Why Do Snowflakes Have Six Sides? The Science of the Snow Crystal
TL;DR: Snowflakes have six sides because ice itself is hexagonal. When water vapour freezes, its molecules lock together in layers of six-sided rings, so a young snow crystal grows as a tiny hexagonal prism. Its six corners poke out into the humid air, grow faster than the flat faces and sprout six arms. Each arm meets the same temperature and humidity on the way down, which is why they match, and no two crystals take the same path, which is why no two look quite alike.
Last updated: 23 September 2026
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Key takeaways
- A snowflake has six sides because the ice it is made of is hexagonal. In ordinary ice, water molecules are arranged in layers of six-sided rings.
- A snow crystal grows straight from water vapour, not from a frozen raindrop. It starts life as a tiny six-sided prism with two flat faces and six side faces.
- The six corners of the prism stick out into the humid air, grow faster and sprout arms. Physicists call this a growth instability.
- The six arms match because they travel together and meet the same temperature and humidity at the same moment. Nothing connects them.
- Temperature decides the shape. Thin plates form near minus 2 C, slender columns near minus 6 C and the big star shaped crystals near minus 15 C.
- The question is more than 400 years old. Johannes Kepler asked it in 1611, and it was not properly answered until the twentieth century.
Why do snowflakes have six sides?
Snowflakes have six sides because ice is a hexagonal crystal. When water freezes under the conditions found on the surface of the Earth, its molecules link up with hydrogen bonds into a regular lattice built from rings of six. Every snow crystal is a single piece of that lattice, so it grows with the same sixfold symmetry from the first few molecules to the finished star.
That is the short answer to why do snowflakes have six sides, and it is the one Kenneth Libbrecht, the Caltech physicist who has spent decades growing snow crystals in the laboratory, gives on SnowCrystals.com: the sixfold symmetry arises from the arrangement of water molecules in the ice crystal lattice.
The longer answer is more interesting, because a hexagonal lattice only explains the number six. It does not explain the arms, the branches, the lace, or why the six arms of one crystal look like copies of each other. For that you have to follow a single crystal on its fall through a cloud.
What does the water molecule have to do with it?
Everything. A water molecule is one oxygen atom with two hydrogen atoms attached, and its charge is lopsided: the hydrogen side is slightly positive and the other side slightly negative. That imbalance lets each molecule grab its neighbours with hydrogen bonds.
The University of Colorado’s snow hydrology notes on ice physics set out what happens when water freezes. In ice, each water molecule forms four hydrogen bonds with its nearest neighbours, the oxygen to oxygen angles are about 109 degrees, and the result is a tetrahedral arrangement. Built up in three dimensions, those tetrahedra place the oxygen atoms in layers of hexagonal rings.
The same notes point out that the hexagonal form, known as ice Ih, is the only kind of crystalline ice found at the surface of the Earth under normal temperatures and pressures. Every icicle, ice cube and hailstone you have ever seen is made of it.
The open, ring shaped lattice has a famous side effect. It holds the molecules further apart than they sit in liquid water, which is why ice is less dense than water and floats. We wrote about that in why does ice float. The snowflake and the floating iceberg are two faces of the same six-sided ring.
How does a snow crystal grow from water vapour?
A snow crystal is not a frozen raindrop. Libbrecht is precise about this: a frozen raindrop is sleet. A snow crystal forms when water vapour in a cloud turns directly into ice without first becoming liquid, and it keeps growing as more vapour condenses onto it.
Molecules arriving from the air do not stick everywhere equally. On faceting, Libbrecht explains that they stick best to rough spots on the surface, where many chemical bonds are free, and less well to smooth areas. The rough patches fill in quickly, and what is left is a set of flat, smooth faces. The simplest snow crystal is a hexagonal prism with two basal facets, the flat top and bottom, and six prism facets around the sides.
That little six-sided prism is the seed of every snowflake. Whether it stays a plain prism, flattens into a plate, stretches into a column or explodes into a star depends on what happens next.
Why does a snow crystal grow six arms?
