What is snowflake?
How do snowflakes get their shape?
Snowflakes start as grains of dust floating in clouds. Water vapor in the clouds sticks to the grains and freezes, forming ice -- the simplest hydrogen-bonded crystal. From there, each snowflake takes a different path, morphing into a unique shape as it travels to the ground. The end result is a "mind-boggling array of shapes."
The snowflakes that settle upon our sleeves and scarves during a snowstorm have more variations in shape than you might think. There's the classic snowflake: a flat plate with branchlike, dendritic arms. Some look like hexagonal prisms; others like hollow pencil-shaped columns or tiny needles.
Scientists use the term "snow crystal" more than snowflake. "A snow crystal, as the name implies, is a single crystal of ice. A snowflake is a more general term; it can mean an individual snow crystal, or a few snow crystals stuck together, or large agglomerations of snow crystals that form 'puff-balls' that float down from the clouds."
A few facts about snow crystals: They are formed from water vapor that condenses directly into ice inside of clouds. They take shape as water vapor molecules from cloud droplets condense and freeze on the surface of a seed crystal, and patterns emerge as these crystals grow. The seed crystal itself forms on a tiny particle, like a speck of dust in the air, which serves as a base for ice growth.
Note: Snowflakes are not created from frozen raindrops. Liquid water that freezes in the atmosphere as it falls to the ground is actually sleet. And hail, Libbrecht said, is just a large piece of sleet that collects water and grows as it travels from the atmosphere to the ground.
Water molecules -- made of one oxygen and two hydrogen atoms each -- are ultimately responsible for the familiar six-sided shape we associate with snowflakes.
Atoms and molecules can hook up in different ways and, in the case of water, they like to hook up into a hexagonal lattice. That underlying structure is how the crystal gets its sixfold symmetry.
Temperature and humidity are the two main factors that influence how the crystalline structure forms. If the temperature and humidity level changes, so does the growth pattern of the crystal.
At low humidity you get simple plates and simple hexagonal blocks. At higher humidity, more branched structures. And temperature also plays a major role in the forming of flakes.
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