Looking across a snow covered landscape can be both pleasing and overwhelming. Your perspective may depend on whether you are about to go skiing or shovel out a buried car. But, there is no denying that individual snowflakes are as beautiful as they are complex.
Most often, when we talk about snowflakes we really mean snow crystals, which are single crystals of ice with six-fold symmetry. A snowflake can also mean a combination of multiple snow crystals.
It is true that no 2 are alike, but they do have things in common with each other. Snow crystals begin as a microscopic speck of dust. Scientists call these small particles condensation nuclei. They supply the surface that a water droplet needs to initially freeze to ice. Once a droplet freezes, it continues growing into a snow crystal as water vapor condenses on its surface.
We know that a glass of water will freeze at 32 degrees Fahrenheit, but dispersed fine droplets can remain liquid in air as cold as -40 degrees Fahrenheit. These supercooled droplets promote the growth of ice crystals.
The path that an individual ice crystal takes on its way to the ground is unique to that crystal. The conditions along that path determine the shape and complexity we see when a snowflake lands on our coat sleeve.
On colder days here in Yellowstone we may get a chance to see a form of snow crystals called diamond dust. These hexagonal ice prisms are only a few 10ths of a millimeter in size and nearly invisible to the naked eye. These small crystals sparkle in the sunlight as if the air is twinkling. Cirrus clouds are made of diamond dust.
The largest ice crystals are called stellar dendrites. They begin as small hexagonal ice prisms with branches that grow simultaneously from the six corners. The side branches are triggered by changes in temperature and humidity.
Ice crystals can also come in column or needle shapes. They never come with 4, 5, 7 or 8 sides.
While talking about snowflakes, Thoreau said, “How full of the creative genius is the air in which these are generated! I should hardly admire more if real stars fell and lodged on my coat.”