Snow and Ice Crystals
Three Dimensional Structure, Metamorphoses of Crystals Revealed by Scanning Electron Microscopy
William P. Wergin
U.S. Department of Agriculture
President Lettieri called the 2080th meeting to order at 8:17 p.m. on November 7, 1997. The Recording Secretary read the minutes of the 2079th meeting and they were approved.
The speaker for the 2080th meeting was William P. Wergin of the Nematology Laboratory in the Agriculture Research Service at the U.S. Department of Agriculture. The title of his talk was, “Snow and Ice Crystals: Metamorphoses of Crystals Revealed by Scanning Electron Microscopy.”
At any one time, snow may cover up to 53% of the land surface in the Northern Hemisphere and up to 44% of the world's land areas. Further, it provides 1/3 of the world's water supply relied on to irrigate our fields, satisfy our thirst and provide a spectrum of commercial and recreational uses. Consequently, it is important to know where and how much of this valuable winter snow resource can be forecast to be available during the following warmer weather and growing season.
We all think we know what snowflakes look like from the popular artwork on greeting cards at holiday time and from pictures in elementary school textbooks. However, these pictures really depict individual snow crystals that agglomerate to form the snowflakes that we see falling from the sky. The most popular image of our snowflake crystal is flat structures with six branching arms and displays a fairy-like beauty. Historically, these images are based on the 1930's photomicrographs of transmitted light-illuminated snowflake crystals made by Wilson A. Bentley. Mr. Bentley was an amateur microscopist who was primarily interested in the esthetic quality of the photographs of the light microscope images that he made. This limited his quest to flat, two-dimensional snowflake crystals called plates or dendrites that could be properly focused and photographed with transmitted light in the relatively narrow depth of field of the light microscope. Mr. Bentley did not address the more numerous three-dimensional snowflake crystals that couldn't be well photographed. Further, he didn't catalog the environmental conditions such as temperature and pressure when the snowflake crystals were observed nor the magnification he used for the various images. Pretty as they are, Bentley's photographs of flat 2-D snowflake crystals had limited use as a scientific tool and represent only a fraction of the 3-D geometry of real snowflake crystals.
Now enters the technology of scanning electron microscopy (SEM) with a longer depth of field than visual light microscopy will allow and which uses electrons to illuminate the subject in lieu of visual light. Low temperature scanning electron microscopy (LTSEM) has been used to record images of snowflake crystals, as well as metamorphosed snow, rime, graupel, frost, ice cores and icicles. Dual images from two SEM's offset by 6° allowed stereoscopic images to be made. Mr. Wergin's used these images during his presentation. Consequently, the audience, reminiscent of a 1950's movie audience wore special 3-D glasses.
Newly precipitated snowflakes consist of complex aggregations of hexagonal plate, prismatic columns, needles and dendrites. Samples of snow from different areas were investigated. These ranged from newly fallen snow on the hood of Mr. Wergin's car in the NIST parking lot to snow in remote areas of Alaska and Colorado. Samples from snowpacks exposed to low and high temperature gradients contained crystals with unique structural features and bonding patterns that resulted from variations in temperature and pressure. These results will help improve the forecasts of the quantity of water that will ultimately reach reservoirs and be available for agriculture. Further, new knowledge of structure and bonding of metamorphosed snow crystals increases our understanding of avalanche conditions and hopefully reduces the risks to people and property.
President Lettieri thanked Mr. Wergin for the society and announced the next meeting. He then made the usual parking announcement and adjourned the 2080th meeting to the Social Hour at 9:45 p.m.