Snowflakes are thin and flat for a reason.

• Position: Your Life

We all have heard that no two snowflakes are alike. Kenneth Libbrecht, professor of physics at the California Institute of Technology, Pasadena, will tell you that this has to do with ever-changing conditions in the clouds where snow crystals form. Now Libbrecht, widely known as the snowflake guru, has shed some light on the grand puzzle in snowflake science: why the canonical, six-armed "stellar" snowflakes wind up so thin and flat.

Scientists have known for more than 75 years that, at conditions typically found in snowflake-producing clouds, ice crystals follow a standard pattern of growth: near -2[degrees]C, they grow into thin, platelike forms; near -5[degrees], they create slender columns and needles; near -15[degrees], they become really thin plates; and at below -30[degrees], they are back to columns. Yet, no one has been able to explain why such changes in temperature yield such dramatic alterations in snowflake structure.

Libbrecht started his observations with the thinnest, largest platelike snowflakes, which form at around -15[degrees] in high humidity. Some of these are about as sharp as the edge of a razor blade. "What I found in my experiments is a growth instability, or...