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Snowflakes form when an extremely cold water droplet touches a pollen or dust particle in the sky and begins to form an ice crystal. As it falls to the ground, water vapor freezes onto the crystal and builds new crystals on top like Lego blocks.
Snowflake crystals are symmetrical or patterned, a reflection of the internal order of the crystal’s water molecules during crystallization. Ultimately, temperature and to a lesser extent humidity determines the basic shape of the ice crystal. You get long needle-like crystals at 23 degrees F and flat plate-like crystals at 5 degrees F.
The intricate shape of a single arm of the snowflake is determined by the atmospheric conditions experienced by entire ice crystal as it falls. At around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes. Most snow particles are irregular in form, despite their common depiction as symmetrical. It is unlikely that any two snowflakes are alike due to the estimated 10 quintillion water molecules which make up a typical snowflake. The crystals grow at different rates and in different patterns depending on the changing temperature and humidity within the atmosphere that the snowflake falls through on its way to the ground.
A crystal might begin to grow arms in one manner, and then minutes or even seconds later, slight changes in the surrounding temperature or humidity causes the crystal to grow in another way. Although the six-sided shape is always maintained, the ice crystal (and its six arms) may branch off in new directions. Because each arm experiences the same atmospheric conditions, the arms look identical but they are never actually identical.
IMAGE CREDIT: Alexey Kljatov.