Because corners win. A hexagonal plate has six corners, and each corner sticks out a little further into the humid air than the flat faces between them. More water vapour reaches the corners, so they grow slightly faster, which makes them stick out further, which makes them grow faster still.
On branching, Libbrecht calls this a growth instability, and it is responsible for the complex shapes we recognise as snowflakes. The same effect repeats on each arm, so sidebranches sprout from the corners of the branches themselves. As a rule, faceting dominates while crystals are small or growing slowly, and larger, faster growing crystals become branched.
The finished star is a balance between two forces. Faceting brings order and keeps the edges straight. Branching brings chaos and complexity. Put the two together and you get a pattern that is intricate and symmetrical at the same time.
Why do all six arms match?
They match because they share a journey. As a crystal tumbles through a cloud it passes through air of changing temperature and humidity, and each change makes the arms grow a little differently. All six arms experience the same change at the same moment, so they grow in step.
Libbrecht is emphatic that nothing synchronises them. There is no hidden force, quantum or acoustic, passing messages from arm to arm. Each arm grows independently under identical conditions, and identical conditions produce near identical results.
This is also why many real snowflakes are not perfectly symmetrical. When growth is hurried, sidebranches can space themselves erratically, and a crystal that collides with another or breaks an arm on the way down arrives lopsided. The perfect stars in photographs are the careful selections of patient photographers.
Why does temperature change the shape of a snowflake?
Temperature decides which way the crystal prefers to grow, flat or long, and humidity decides how elaborate it becomes. In the 1930s the Japanese physicist Ukichiro Nakaya grew snow crystals in a refrigerated laboratory at different temperatures and humidity levels and mapped the results into what is now called the snow crystal morphology diagram.
The morphology diagram shows a surprising back and forth. Thin plate-like crystals appear near minus 2 C. Slender columns and needles grow near minus 6 C. Large thin plates return near minus 15 C, and that narrow band is where the biggest, most photogenic star shaped crystals grow. On top of that, low humidity produces simple, slowly grown forms, and high humidity produces complex, branched forms.
The Royal Meteorological Society’s MetLink guide to snowflakes describes the same pattern from the ground. When it is very cold, snow tends to be fine and powdery, with simple rod or needle shapes. Closer to 0 C, flakes are larger and more complex. It groups snowflakes into six main shapes: plates, stars, columns, needles, dendrites and capped columns.
The clearest proof is the capped column, which Libbrecht’s guide to snowflakes describes as shaped like two wheels on an axle. It forms when a crystal travels through different temperatures as it grows: first a column forms around minus 6 C, then plates grow on its ends around minus 15 C. Every snowflake is a record of the air it fell through.
Are any two snowflakes exactly alike?
Almost certainly not in nature, and the reason follows from everything above. The final shape of a snow crystal is set by the exact path it took through the cloud, and no two crystals follow exactly the same path.
The idea was made famous by Wilson Bentley, a farmer from Jericho, Vermont. According to the Jericho Historical Society, on 15 January 1885 he became the first person to photograph a single snow crystal. He went on to photograph well over 5,000 of them without finding two the same, and in 1925 he wrote that every crystal was a masterpiece of design.
The laboratory has since added a twist. Libbrecht has grown what he calls identical-twin snowflakes by exposing a pair of tiny seed crystals to nearly identical changing conditions. That is roughly what would happen if two crystals fell side by side through the same air. They come out strikingly similar, which proves the point from the other direction: same path, same shape.
Is a snowflake the same thing as a snow crystal?
Not quite. A snow crystal is one single crystal of ice with its molecules lined up in one hexagonal array. A snowflake is a looser word for anything that falls from a winter cloud, including clumps. Libbrecht notes that hundreds or even thousands of snow crystals can collide and stick together in mid air, forming the soft puff balls we usually call snowflakes.
The difference shows up in the record books. On Libbrecht’s fun facts page, the largest snow crystal ever photographed measured 10 mm from tip to tip, while the smallest, diamond dust crystals, can be as thin as a human hair. MetLink lists a far larger snowflake, reported as 38 cm wide at Fort Keogh, Montana, on 28 January 1887. A flake that size can only be a cluster of many crystals stuck together, not a single six-sided star.
