How Do Snowflakes Form? Why Every One Has Six Sides
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Every snowflake that has ever fallen has had six arms, or six sides, or six of something, and no two that have been examined have been quite alike. Both facts follow from the same cause: the way water molecules pack when they freeze, and the path each crystal takes through a cloud. A snowflake is a record of its own journey, written in ice, and the man who photographed five thousand of them on a Vermont farm at the turn of the twentieth century called them tiny miracles of beauty, which is sentimental and also a fair description of the physics.
It starts with dust
Water vapour in a cloud does not simply freeze when the air drops below zero. Pure water droplets can stay liquid to nearly minus forty, because freezing needs a starting point, and in a cloud the starting point is a speck of something else, a grain of clay, a fragment of soot, a bacterium, on which molecules can settle in an orderly way. Once a tiny ice crystal exists, it grows: water vapour in the surrounding air deposits directly onto it as ice, skipping the liquid stage, and the crystal draws in vapour from the supercooled droplets around it, which evaporate to feed it. A snowflake is not a frozen raindrop. It is built from vapour, molecule by molecule, over the fifteen minutes to an hour it spends falling through the cloud.
Why six
A water molecule is a bent shape, an oxygen atom with two hydrogens at an angle of about 104 degrees, and when molecules freeze they link hydrogen to oxygen in a lattice whose most stable form is a stack of hexagonal rings, like a honeycomb. The six-fold symmetry of the lattice is invisible in a block of ice but shows in a growing crystal, because the six corners of a hexagonal plate stick out slightly further into the vapour than the flat sides, gather molecules faster, and grow into arms, and each arm's edges do the same at smaller scale, producing the branching that makes a star. Whatever the crystal does, it does it six times at once, because the six corners are in identical surroundings.
What decides the shape
The habit of a snow crystal, whether it grows as a flat plate, a branching star, a column or a needle, depends on the temperature of the cloud and on how much excess vapour the air holds, and the dependence was mapped in the 1930s by the Japanese physicist Ukichiro Nakaya, who grew snowflakes on a rabbit hair in a cold laboratory and drew the first diagram of it:
- •Around minus 2 degrees: thin plates
- •Around minus 5: needles and hollow columns
- •Around minus 15: the large, branching stars of Christmas cards, which grow fastest of all and only in a narrow band of temperature
- •Below minus 22: plates and columns again, and at the coldest temperatures small solid prisms
- •More moisture at any temperature: more branching and more delicate structure; less moisture: simpler, solid shapes
Why no two are alike
A crystal that falls through a cloud passes through changing temperature and humidity, and because all six arms experience the same changes at the same time, they grow in the same way and the flake stays symmetrical. But no two crystals take the same path, and a star that grew plates at one height, branches at another and side-arms at a third has a history that will not be repeated. A large stellar crystal contains around a hundred quintillion water molecules, and the number of ways of arranging its branches is so much larger than the number of snowflakes that have fallen on Earth that two matching ones are, in practice, impossible. Small, simple prisms grown in stable conditions can look identical under a microscope, and researchers have made twin crystals in the laboratory by growing two side by side, but the elaborate flakes of a snowstorm are each unique.
From cloud to ground
What reaches the ground is rarely a single crystal. Flakes collide and stick, especially near freezing when their surfaces are slightly wet, and the large fluffy snowflakes of a mild snowfall are clusters of dozens or hundreds of crystals. A snowflake can also fall through a layer of supercooled droplets that freeze onto it as it goes, coating it in rime until it becomes a pellet of graupel, or through warm air that melts it to sleet or rain. Fresh snow is around ninety percent air, which is why it insulates the ground, muffles sound and can be squeezed to a tenth of its volume, and why ten centimetres of it is worth about a centimetre of rain. Wilson Bentley, the Vermont farmer, caught his flakes on black velvet and photographed them through a microscope before they could melt, from 1885 until his death in 1931 of pneumonia caught walking home through a blizzard; Nakaya, who put the shapes on a chart, called snow crystals letters sent from the sky.
The takeaway
A snowflake grows from a speck of dust in a cloud as water vapour deposits directly onto it as ice, and it has six-fold symmetry because water molecules freeze into a hexagonal lattice whose corners grow fastest. Temperature and humidity decide whether it becomes a plate, a column, a needle or a branching star, and because each crystal falls through its own sequence of conditions, its arms match each other while no two flakes match.