Physics & Math

William Wergin did not set out to be the king of snow. He always thought of himself as more of a botanist and a nematode guy, studying how parasitic soil-dwelling worms interact with crop plants. Then in December 1993, he and Eric Erbe, a colleague at the U.S. Department of Agriculture’s Electron Microscopy Unit in Beltsville, Maryland, started experimenting with a newly configured low-temperature scanning electron microscope. The custom-built device keeps samples chilled to –320 degrees Fahrenheit, making it possible to flash-freeze nematodes or insects and then magnify them enormously to observe their behavior at a single moment in time. Wergin and Erbe couldn’t get their hands on a suitable agricultural sample, so they decided to try their new toy on some of the snowflakes falling outside.

“We had nothing else to image,” Wergin says. The two researchers collected flakes on a copper plate and brought the crystals indoors to their microscope. Fellow USDA researcher Al Rango, now at the department’s Jornada Experimental Range in Las Cruces, New Mexico, stopped by the lab and was stunned by the results. “I’d seen a lot of snow-crystal imagery, but I had never seen crystals this way before,” he says. He realized the views could lead to better understanding of winter snowpacks. In time, other researchers came calling, clamoring for help with everything from studies of glacial ice structure to modeling carbon dioxide frost on Mars. Wergin found himself spending more of his spare hours learning about the intricacies of snow. In late 2000 he retired from the USDA, but he still divides his time between his old work and his frosty sideline.

Since that first glimpse of what the low-temperature scanning electron microscope can do, Erbe has developed procedures for harvesting and preserving snow crystals. The sequence goes like this: Get some copper sample plates, each about the size of a penny. Coat the surfaces with an adhesive such as Tissue-Tek, sticky gunk that biologists use to affix cells to microscope plates, and chill the plates to freezing. Let snow fall onto the plate or scrape some onto it and quickly plunge it into a Styrofoam tray filled with liquid nitrogen. Slide the specimens into a brass storage tube and put it in a thermoslike container in the lab in Beltsville, where cryogenic containers can keep crystals intact for a decade or longer. Finally, prep the crystals with a layer of platinum less than a millionth of an inch thick, which clarifies and intensifies the image produced by the microscope.

Wergin and Erbe have imaged tens of thousands of snow crystals from as nearby as the parking lot outside their Beltsville laboratory to as far as away as Alaska. On the pages that follow is a sampling of the staggering range of forms, including grape-shaped blobs, six-sided plates, shattered rods, and pockmarked stars. All illustrate the fantastically complex, protean nature of water.