Snow has been puzzling physicists for generations and if you pay attention to it this winter, you’ll begin to understand why.
Sometime over Christmas or early in the New Year, on a still, cold night, you’ll see pencils of light from street lamps shining into the night sky like search lights.
Snow crystals will be falling very gently down through the beam of light.
The crystals are like pieces of pencil-shaped, fibre optic cable and the light is shining up through them as they fall.
They aren’t really flakes — they’re hollow snow columns, slipping vertically through the perfectly still air.
Even the slightest breeze will break the magic: the crystals will tumble chaotically; the searchlights will be gone.
Snowflakes and crystals begin to form on single, microscopic dust particles as high as 10 km above the Earth. Water vapour condenses around the dust as microscopic droplets that freeze into equally microscopic crystals.
They drift downward and grow as vapour freezes onto different sides and edges of their surface.
The two-legged pyramid structure of each water molecule, fitting like jigsaw pieces with other molecules, causes all of the shapes to be six-sided.
The ever-changing conditions that each crystal experiences as it falls make all the difference to its final shape. Temperature, humidity, wind, and atmospheric pressure all influence the shape of a snow crystal.
Thin plates and stars grow around -2 C, while hollow columns and needles form between -5 C to -10 C. Thicker and six-pointed plates, as well as leaf-like shapes called dendrites, form near -15 C. And finally, combined column and plate forms are made around -30 C.
Another factor, humidity, makes a difference in the location of growth. Dry air encourages it across the flat surfaces. Higher humidity shifts growth to the edges and tips of the crystal. More water vapour present in the air influences crystals to grow at a faster rate, and with more complexity.
So, how do we answer the question about the snowflake diversity? Are they all truly different, as the saying goes?
The answer is yes, fully formed snowflakes are all different.
Snowflake researcher Jon Nelson has said undeveloped crystals “sometimes do reach the ground. And in that case, there’s not much detail to distinguish any two.”
If you want to compare fully grown flakes or crystals, Nelson warns that “if you had a million snow crystals photographed for comparison and could compare two of them every second, you’d be there for nearly a 100,000 years.”
Not even physicists have been that patient.
Alexander Johnston is a student in the joint Laurentian University/Science North Science Communication Graduate Program – and makes a wicked snow angel. Send your science question to firstname.lastname@example.org.
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