The complex shapes and uniqueness of snowflakes have confounded scientists for hundreds of years. In the past, it was generally recognized that the formation of a snowflake is a two-step process: making a single crystal and then having it grow.

This process begins as a microscopic speck of dust is trapped in a molecule of water vapor inside the winds of a winter storm. As the particle is frosted with droplets of supercooled water, it becomes heavier and begins its plunge to earth. The falling ice crystal is sculpted by the varying temperature and humidity—lengthening here, a spiky branch pushing out there-until it grows into a shape as unique as a person's fingerprint.

But in the past few years, as…