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At the Beltsville Agricultural Research Center (BARC) in Maryland, you will find the US Department of Agriculture’s Low Temperature Scanning Electron Microscope (LT-SEM). One of the LT-SEM’s uses is for the study of snow crystals. Hydrologists study photographs of the grain sizes, shapes and associations in relation to passive microwave remote sensing in an effort to determine the water content of the winter snow pack. This information is critical to the determination of the nation’s water supply as well as protection from flooding.
Dislodged Crystals
Samples of snow, ice and associated life forms are collected by dislodging the crystals or biota from the face of a snow pit or the surface of the snow onto copper metal sample plates containing precooled methyl cellulose solution. Within fractions of a second these plates are plunged into a reservoir of liquid nitrogen which rapidly cools them to -196°C and attaches these pre-frozen materials to the plates. Due to the low surface tension of liquid nitrogen and the extreme hardness of materials cooled to these temperatures, very fragile samples can be shipped by aircraft, in dry shipping dewars from study sites throughout the US.
Beltsville Electron Microscopy facility
After arrival at the Beltsville Electron Microscopy facility, the copper plates can be stored at -196°C in storage dewars. Selected samples are transferred to the preparation chamber for sputter coating with platinum. This renders them electrically conductive and they are placed on the pre-cooled (-170°C) stage of a Hitachi S-4100 field emission Scanning Electron Microscope where they are imaged and photographed.
