Low-temperature scanning electron microscopy of irregular snow crystals

TitleLow-temperature scanning electron microscopy of irregular snow crystals
Publication TypeConference Proceedings
Year of Publication2002
AuthorsWergin W.P., Rango A., Foster J., Erbe E.F., Pooley C.D.
Conference NameJournal of Microscopy and Microanalysis
Number of VolumesSuppl. 2
Date PublishedDecember 10, 200
ARIS Log Number133885
AbstractSnow 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.