MTPR

Are All Snowflakes Unique?

Feb 1, 2015

Snowflakes on carpet. (CC BY 2.0)
Credit Flickr user, Alexey Kljatov

"It almost makes you dizzy to look straight up into falling snow. People love watching things fall through the air: autumn leaves, fireworks, even skydivers wafting to the ground on their parachutes. On this winter day, I begin to wonder if the grammar school adage is true. Are all snowflakes unique?

Most snowflakes start in a snow cloud, which is a patch of cold air supersaturated with water. Mixed in the water are particles of dust, volcanic ash, or specks of pollution which are the "seeds" of a snowflake. Water molecules collect on these seeds and freeze into an ice crystal. The shape of that crystal - its needles, columns, and plates - is determined by how cold and saturated the air was when it froze. Water molecules continue to gather and freeze until the ice crystal is heavy enough to fall. As the crystal falls, wind blows it through layers of temperature and moisture, sculpting it further. Collisions with other snowflakes often cause several to clump together. Depending on the direction of the wind, the snowflake may take an hour to reach the ground.

But in that hour of history, the snowflake has its own special adventure, full of false starts, updraft detours, collisions with other snowflakes that result in the loss of an "arm," and dangerously close encounters with water droplets. Each experiences chisels the snowflake until its beauty is like no other. One scientist noted: "As a metaphor for human experience, the comparison is almost irresistible: one unique path, one unique individual."

But just for fun, let's split hairs over the conceptual definition of "unique." Here's a scientific counter-argument to the "none are alike" idea from Duke University professor of mechanical engineering, Adrian Bejan. His work explains the theoretical physics and engineering of how snowflakes grow.

(Broadcast: "Fieldnotes," 2/1/15. Listen weekly on the radio, Sundays at 12:55 p.m., or via podcast.)