Plate 08 · Matter
Why Snowflakes Have Six Points
Every snowflake grows from one tiny six-sided crystal of ice. Its shape records the path it took through the cloud.
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Six Begins With Water
A snowflake is not a frozen raindrop (that is sleet, or ice pellets). It is ice that grows straight from water vapor, molecule by molecule, without first becoming liquid.
When water molecules freeze into ice, they line up in layers of hexagons. A snowflake’s six-fold symmetry comes from that arrangement.
Model snowflake · on black velvet
From One Small Crystal
It starts as a tiny six-sided crystal of ice. Water vapor from the air around it sticks to its surface, layer by layer, and it grows.
The Corners Lead
The six corners of the hexagon stick out a little farther into the vapor-rich air, so they grow a little faster; growing faster, they stick out further, and grow faster still.
That runaway growth sends an arm out from each corner, and the arms sprout side branches.
Temperature Decides the Shape
Beginning in the 1930s, the Japanese physicist Ukichiro Nakaya and his colleagues found that a snowflake’s shape depends on the temperature and humidity it grows in. Thin plates appear near −2°C, slender columns near −6°C, and large thin plates again near −15°C; when the air is humid and growth is fast, the shapes grow more complex, with fernlike branches.
The chart is known as the Nakaya diagram. The three flakes on the right come from one model with two of its settings changed.
Right: a plate, a broad-armed star and a branching star from the model
No Two Quite Alike
As a snowflake falls through a cloud it passes through air of different temperatures and humidities. Its six arms go through the same changes at almost the same moment, so they resemble one another; two flakes take different paths, so they come out different.
These two model flakes follow the same rules; they simply met different conditions on the way down.
Letters From Heaven
In 1936, in a cold laboratory at Hokkaido University, Nakaya grew a snow crystal on the tip of a single rabbit hair — the first snowflake made in a laboratory.
Snowflakes, he wrote, are letters sent from heaven. Read the shape of one, and you can tell what temperatures and humidities it met on its way down.
Your Turn
Change the model’s two settings and grow a snowflake again from a single crystal. Will it come out as a plate, a broad star, or a fine branching one?
Try thisSlide “How readily ice takes it up” to quickly: do the arms fatten or thin?
What the model leaves out
The snowflake is grown with Reiter’s local cellular model (2005): each cell of a hexagonal grid holds some water; ice and its neighbors take in vapor, and the rest of the vapor spreads around. The model grows only in a plane, so the flake’s thickness is illustrative, and its two settings (the vapor in the air far away, and how readily ice takes it up) do not map one to one onto real temperature and humidity. How temperature and humidity shape real snowflakes comes from the observations of Nakaya and others.
A common misreading
Snowflakes are not frozen raindrops; they are ice crystals that grow directly from vapor. And “no two snowflakes are alike” applies to large, complex flakes. Small, simple ice crystals can look very much alike. And many falling snowflakes are clumps of several crystals; this page follows a single one.
Sources
Follow the question
Plate 17Why Ice FloatsFreeze most liquids and the solid sinks in its own liquid. Ice floats. The secret is in the little hexagons water molecules build.
Plate 30Why Clouds Don’t FallAn ordinary cumulus cloud holds hundreds of tonnes of water, yet it floats. The secret is how astonishingly small its droplets are.
