The Science of Snowflakes

Growing up in Pennsylvania, I had my fair share of snow days. 

Most other kids at school wished for a huge blizzard, manifesting the highly-coveted snow day. But me? I dreaded the thought of having to deal with the cold, understandably illustrating why weather was such a huge factor in my college application process. 

Despite my lifelong disdain towards snow, the one thing that kept my morale high throughout the winter was the simple act of sitting by a window and watching the snow fall onto my front lawn– with my feet near the heater and a cup of hot chocolate in my hands, of course. 

It is my firm belief that snowflakes are some of the most mesmerizing phenomena occurring in the natural world. Even though the flakes are quite small, their intricate details shine at a microscopic scale, revealing lacy mandala patterns. Each one is truly unique, but this is no random accident. 

Snowflake formation reveals profound insights into crystallization in both nature and technology. 

Long before modern laboratories, ancient Chinese scholars in 135 B.C. were already contemplating the snowflake’s interesting hexagonal quality, different from the pentagonal pattern innate to flowers and trees. 

Centuries later, in 1611, Johannes Kepler — a well-known German polymath -– became curious about the six-pointed nature of snowflakes. In his study, he remembered how an old friend, Thomas Harriot, had written to him about his travels at sea and the efficiency of the hexagonal shape when packing cannon balls on his ship.  

Kepler thought that this might explain what happens to water in the sky: it freezes at the ideal temperature, forming hexagonally shaped snowflakes. 

However, Kepler set aside the question of the hexagon’s efficiency, choosing instead to concentrate on the work that would culminate in his three laws of planetary motion.

Now, scientists know that the hexagonal structure makes ice less dense than liquid water, which is why my elementary school’s local creek always had a layer of ice at the top whenever we’d return to campus after a snow day.

It wasn’t until almost 300 years later that a Japanese researcher, Ukichiro Nakaya, began studying snowflakes to better understand the two types: flat stars and columns. 

Ukichiro  created his own snowflakes in a lab by suspending frost crystals on rabbit hairs in refrigerated air. From there, the crystals grew into full snowflakes. He found that by manipulating environmental factors, like temperature and humidity, he could change the type of snowflake. Flat stars typically formed from -2 to -15 degrees Celsius, while columns formed from -5 to -20 degrees Celsius. 

Ukichiro also made the correlation between humidity and the actual design of star plates. In low humidities, he found that the snowflakes formed fewer branches, whereas higher humidities produced more decorative snowflakes.

The speed at which each snowflake component grows also plays an important role in what type it will be. A plate-type snowflake will have edges that grow outwards at a faster rate than their centers growing upwards. Columns are the exact opposite with their centers expanding upwards faster than the edges.

Even with all of this knowledge, a question remains: why exactly do snowflakes do this and how do these environmental factors change their behavior?

A key idea in answering these questions is in surface energy driven molecular diffusion. 

Surface energy driven molecular diffusion occurs when molecules move along a surface to lower the overall surface energy. In the context of snowflake formation, the process explains how the growth of a snow crystal depends on the initial conditions of its formation and the subsequent behavior of the molecules that bind to its surface. 

When water vapor begins to freeze, the molecules coordinate with each other and form the snowflake’s hexagonal structure. Each oxygen atom is connected to four hydrogen atoms (two on each side) forming the detailed lattice structure. Then, water vapor settles on a corner of the hexagon and diffuses out (to form a plate) or up (to form a column), with the temperature determining the direction of diffusion. 

This process occurs because the water ice is typically found near its melting point which forms a quasi-liquid layer (QLL). QLLs behave differently at the centers and edges as a function of temperature. This phenomenon is called pre-melting, and only occurs in ice, though it is not yet fully understood by researchers.

Scientists in the field of condensed matter physics also study these crystallization properties and examine crystal growth in a generalized manner. 

One such researcher, Professor Meenesh Singh at the University of Illinois,  co-published a paper describing the crystal growth in solvent crystallization. Instead of researching how crystals form by freezing, they looked at how they formed via organization within a liquid.  

Although the answer to why crystals and snowflakes behave and form the way they do is still a mystery, their research is important because it has been used to create crystalline materials such as pharmaceuticals, agrochemicals, catalysts, semiconductors and metal–organic frameworks.

When I went back home this past winter break, I sat by the same window and watched the snow settle onto my front lawn, just like I did so many years ago. Now, instead of seeing the cold as an inconvenience, I see it as a reminder that even the simplest moments can hold extraordinary complexity. 

Sometimes we can get caught up in large-scale events like blizzards and snow days. Yet, the more profound takeaway is how each snowflake that falls carries with it a storied history and unanswered questions.