Particulate matter in the atmosphere can alter the shape of snowflakes!
How can triangular snowflakes form? Flurries of questions about mysterious triangle-shaped snowflakes may soon subside, thanks to new research on snowflake formation. Snowflake enthusiasts have spotted triangular snowflakes in the wild. The snowflake scientific literature, which goes back almost two centuries, is also full of such sightings, but no one has explained why.
What are the most common snowflake forms?
Most snowflakes are hexagons because of the arrangement of hydrogen bonds in the water molecule. But the following study: (http://arxiv.org/abs/0911.4267) suggests that after hexagonal flakes, oddball triangular flakes are the most prevalent.
How can triangular snowflakes form?
To address the mystery, the researchers created snowflakes in the laboratory and recorded the shapes. In conditions that simulate natural snowfall, the vast majority of flakes were the standard hexagons. But more of them were triangular than a statistical model had predicted. Some of these flakes still have six sides but an overall triangular shape, created by three short edges and three long ones.
Researchers propose an aeronautical reason for the triangular geometry. Tiny impurities, such as dust particles, can cause one edge of the falling snowflake to tilt up as it falls. The snowflake sides that are pointed down grow faster as the wind blows by, leading to a stable triangular pattern. Once a triangle shape gets started, the snowflake remains triangular despite any later bumps as it falls.