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Water and Ice

How Do Snowflakes Form?

close-up photo of a snow flake

Credit: Kenneth G. Libbrecht

No two snowflakes…

They say that no two snowflakes are the same. That may be true, but snowflakes share some striking similarities. Take a look at these snowflakes:

Credit: Kenneth G. Libbrecht

See a pattern? It may not be immediately clear, but they are all symmetrical in a similar kind of way. The fancy way to say this is that they all have “six-fold radial symmetry.” In other words, if you draw six evenly spaced lines out from the center of the flake, you will notice that the shape on that line is repeated on the five other lines.

Why do they have such a pattern? And if they all have such a similar pattern, why is it so inconceivable that two snowflakes be identical? To answer both questions, you have to know how a snowflake forms.

The birth of a snowflake

Snow is not simply a frozen droplet of water falling from a cloud. What makes a snowflake different is that it forms slowly, and that it grows in the cloud.

A snowflake is born when water vapor travels through the air and condenses (changes from a gas to a solid) on a particle. There it forms a slowly growing crystal. There are two basic ways that the vapor can condense. Each way plays a big role in the shape that the snowflake will eventually take.

illustration of basic six-sided prisms.

A collection of basic six-sided prisms. The facets are the flat surfaces.

The first way is to form what are called ‘facets.' A facet is essentially a flat face on a 3D shape, like a prism. They form naturally when a crystal grows. In ice crystals the shape they take mirrors the shape of the molecules forming the crystal. The crystal structure of frozen ice is a six-sided shape. Therefore an icy facet is six-sided as well. That is where the symmetry in a snowflake comes from.

The second way to grow a snowflake is to form branches. Not surprisingly, this is what creates those beautiful tree-like structures. Branches form because water vapor will condense on the first thing it touches. If there is a small bump on a flake’s surface, the vapor will condense there instead of traveling any further. Now the bump is bigger and even more likely to ‘catch’ water vapor at that point. The process repeats itself and a branch is formed!

Chaos ensues…

While the snowflake generally starts as a prism with six facets, its growth can switch back and forth between creating facets and forming branches. And both processes can occur at the same time. Nearly imperceptible changes in temperature and the amount of water in the air change how the molecules act and how they condense.

Imagine a growing snowflake in a cloud. As it blows back and forth, it experiences all sorts of changing conditions. There are different temperatures and moisture levels in different parts of the cloud. There are also different conditions at the microscopic level. The order in which it experiences those changes and how long each set of conditions lasts determines the shape it makes.

How likely would it be for two snowflakes to experience the same exact of conditions all the way down to the microscopic level? Astronomically unlikely! That’s why you will never find two truly identical flakes!

photo of NYC during the blizzard of 2015

Numerous snowflakes fall on the streets of New York City during the Blizzard of 2015. Credit: Anthony Quintano.