The same page offers a number that is hard to picture: averaged over a typical year, about a million billion snowflakes fall somewhere on Earth every second. Libbrecht is careful to call that a rough estimate.
Who first asked why snowflakes are six-sided?
The first serious attempt came from the astronomer Johannes Kepler. As St Edmund Hall, Oxford recounts, in 1611 Kepler was crossing the Charles Bridge in Prague when snowflakes landed on his coat, all six cornered. He turned the question into a short essay and gave it to his patron as a New Year present: The New Year’s Gift, or On the Six-Angled Snowflake.
Kepler could not see molecules, but he guessed that the shape might come from the efficient packing of tiny identical parts, the way cells pack in a honeycomb or seeds in a pomegranate. That is not far from the modern answer. The complete explanation had to wait for the twentieth century, when Nakaya first grew artificial snow crystals in 1936 and mapped how temperature and humidity shape them.
Why is snow white if ice is clear?
Because a snowbank is a pile of countless tiny mirrors. A single snow crystal under a microscope looks clear, like a small piece of shaped glass. On why snow is white, Libbrecht compares it to a pile of crushed glass, salt or sugar: each grain is transparent, but light bounces from surface to surface so many times that almost all of it is reflected and little is absorbed. Anything that reflects nearly all light looks white. Clouds are white for the same reason, with water droplets doing the reflecting.
How much snow does the UK actually get?
Less than the Christmas cards suggest. The Met Office’s guide to snow gives the UK an average of 13.0 days of lying snow a year over the 1991 to 2020 period, with Scotland the snowiest nation at 23.0 days. The highest average at any UK station is Cairngorm Chairlift, with 75.2 days of snow lying a year, followed by Braemar with 47.6.
The Met Office also explains why British snow so often arrives as big, wet clumps. Flakes that fall through cold, dry air stay powdery and do not stick together. Flakes that fall through moist air slightly warmer than 0 C melt at the edges and glue themselves into large flakes, exactly the aggregate snowflakes described above.
Snow is also water in storage. A snowy spell in the hills becomes river flow and ground moisture when it melts, and a hard frost in the valley can freeze exposed pipes in buildings. Our guide to preparing buildings for snow warnings covers that practical side.
Frequently asked questions
Why do snowflakes have six sides and not four or eight?
Because the ice they are made of is hexagonal. In ordinary ice, water molecules bond into layers of six-sided rings, and a snow crystal is one continuous piece of that lattice. It grows as a six-sided prism, and its six corners become the six arms.
Are all snowflakes six-sided?
The underlying crystal always is, but the visible shape varies. Columns and needles are six-sided in cross section, and plates and stars show the symmetry clearly. Broken crystals, clumps of many crystals and small triangular crystals can hide it, and two six-branched crystals that collide and grow together can even make a twelve-branched snowflake.
Is it true that no two snowflakes are alike?
In nature it is almost certainly true for complex crystals, because each one follows its own path through changing temperature and humidity. In the laboratory, physicists have grown near identical twin crystals by giving two seeds exactly the same conditions.
What temperature makes the best snowflakes?
The large, classic star shaped crystals grow in a narrow band around minus 15 C when the air is humid. Near minus 6 C crystals grow as columns and needles, and near minus 2 C as thin plates.
Who discovered why snowflakes are hexagonal?
Johannes Kepler asked the question in 1611 and guessed it came from tight packing of tiny parts. The modern answer, a hexagonal ice lattice plus the growth effects mapped by Ukichiro Nakaya in the 1930s, came in the twentieth century.
One last thought
Every snowflake is a weather report written in ice. Its six arms tell you the lattice underneath, its plates and columns tell you the temperatures it fell through, and its lace tells you how humid the cloud was. Catch one on a dark sleeve this winter and you are reading the sky from a few thousand metres up.
We watch water all day, in pipes, tanks, rivers and sewers. It still surprises us. For more short, sourced stories about water itself, read how a raindrop is not shaped like a teardrop, why hailstones are layered and how much a cloud weighs.