Is It True That No Two Snowflakes Are Alike?

It's one of the first science facts many of us learn: no two snowflakes are alike. It turns out to be very nearly true, and the story of how we know is a lovely one.

The snowflake man

In 1885, Wilson Bentley, a farmer's son from Jericho, Vermont, became the first person known to photograph a single snow crystal, by attaching a microscope to a bellows camera. Over the next four decades he photographed more than 5,000 snowflakes, catching each one on a black velvet tray and working quickly in the cold before it melted. He never found two that matched, and his 1931 book Snow Crystals made his images famous.

Why snowflakes have six sides

When water freezes, its molecules lock into a hexagonal pattern. That's why snow crystals grow with six-fold symmetry, as six-sided plates, stars, columns and needles.

Why each one is different

In the 1930s, Japanese physicist Ukichiro Nakaya grew snow crystals in his lab and showed that their shape depends on the temperature and humidity where they form. Flat plates grow in some conditions, long needles in others, and the branching "stellar" shapes we draw on cards form when the air is especially humid.

As a snowflake falls, it passes through slightly different air, and each change adds new growth. All six arms experience nearly the same conditions at the same moment, which is why they look alike. But no two snowflakes take exactly the same path through the sky, so each grows into its own pattern. With trillions of molecules in each crystal, an exact repeat is practically impossible.

The famous near-match

In 1988, scientist Nancy Knight of the National Center for Atmospheric Research reported two snow crystals from a Wisconsin storm that looked identical under a microscope. They were simple hollow column shapes, though, and at the level of molecules they almost certainly differed.

Snow day? Pass the time with our myth or fact quiz, or count down to the holidays with the Christmas countdown.