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United States Department of Agriculture

Agricultural Research Service

Title: The Structure and Metamorphism of Snow Crystals As Revealed by Low Temperature Scanning Electron Microscopy.

Authors
item Wergin, William
item Rango, Albert
item Erbe, Eric

Submitted to: Proceedings of the Eastern Snow Conference
Publication Type: Proceedings
Publication Acceptance Date: October 15, 1996
Publication Date: N/A

Interpretive Summary: Snow provides advantages and disadvantages for the US consumer. On one hand, one-third of the water used for irrigating crops and producing hydroelectric power is provided by snow. Alternatively, winter precipitation, such as snow, ice and sleet, is hazardous to ground and air transpotation. Methods currently used for calculating the amount of water that is present in snow are inaccurate resulting in multimillion dollar losses in agricultural efficiency. To help remedy this problem, models are being developed to estimate more accurately the amount of water that is present in the winter snowpack. One factor in the model is the shape of the snow crystal. ARS scientists have developed a method for exaiming winter snow with an instrument called a scanning electron microscope. With this method, snow can be collected and examined in the laboratory to reveal information about the basic shapes, sizes and growth of crystals. This information will be used by scientists to improve modeling systems that predict the amount of water in the snowpack and benefit farmers who require irrigation water.

Technical Abstract: A scanning electron microscope (SEM) equipped with a low temperature (LT) stage was used to observe precipitated and metamorphosed snow crystals. The samples consisted of precipitating snow and snow collected from pits that were excavated in snow fields. The samples were collected on copper plates, frozen in liquid nitrogen and transferred to a storage dewar that was shipped to Beltsville, MD. Examination of the samples with LTSEM revealed that precipitated snow consisted of aggregations of needles, columns, plates and dendrites. Metamorphosed snow sampled from the snowpits contained rounded, sintered, faceted, and clustered forms of crystals. The development of these unique crystalline forms were associated with temperature and vapor pressure gradients or with melt-freeze cycles that occurred in the snowpits. These cycles were also associated with the appearance of "red snow" resulting from the presence of green algae. This study indicates that LTSEM is a feasible technique for observing snow that can be sampled at remote locations. LTSEM can be used at high magnifications to illustrate the structural features of precipitated snow crystals and their metamorphosed conditions.

Last Modified: 4/23/2014