As I write these words, in sleepy small-town La Pocatière, Quebec, the snow is falling. For mid-December, this is unsurprising in my part of the world, as is the case in many other parts of the Northern Hemisphere. Christmas is just around the corner, and it certainly feels like the holiday season as I admire the white trees outside.
Some of the snowflakes are sticking to the window and I’m amazed, just as I was as a young child, to see that no two are alike. But while many years ago I would have passed this off like magic, I now know that the natural wonder of snowflakes can be explained by science.
Snowflakes are so much more than frozen water – that’s why it’s preferable to chill your favorite drink with ice cubes rather than a handful of snow. They begin as tiny particles of dust or pollen or microbes, floating high above Earth. When temperatures fall below freezing, water vapor gathers around these particles, forming a single, frozen nucleus.
Water molecules are composed of two hydrogen atoms and one oxygen atom, bonded together at an angle of 104.5 °. (Visualize this mechanism here.) Like a magnet, the hydrogen atoms carry a slight positive charge, while the oxygen atoms carry a slight negative charge. The hydrogen atoms of one molecule become attracted to the oxygen atoms of nearby molecules, and a series of water molecules bond together to form a perfect hexagonal structure. As atmospheric water molecules come together to form a snowflake through deposition (when a gas directly freezes to become solid), they always follow this six-sided template. Hence the characteristic six arms of a snowflake.
As the initial hexagonal ice crystal makes its way to the ground, it grows its six distinctive branches. The precise humidity and temperature of the air in which these arms are formed determine their shape. As the arms of a snowflake grow at the same rate, while experiencing the exact same atmospheric conditions, they are identical. But as other snowflakes grow in different conditions, their final structures diverge. Even the slightest changes in humidity or temperature can lead to a drastically different crystallization pattern.
In this vast snow-covered landscape, it’s true that no two snowflakes are identical
The unique, symmetrical patterns of snowflakes reflect the internal arrangement of the crystal’s water molecules, besides the initial six-faced model. At warmer temperatures snowflakes grow more slowly, providing more time for water molecules to arrange themselves into smoother structures. At colder temperatures, water molecules hurriedly fuse together in intricate shapes.
Closer to the freezing point, snowflakes are in the shape of thin hexagonal plates, long needle-like crystals, or hollow cylindrical columns. Around – 10 °C, hexagonal forms with indentations begin to emerge. Just a few degrees colder and snowflakes are lacy, white structures known as dendrites, whose complex crystalline geometries provide many faces to reflect light. It is this type of snowflake that children (and some adults) enjoy cutting from paper or piping onto gingerbread for homemade festive décor.
The sad effects of warming winters
With increasingly warmer air, snowflakes are given less opportunity to crystalize into pretty structures as they fall to the ground. Or, with more water in the atmosphere in general, snowflakes are more likely to fuse together in the air, creating mega Frankenstein-fractals. A third scenario: warmer temperatures closer to the Earth’s surface may cause snowflakes to partially melt before they can be observed.
These conditions may lead to a stickier snowpack, ideal for skiing or building snowmen, or other wintertime activities. But wetter snow melts faster, resulting in a shorter period to enjoy these pass-times. This also increases the likelihood of springtime floods and reduces the period in which soil can absorb nutrients, which is critical to budding vegetation. And to rub salt into the wound, wet snowflakes are less white. Not only does this reduce our chances at white Christmases, but this snow also reflects less sunlight – exacerbating the effects of global warming.
Idyllic wintery scenes – such as this snow-covered tree-lined lane – are increasingly fleeting
A frozen time-capsule
In certain areas of the world, snow never melts. Permanently frozen areas include the North and South Pole regions, and Alpine and Himalayan mountaintops. Select Carpathian, Caucasus, Eastern Rift, and Rocky Mountain peaks are also within the snow line, as is the volcanic cone of Mount Fuji.
These aren’t just bucket-list-topping destinations for the most daring among us, but for scientists too – pioneering adventurers in their own right. Within the frozen water crystals that blanket these snow-capped summits and glacial fields lies precious insight into the history of our planet.
The particles at the cores of snowflakes can provide timelines for events that cause air pollution. Natural events may be volcanic eruptions, wildfires, or tsunamis; even stardust from extraterrestrial supernovae have been found encapsulated in snowy crystals. Notable anthropogenic causes are nuclear disasters. Centuries, or even millennia from now, scientists could theoretically use permanent snow to determine when we first began burning fossil fuels or producing plastics… provided the planet never warms to the point where the snow line disappears altogether. The bottom line: snowpack is an excellent indicator of atmospheric health, and the health of our planet as a whole.
The archival treasures to be found at the heart of a snowflake, their truly unique fractal structures, and the blissful festive ambiance they provide – there’s so much more to snowflakes than meets the eye. This won’t necessarily keep me from griping about shovelling my car out from beneath 30 cm of snow, which is still a common occurence in my little Canadian town. But I hope to always make time to catch a few in my hands or to admire a snow-capped mountaintop in the distance, now with renewed wonderment for the frozen, natural world.