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United States Department of Agriculture

Agricultural Research Service

Title: Low Temperature Scanning Electron Microscopy of Irregular Snow Crystals

Authors
item Wergin, William - RETIRED-ARS
item Rango, Albert
item Foster, James - NASA
item Erbe, Eric
item Pooley, Christopher

Submitted to: Microscopy and Microanalysis
Publication Type: Proceedings
Publication Acceptance Date: July 15, 2002
Publication Date: December 10, 2002
Citation: WERGIN, W.P., RANGO, A., FOSTER, J., ERBE, E.F., POOLEY, C.D. LOW TEMPERATURE SCANNING ELECTRON MICROSCOPY OF IRREGULAR SNOW CRYSTALS. MICROSCOPY AND MICROANALYSIS. 2002. V. 8(2). P. 722-723.

Interpretive Summary: NOT REQUIRED

Technical Abstract: Snow crystals occur in eight basic types: columns, needles, plates, dendrites, irregular crystals, graupel, hail and ice pellets. Most of these types have been described and photographed. However, our understanding of "irregular crystals" remains vague because the limited resolution and depth-of-field associated with the light microscope have prevented investigators from fully characterizing and clearly illustrating the features of these crystals. Our study used a field-emission scanning electron microscope (FESEM) equipped with a cold stage to document the structural features, physical associations, and atmospheric metamorphosis of irregular snow crystals. Snow samples, consisting of freshly fallen snowflakes, were collected from six different locations where air temperatures ranged from -5 degrees C to 0 degress C. Crystals believed to correspond to irregular crystals appeared as short, irregular hexagons measuring 60 to 90 um across when viewed from the a-axis. Their length (c-axis) rarely exceeded the diameter. The irregular crystals were occasionally found as secondary particles on other larger forms of snow crystals. However, they most frequently occurred as aggregates consisting of more than 100 irregular crystals. In the aggregates, the irregular crystals had their c-axes oriented parallel to one another and collectively tended to form columnar structures. Occasionally, the aggregates exhibited rounded facets along one side, suggesting atmospheric metamorphoses and unidirectional fall. In extreme cases of metamorphoses, the aggregates would be difficult to distinguish from graupel. Frost consisting of irregular crystals was also encountered, suggesting that atmospheric conditions that favor this form of growth also occur terrestrially. Results obtained with low temperature FESEM suggested that crystals which were previously designated as "irregular crystals" have distinctive features that can be used to characterize and distinguish them from other types of snow crystals.

Last Modified: 11/24/2014
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