In the northeast, winter is a season of cold. It is a time when outdoor activities are dictated more than ever by the weather. As children, snow is the stuff of dreams. It can be mounded, piled, thrown, shaped, dug out, eaten, colored, ridden on, and experienced in a million ways. As if that isn’t enough, the enduring hope of snow day is ever present, and the moment when you first wake up to that brightness of reflected sunlight through your windows, when you wake up and know it will be a day for only fun, is the lightest feeling in the world. As adults, snow is polarizing: you love its beauty or hate its inconvenience. Everyone has heard that every individual snowflake is unique. But is this true? And if so, why?“Out of the bosom of the Air,
Out of the cloud-folds of her garments shaken,
Over the woodlands brown and bare,
Over the harvest-fields forsaken,
Silent, soft, and slow
Descends the snow.” -Henry Wadsworth Longfellow
First off: a definition of terms. Snowflake is a more general term, and can refer to an ice crystal, a single crystal of ice, or to larger clumps of ice crystals that fall in agglomerations. An ice crystal is not a frozen raindrop. Liquid water that freezes and falls to the ground becomes sleet. Rather, ice crystals are formed when water vapor in the clouds condenses directly into ice. As you may (or may not) remember from chemistry class, ice crystals, each molecule of which is made of an oxygen atom and two hydrogen atoms, form what is called a hexagonal lattice. The six-sided symmetry of this lattice is what gives snowflakes their six-sided symmetry. Elaborate patterns emerge as the ice crystals grow in the clouds.
The most basic form of a snow crystal is a hexagonal prism, which includes two basal facets and six prism facets. Different shapes can occur under different conditions, and the form of a snowflake depends very heavily on temperature and humidity. The growth of these crystals relies on a balance between branching and faceting. Branching is due to a property of water molecules and the way they travel through the air: they must diffuse, and thus the surface of a crystal a water molecule reaches first is the one one which it condenses. A small bump on an otherwise-homogeneous structure is thus more likely to result in a crystalline branch, and once a branch has formed, it is more likely to grow longer. Facets, on the other hand, are due to the fact that some surfaces of ice crystals grow more slowly than others. Unlike a bump on a crystal surface, inviting a water molecule to bond, the flat surface of a facet is more likely to remain smooth because there is nothing for the water molecule to attach to. Therefore, this surface grows more slowly.
Below, see a general guide for the most common forms of snowflakes:
For all that we understand about crystalline structure and how ice crystals form, there is even more we don’t know. Why do the most complex shapes form at high humidity? Why do the shapes of snowflakes go from plates to columns as the temperature lowers? Usually, each arm of a snow crystal is different from its other arms. Air current turbulence causes this: as crystals are blown about, water molecules can condense unevenly as each corner of a snow crystal experiences a different environment in the atmosphere. So there is truth in the unique design of snowflakes, and it is ultimately due to the randomness of nature, building a snow crystal as it is spun and whipped through the air inside a cloud. John Hallett, director of the Ice Physics Laboratory in Reno, NV, explains that, “[A snowflake’s] final shape is a history lesson of how the thing grew. The outside edge of the crystal is where it grew last, and as you go inward you can tell where it was before.”“Before I melt,
Come, look at me!
This lovely icy filigree!
Of a great forest
In one night
I make a wilderness
Of white” -Walter de la Mare
The winter has come, and with it, the cold. Snow has been sparse in this part of the country the last few years, and all we can do is watch and wait. Will we awake tomorrow morning with the light bright and snow-shattered through our windows, with a kiss of frost on the glass from a storm like those when we were children? If so, take a minute and think about how these amazing ice creations came to be. And then go make a snowman.
Griffin, Julia 2011. The science of snowflakes, and why no two are alike. PBS Newshour. Link.
SnowCrystals.com. Kenneth G. Libbrecht 1999. Caltech. Link.
“Snow-Flakes,” by Henry Wadsworth Longfellow. Read it here.
“Snowflakes,” by Walter de la Mare.