Have you ever wondered how snowflakes get their beautiful shapes?
If no two snowflakes are the same, then what’s up with these?
These are microscope images of lab-grown snowflakes engineered to be identical. The process is simpler than you may think, and tells us a lot about how snowflakes get their shapes!
Some assembly required
Snowflakes are an example of what scientists call “self-assembly.” This means that the beautiful shapes of snowflakes arise basically out of nowhere. There’s no one slowly and carefully arranging the water molecules into the right structure. Instead, the water molecules put themselves into the snowflake shape - that’s why it’s called self-assembly! Self-assembly is a super important process in physics, because a lot of the structure we see in nature doesn’t have any external hand driving it. Instead, nature is putting itself together, assembling into some ordered structure out of the soupy chaos of the universe.
The structure of snow
So how does a snowflake grow? First you need the right conditions - freezing temperatures, supersaturated humidity, and something for the snowflake to form on, called a nucleation seed, which could be a piece of dust or soot in the atmosphere. Supersaturation means that the humidity in the air is so high that there’s too much water in the air. At really humid conditions, water actually wants to come out of the air and solidify onto an ice crystal. These conditions are pretty specific, which is why it takes a particular kind of weather to get a snow day!
The initial shape of the crystal is set by how water molecules stick together. Water is polar, which means it has different charges on the different sides of the molecule. When water molecules get close together, the like-charged sides repel and the unlike-charged sides want to come together to form what's called a hydrogen bond. This means that water wants to form a particular shape when it solidifies, where the molecules form a hexagon pattern.
This hexagonal lattice of ice is what gives snowflakes their 6 sided symmetry. Since the smallest seed crystal is in a hexagonal lattice, when incoming water molecules attach onto that seed, the place they stick is set by that initial shape. Isn’t it crazy to think that symmetry at the atomic scale determines symmetry that we can see?
Once the snow crystal has formed, the way it grows depends on two things- humidity and temperature. But these conditions are constantly changing as the snowflake is blown through a cloud. Maybe first it’s in a region where conditions favor flat, platelike growth but then it gets blown to a region that favors spiky ice growth. These changing conditions as the snowflake forms are how you get these beautiful, branching patterns that grow until the snowflake is heavy enough to fall.
In nature, there are a ton of different shapes that snowflakes can make, depending on the particular conditions of the cloud they’re forming in. They can be long and skinny, like columns. Or they can be big and flat like plates. Most commonly, though, the snowflakes we actually see when we catch them outside aren’t actually as perfect as the lacy, six-sided snowflakes we imagine. This is because in the chaotic process of growing, snowflakes and snow crystals often run into each other and merge into a big, gunky chunk. But in certain weather conditions, we can find a whole zoo of snowflake types out in the wild!
Do you want to build a snowflake?
Now that scientists understand the physics of snowflakes, it’s possible to reverse engineer that process to make custom lab grown snowflakes! All we need to do is seed a snowflake under a microscope and precisely change the temperature and humidity, and we can grow these beautiful, complicated shapes. If we grow two snowflakes in the same exact conditions, we end up with identical snowflakes. Lab-grown snowflakes are beautiful, but they’re also a really cool tool for understanding what conditions lead to different shapes of ice crystals and watching the self-assembly process in action.
At first glance, the unique, lacy shapes of snowflakes seem super complicated. But those delicate patterns are actually completely determined by the path of changing temperature and humidity that a snowflake takes in its journey through the cloud. So even though the snowflakes seem chaotic, there are deterministic physical laws here that govern their beautiful shapes, which is pretty cool.
Written by Caroline Martin
Edited by Ella King
Illustrations by Sarah Helke
Sources and Further Learning
Snow Crystals by Kenneth Libbrecht
Why are Snowflakes Like This? by Veritasium
Towards a Grand Unified Theory of Snowflakes by Rebecca Boyle
A Quantitative Physical Model of the Snow Crystal Morphology Diagram by Kenneth Libbrecht
Learn more about the beauty of snowflakes and self-assembly!
Create (5 - 10 minutes): As we learned, snowflakes usually grow with 6 sided symmetry because of the symmetry of water molecules. Make your own (scientifically accurate!) paper snowflakes with 6 sides. How close can you make your paper snowflakes to the real thing?
Connect (20 - 30 minutes): When ice grows, it doesn't just grow into the beautiful lacy shapes of snowflakes. There are tons of shapes ice can grow into, including a strange phenomenon called "ice spikes." You can make your own ice spikes at home using just a normal freezer and distilled water. Try it at home by following the instructions here.
Experiment (30 - 45 minutes): Snowflakes are beautiful natural forms of art, but they don't last very long. If you have the right snowy conditions at home, you can try to preserve your own snowflake. This can be tricky, but if you get it just right, then your "fossilized" snowflake will last basically forever.