Probing Question: Is each snowflake really unique?

snowflake
Fred Widall

Do you remember seeing your first snowflake? Maybe it was caught on your mitten, suspended atop the wool fibers so you could see every detail—graceful spires radiating from the center, so tiny and yet so intricately formed. Snow—whether a child's snowman or a dirty snow bank along the roadside—is composed of millions of these miniscule masterpieces, each one different from the next.

Or so we've been told. How do we really know that no two snowflakes are alike? Ask a meteorologist, and you may find that the snowflake's fabled uniqueness is a matter of semantics.

"It depends upon how we define snowflake," says Hans Verlinde, associate professor of meteorology at Penn State. "Let's be specific, and define a snowflake as a single, vapor-grown ice crystal. I would say with a great deal of confidence that all crystals are different on a molecular level, purely because there are differences in the atomic structure of the atoms making up a water molecule, and hence, in the water molecules themselves."

However, says Verlinde, two molecularly different ice crystals may look nearly identical, even under a microscope, making the question of whether every snowflake is unique more complicated. "You have to decide what you mean by 'unique.' Molecularly, yes. But if you constrain your definition to visual distinctions, you will find some that are very similar looking, and you'll have a different answer to your question," says Verlinde.

A snowflake is formed when the temperature drops below 32 degrees Fahrenheit, and the water vapor in the cloud crystallizes into ice. The shape and structure of the crystal are determined by conditions inside the cloud, such as temperature and amount of available water vapor. "But beyond the details of how the ice crystal initially formed," explains Verlinde, "how molecules incorporate themselves in the crystalline structure and fluctuations in the immediate environment both have some influence."

Snowflakes all start in the shape of frozen water crystals, and at their most basic may be a simple hexagonal plate. But as a snowflake makes its way through the sky, the different conditions it encounters affect its growth. Dirt or dust particles can cause the pattern to alter, as can temperature and amount of vapor in the air. Lower vapor concentrations cause a snowflake to grow more slowly and produce less intricate specimens, while the lacey shapes we most associate with snowflakes are more likely to form in higher vapor concentrations, or in colder conditions such as those in high-altitude cirrus clouds. "Ice crystals take on more complex shapes—called polycrystals—in these low temperature and vapor concentration environments," says Verlinde.

We cannot know for certain that every snowflake is unique, simply because we cannot observe them all, he notes. "But we can address the probability of finding two identical ice crystals, which is vanishingly small," he notes. "The bigger the crystals get, the greater the freedom for different growth paths, and the lower the probability of finding identical crystals even at the macroscopic visual level."

Two of Verlinde's colleagues, Dennis Lamb and Jerry Harrington, conduct research in Penn State's cloud chamber to explore ice crystal growth processes. "The chamber allows them to grow crystals in conditions similar to cirrus cloud environments hitherto unexplored in a laboratory," notes Verlinde. "Cirrus clouds are known to play a large role in earth's energy budget, and hence climate. The molecular level processes determine the shape of the ice crystals, which determine the characteristics of the clouds, which control the radiative properties of clouds and the role of cirrus in climate—it's fascinating!"

So while there is no guarantee that the snowflake on your mitten never had a doppelganger, these crystals have more to offer than their uniqueness and beauty, Verlinde reminds us. The soft white dusting on your coat is composed of cogs in a complex meteorological system that affects the entire planet. Think of that the next time you shovel your driveway.

Hans Verlinde, Ph. D., is an associate professor of meteorology in the College of Earth and Mineral Sciences. You can reach him at verlinde@essc.psu.edu

Last Updated February 18, 2009