Every snowflake starts as a speck of dust or pollen high in a cloud and ends as a tiny sculpture of ice. Why do snow crystals have six sides? Why do their arms match, and why is every one different? The answers come from the shape of the water molecule and the journey each crystal makes through the cloud.
How a snowflake forms
According to NOAA’s explanation of snow-crystal formation, a snowflake begins when an extremely cold water droplet freezes onto a particle of pollen or dust in the sky, creating a primary ice crystal. As the crystal falls, water vapour freezes directly onto its surface and it grows. A typical snowflake contains around 1019 water molecules, ten billion billion of them.
Strictly speaking, a snow crystal is a single crystal of ice, while a snowflake can also be a cluster of many crystals stuck together. The largest flakes ever claimed, recorded by Guinness, fell at Fort Keogh in Montana in January 1887 and were said to be 38 cm (15 inches) wide. Single crystals as large as a dime, about 18 mm, have been seen.
Why snowflakes have six sides
The sixfold symmetry comes from ice itself. When water freezes, its molecules lock into a hexagonal lattice, so a growing crystal starts as a minute hexagonal prism. Six arms, or dendrites, then grow out from the corners of that tiny hexagon.
Snow is white for a related reason: ice is clear, but the countless small facets of the crystals scatter sunlight in all directions.
Plates, columns and stars: the Nakaya diagram
In the 1930s the Japanese physicist Ukichiro Nakaya grew snow crystals in his laboratory and discovered that their shape depends mainly on temperature, with humidity playing a supporting role. His findings, now drawn as the Nakaya diagram, are explained in detail by the physicist Kenneth Libbrecht at snowcrystals.com:
- Around −3 °C: thin, flat plates.
- Around −5 to −6 °C: needles and columns.
- Around −15 °C: large, branching stellar crystals, the classic “star”, especially in very humid air.
- Changing conditions: a crystal that starts as a column and then falls into a plate-growing layer can end up as a capped column, a column with a plate at each end.
Low humidity gives simple prisms; high humidity gives elaborate branches. Picture-perfect symmetry is actually rare: fewer than one snowflake in a thousand shows the ideal sixfold shape, and irregular crystals are far more common.
Matching arms, and why no two are alike
The six arms of a snow crystal do not communicate. Libbrecht explains that they look alike because they grow at the same moment in the same tiny parcel of air, so every change in temperature and humidity affects all six in the same way. He compares them to neighbours who all open umbrellas when it starts to rain without consulting one another.
Each crystal, though, follows its own path through the cloud. Small changes in temperature and humidity can alter how an arm grows within minutes or even seconds, so no two crystals live through exactly the same sequence of conditions. That is why it is so unlikely that any two snowflakes are identical.
Wilson Bentley revealed this variety to the world, photographing snow crystals through a microscope from 1885 onwards. Today anyone can try: catch flakes on a dark, cold cloth and look with a magnifier or a phone macro lens before they melt, as Alexey Kljatov does in his remarkable home-made macro photography.
Explore more: wind-shaped snow rollers and ice patterns on soap bubbles.
Related reading
- Ten Ways to Understand Snow: Crystals, Records and Winter Landscapes
- A Snowflake Field Guide: Plates, Columns and Branches
- Alexey Kljatov’s Snowflake Photography: Tiny Crystals, Resourceful Equipment
- 18 Snowflake Photographs: Patterns Worth a Closer Look
- Stunning Snow Patterns by Simon Beck (Snow Art)
- Lake Baikal Ice Percussion with irkDarbuka
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