Snow crystalsappear when water vapor condenses directly into ice. A snowflake describes asingle snow crystal or a group of snow crystals. The foundation of a snowflakebegins with a water droplet.
Water droplets in clouds are unique in that theydo not freeze immediately when the temperature decreases below zero degreesCelsius. Alternatively, they stay in their liquid form in a supercooled state.At negative forty degrees Celsius pure water droplets will freeze. Waterdroplets with dust, on the other hand, start freezing at a much highertemperature of negative six degrees Celsius because the dust allows for a solidsurface to begin the freezing process. Individual frozen droplets will grow asthe water vapor condenses onto the surface, solidifying into a crystal-latticestructure.
When the snow crystal accumulates enough weight, gravity will pullit down.There is truthwhen people say that no two snowflakes look alike. Each snowflake has a uniqueshape and pattern. To start, the angles between atoms in a water moleculecreate a hexagon shaped ice lattice, causing six-fold symmetry.
Shapes ofsnowflakes can range from thin hexagonal columns to slender hexagonal plates.One example of a particular shape of snowflake is bullet rosettes, named forits bullet shaped columns. Star-shaped or stellar snow crystals are anothershape. These snow crystals have six primary branches which are embellished withridges or patterns. These could have additional side branches that would beseparated by multiples of sixty degrees and run parallel to their adjacent rowsof branches. One special shape of snowflakes is called capped columns. Theseare created when a snow crystal begins to form as a column and changes toforming into a plate growth. The shape of a snowflake emerges with time aswater vapor condenses to the crystal surface.
The lattice structure helps toexplain the six-fold symmetry, but fails to explain the complex shapes ofsnowflakes. The crystal shape formed is dependent on the temperature andhumidity of the surroundings. At negative two degrees Celsius thin plate-likecrystals appear.
At negative five degrees Celsius slender needles form. Atnegative fifteen degrees Celsius large, thin plate-like crystals develop. Atnegative twenty-five degrees Celsius and below the crystals grew as shortcolumns. To put it simply, small crystals have simple shapes and large crystalshave complex shapes. Complex shapes are formed because snow crystals get whirledaround the cloud so the temperature and humidity change rapidly. Faceting is an importantaspect in guiding growth of a snow crystal.
When a water droplet freezes it expandscrystals, forming facets because some crystalline surfaces accumulate material moreslowly than others. Sometimes water molecules attach to the prism surface, producingthin plates of ice. Other times, they attach to the basal facets, forming columns.The two main contributors in growth are faceting and branching. Faceting providessymmetry, while branching provides complexity.
The knife-edge instability is anothercontributor to growth. This hypothesis is characterized by an initial thicknessof 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, butwith today’s technology that might not be true for much longer.