow. The results have application to research activities that attempt to forecast the quantity of water in the winter snowpack and the amount that will ultimately reach reservoirs and be available for agriculture and hydroelectric power.