If middle schoolers are cutting out paper snowflakes for holiday decorations and one student insists on cutting white triangles, that’s okay. The six-sided snowflake is most often depicted, but three-sided snowflakes are not uncommon. They’ve been observed in nature for hundreds of years. Noted snowflake photographer W.A. Bentley (celebrated in the Caldecott Medal Book Bentley’s Snowflakes) and other scientists recorded them.
Physicists Kenneth G. Libbrecht and H. M. Arnold have created triangular snowflakes, as well as hexagons, in their laboratory at the California Institute of Technology. They found that the majority of flakes grown in a vapor diffusion chamber were hexagons but more than they had estimated became triangles, leading them to look for the trigger that turns hexagons into triangular shapes.
According to the authors in their published report, ”We have been studying the detailed physics of snow crystals as a case study in crystal growth, with the hope that developing a comprehensive mechanistic model for this specific system will shed light on the more general problem of structure formation during solidification.”
A single small growth perturbation on the forming hexagon flake resulted in a distorted, or triangular, shape under certain aerodynamic conditions. The perturbation caused the falling flake to tilt up. The airflow around the crystal produced instability in the growth of the facets, creating the triangular shape. After the triangular shape is initially formed, the flake stays triangular during the rest of its fall.
Libbrecht and Arnold point out, “The growth of triangular snow crystals is another piece in the puzzle that describes the many interconnected mechanisms by which complex structures emerge spontaneously during solidification.”
For all those interested in snowflakes, Libbrecht created the web site SnowCrystals.com, with photo galleries of real and synthetic flakes, frequently asked questions, tips on photographing crystals and preserving them, snow activities for all age groups, and more. Libbrecht used a specially designed snowflake photomicroscope to photograph flakes. In 2006, the U.S. Postal Service used his photos as a set of commemorative stamps.
More Snowflake Studies. At Purdue University, a Ph.D. candidate in chemistry, Travis Knepp, has been growing ice crystals in his lab, subjecting the crystals to temperatures ranging from 110 degrees Fahrenheit down to minus 50 degrees. A press release from the university reports that Knepp’s experiments are part of his study of ground-level ozone depletion in the Arctic.
Knepp explains, “Most people have probably heard of ozone depletion in the North and South Poles. This occurs in the stratosphere, about 15 miles up, What people don’t know is that we also see ozone levels decrease significantly at ground level.” The complex chemical reactions that take place on the snow crystal’s surface cause the release of chemicals that reduce ozone at ground level. “How fast these reactions occur is partially limited by the snow crystals’ surface area,” he said. His findings are published in the journal Atmospheric Chemistry and Physics.