The science of snowflakes

Snow crystal. Image credit:

The season of festive cheer is descending upon us, with our thoughts already wandering to those things that are iconically entangled with Christmas time. One such quintessential facet of any dreams of Christmas are scenes of snow falling all around us, which for almost all of the country (bar Edinburgh!) is currently less of a dream and more of a reality. Whilst causing aggravation for people trying to commute to work and elation for schoolchildren who can take the day off school, falling snow produces one of nature’s most beautiful complexities, the snowflake.  In fact, to be thoroughly rigorous it must be mentioned that what we all term a snowflake, as shown in the article image, is actually known as a snow crystal within the scientific community, with the name snowflake describing a conglomeration of such snow crystals.

Snow crystals are formed when a minute atmospheric particle, usually dust or pollen, becomes coated with water vapour. The water vapour then freezes on the particle, bypassing the liquid phase and forming a crystalline lattice structure. This structure is what differentiates snowflakes from sleet, which is created when raindrops freeze as they fall from the sky.  The process of crystallising and bypassing the liquid phase is what gives the snow crystals their remarkable regularity, with the crystals having six faces due to the hexagonal bonding structure of the water molecules.

There are several different mechanisms that control the growth of a snow crystal. The flat appearance of the surfaces arises due to water vapour molecules that come into contact with the falling crystal having an energetic preference to bond to rough parts of the snowflake due to the higher availability of unpaired chemical bonds. This has the effect of smoothing out the surfaces of the crystal as it falls through the sky. As the vertices of the hexagonal crystal protrude into the air further than the flat faces, they come into contact with more water vapour, causing those areas to grow faster. This then creates a positive feedback loop, as the more the protrusions grow, the greater the difference in water vapour contact rate compared with the faces of the crystal. This process is what causes new branches on a snow crystal to form.

The shape of a snow crystal is determined by the atmospheric conditions it experiences as in descends from the sky. The most aesthetically pleasing snow crystals are created in a narrow temperature band from -13°C to -17°C, with high atmospheric humidity to encourage more branching. Low humidity in the atmosphere means slow crystal growth and as such cause the snow crystal to have a simpler shape due to the branching mechanism being inhibited. The fact no two snowflakes can experience the exact same set of conditions as they fall means that no two snowflakes can be exactly the same. So I don’t know about anyone else, but I’m dreaming of a Christmas with a -13°C to -17°C temperature range and high atmospheric humidity. Just like the one we had last year.


This article was written by James Hitchen and edited by Bonnie Nicholson.

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