Snow is the white stuff that falls from the sky during snowfall and settles on the ground forming snowpack. It is made up of millions and millions of ice crystals called snowflakes... small, magically crafted flakes of ice that are both beautiful and fascinating. Snowflakes should not be confused with frozen raindrops which are called ‘sleet’ ~ neither should they be mixed up with ‘hail’, which is formed from condensing water. Snowflakes are individually sculpted by Nature through an extraordinary process called ‘sublimation’.

Sublimation

Sublimation is the process of a substance changing from its solid state directly to its gaseous state without becoming a liquid, and vice versa. Common old water is one of those substances which sublime, and snowflakes are only created when vapour condenses directly into ice.

In order for the sublime condensation process to take place there has to be a solid surface for the water to condense onto, so all snowflakes start out as aerosols or "air salts". Without these microscopic dust particles in the air, water vapour cannot condense and remains vapour in a supersaturated state. The nucleus of of every snowflake is a tiny speck of dust, but once water vapour begins to condense on this surface it creates a further solid surface (ice is dry) for more water vapour molecules to condense upon, and so the snowflake grows.

Snowflake Growth

Ice is a crystalline material, which means that the water molecules are arranged in a regular lattice. Even when a snow crystal has a complex shape, its water molecules are aligned throughout its entire construction, giving snowflakes their distinctive features such as six fold symmetry and their different branches and side branches. This process is called ‘faceting’.

Early in their life, snowflakes form a simple eight sided, hexagonal crystal with six ‘prism facets’ around them, and two ‘basal facets’ on top and bottom. Depending on the temperature and humidity they can be broad and flat (like hexagonal coins), long and thin (like pencils), or anyting in between. At this point they are still microscopic, and the morphology of their subsequent growth is again determined mainly by temperature and humidity.

Although they are three dimensional (what isn't) their continued growth follows only two of the three dimensional planes at any one time. Thus you get flat plate like crystals (such as plates and stellar dendrites) or hollow hexagonal column like crystals (such as needles and hollow columns) ~ or a mixture of the two combined or crossed (such as capped columns, cups, bullet rosettes, split plates and split stars). Exactly why snow crystals grow this way remains a scientific puzzle.

Once they reach a certain size they become too heavy for the atmosphere to support them and they fall out of the sky in snowfall.

Infinite Variety

It is said that no two snowflakes are the same, and when you look closely at their construction it is easy to see why. It is also true that they are not perfectly symmetrical, close inspection revealing this fact. Remember that even the smallest microscopic snowflake is made of many thousands of water molecules in a constant state of metamorphosis.

Is is possible to predict what forms of crystal will grow under different mixtures of temperature and humidity. This is known as ‘snowflake morphology’, and the same science allows us to observe a snowflake and divine the weather conditions under which it was formed. When you think about this carefully, and consider the chaotic nature of the clouds, it becomes easier to accept that no two snowflakes are the same¹.

It is also worthy of note that, amazingly, there are some snowflakes that have triangular forms, and others that are twelve branched.

Exceptions

Sometimes, a snowflake will grow due to supercooled² water droplets freezing to its surface. This is not an example of sublimation, and the resulting deposit is called ‘rime’. Heavily rimed snow is called ‘graupel’.

Not all snowflakes are formed in clouds. The winter equivalent of dew is called ‘hoar frost’ or ‘surface hoar’. These are true ice crystals because they are formed by condensing vapour ~ sublimation. They are feathery, fern like crystals that generally follow the flat plate growth pattern. They are often larger and easier to observe than snowflakes.

Another type of sublimation ice crystal is ‘depth hoar’ which forms in lower layers of the snowpack when there is a large temperature gradient between the bottom and top layers. This type of crystal generally follows the column growth pattern and is associated with an increased risk in Avalanche, as it creates poorly bonded layer in the snowpack.

Further information.

This entry is dedicated to Ken Libbrecht and his excellent book "Field Guide to Snowflakes", which has a stunning collection of snowflake photographs and detailed explanations of how snowflakes manifest themselves. This information is also available online at his excellent web site SnowCrystals.com