January was once again cold and blue, but at least this year it also brought snow for rainy Manchester. That one day of snowmen outside the library and slush on the roads got us wondering about snow, in particular the impressive formation of snowflakes as they fall. If you too have ever been interested in the answer to how such intricate patterns naturally occur, here is a how-to manual for the construction of the delicate ice crystals.
From dust to snowflakes
Snowflake formation involves a few different steps, determined by a mixture of mathematical probability, atmospheric variation, chemical bond angles and, as always, some physics for good measure.
First of all, snowflakes need a place to start – minuscule dust and pollen particles in the atmosphere provide this. In the glacial winter sky, water molecules can freeze around these particles, creating a small ice crystal which forms the basis of the snowflake.
The next step is the arrangement of the hexagonal core. Water molecules freeze together to create an ice sheet with hydrogen bonds forming between each molecule. The molecules create a hexagonal shape as a result of the 104.5 degree bond angles between the two hydrogen atoms in water. Then, as these hexagons join together, they begin to form a larger hexagon.
This process continues. Hexagons join together evenly as there is an equal probability of a new water molecule colliding at any of the six symmetrical hexagonal sides. In this way, the core of the iconic snowflake shape is formed.
The final consideration is how the rest of a snowflake, with its intricate pattern of connecting arms, is created. Branching occurs because the six vertices of the hexagonal core stick out further than the sides, so are more likely to interact with surrounding water molecules and form hydrogen bonds. This leads to increased growth at these points and the formation of six symmetrical arms. Smaller branches will grow off these main branches as water molecules continue to collide, leading to a complex, symmetrical pattern.
These steps explain the basic template of a snowflake, however, the reality is that the exact shape of each flake is famously unique and determined by the surrounding temperatures and conditions at the time of formation. The shape of the growing snowflake is so sensitive and temperamental that it can change its course mid-formation if the temperature varies even slightly.
Despite this, conditions at each of the six sides of the snowflake are always similar enough to make the snowflake symmetrical. No two snowflakes share the same path to the ground, meaning each long journey from the sky has slightly different conditions, leading to unique arrangements of water molecules. Taking into account the number of possible branching points in a growing flake, and the probability of identical environmental conditions, it is highly probable that no snowflake pattern will ever be repeated.
So next time you catch a snowflake on your coat, take a minute to appreciate its one-of-a-kind beauty; if it’s a particularly strange-looking flake, know it may have experienced some turbulent conditions on its way down.