Page Banner

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: Journal of Microscopy Research and Technique
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 20, 1996
Publication Date: N/A

Interpretive Summary: Snow provides at least one third of the water used for irrigation and the growth of agricultural crops. Predicting the amount of water present in winter snowpack is an essential forecast activity needed to estimate the amount of water that reaches reservoirs and agricultural fields. Because currently used methods for calculating such volumes of water are inaccurate, multimillion dollar losses in agricultural efficiency result each year. Therefore, ARS investigators are developing a new method based upon the examination of snowflakes highly magnified in an instrument called a scanning electron microscope. In the present study, both freshly fallen snowflakes and older snow were placed on special holders that were transferred to a laboratory for examination. Photographs of individual snow particles revealed them to consist of thin hexagonal plates, hollow columns, needle-like crystals, or branching, tree-like crystals. Photographs of the older snow samples revealed that time and temperature changes influenced the basic shape of the crystals. Because this new technique yielded accurate measurement of snow particles, it will be used by scientists developing computerized modeling systems for predicting the amount of water in snowpack. These predictive systems will in turn benefit growers who consume irrigation water.

Technical Abstract: Low temperature scanning electron microscopy (LTSEM) was used to observe precipitated and metamorphosed snow crystals commonly known as "snowflakes". Snow was collected from sites in Maryland, West Virginia, Colorado and Alaska. The samples consisted of freshly precipitated snowflakes as well as snow collected from snowpits measuring up to 1m in depth. The samples were frozen in LN2 and then transferred either to a cryosystem mounted on an SEM or to a storage dewar. LTSEM observations revealed that precipitated snowflakes, consisting of plates, columns, needles and dendrites, did not appear to be affected by storage. Snowpacks that had been exposed to temperature gradients contained crystals with unique structural features and bonding patterns that resulted from variations in temperature and pressure. Melt-freeze cycles were also associated with the appearance of "red snow" resulting from the presence of cells believed to be an alga. This study indicates that LTSEM is a feasible technique for observing snow that was sampled at remote locations and determining the shapes of precipitated and metamorphosed snow crystals. The results assist research activities that forecast water in the winter snowpack and predict the amount that will be available for agriculture.

Last Modified: 4/17/2014
Footer Content Back to Top of Page