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Snow crystals
appear when water vapor condenses directly into ice. A snowflake describes a
single snow crystal or a group of snow crystals. The foundation of a snowflake
begins with a water droplet. Water droplets in clouds are unique in that they
do not freeze immediately when the temperature decreases below zero degrees
Celsius. Alternatively, they stay in their liquid form in a supercooled state.
At negative forty degrees Celsius pure water droplets will freeze. Water
droplets with dust, on the other hand, start freezing at a much higher
temperature of negative six degrees Celsius because the dust allows for a solid
surface to begin the freezing process. Individual frozen droplets will grow as
the water vapor condenses onto the surface, solidifying into a crystal-lattice
structure. When the snow crystal accumulates enough weight, gravity will pull
it down.

There is truth
when people say that no two snowflakes look alike. Each snowflake has a unique
shape and pattern. To start, the angles between atoms in a water molecule
create a hexagon shaped ice lattice, causing six-fold symmetry. Shapes of
snowflakes can range from thin hexagonal columns to slender hexagonal plates.
One example of a particular shape of snowflake is bullet rosettes, named for
its bullet shaped columns. Star-shaped or stellar snow crystals are another
shape. These snow crystals have six primary branches which are embellished with
ridges or patterns. These could have additional side branches that would be
separated by multiples of sixty degrees and run parallel to their adjacent rows
of branches. One special shape of snowflakes is called capped columns. These
are created when a snow crystal begins to form as a column and changes to
forming into a plate growth. The shape of a snowflake emerges with time as
water vapor condenses to the crystal surface. The lattice structure helps to
explain the six-fold symmetry, but fails to explain the complex shapes of
snowflakes. The crystal shape formed is dependent on the temperature and
humidity of the surroundings. At negative two degrees Celsius thin plate-like
crystals appear. At negative five degrees Celsius slender needles form. At
negative fifteen degrees Celsius large, thin plate-like crystals develop. At
negative twenty-five degrees Celsius and below the crystals grew as short
columns. To put it simply, small crystals have simple shapes and large crystals
have complex shapes. Complex shapes are formed because snow crystals get whirled
around the cloud so the temperature and humidity change rapidly.

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Faceting is an important
aspect in guiding growth of a snow crystal. When a water droplet freezes it expands
crystals, forming facets because some crystalline surfaces accumulate material more
slowly than others. Sometimes water molecules attach to the prism surface, producing
thin plates of ice. Other times, they attach to the basal facets, forming columns.
The two main contributors in growth are faceting and branching. Faceting provides
symmetry, while branching provides complexity. The knife-edge instability is another
contributor to growth. This hypothesis is characterized by an initial thickness
of the plate, but then gradually thins out, causing a rapid growth rate at the edge.
There is still a lot we do not know about how the shapes of snowflakes form, but
with today’s technology that might not be true for much longer.

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