Why do snowflakes have six points?

How it works

Clouds are full of droplets far thinner than a hair. High in freezing air, a droplet can freeze around a speck of dust or pollen to make a tiny ice crystal. Water vapour in the cloud keeps sticking to the crystal, so it grows bigger and bigger until it is heavy enough to fall as a snowflake.

When water freezes, its molecules link hands in six-sided shapes, a bit like the cells of a honeycomb. A crystal that starts with this shape grows fastest at its six corners, so it sprouts six arms. That is why snowflakes almost always have six points or six sides.

As a snowflake falls, it drifts through air that is a little warmer or colder, wetter or drier, and each change makes its arms grow a different pattern. No two flakes take exactly the same path, so no two look exactly alike. Temperature also changes the style: some flakes look like stars, others like tiny columns.

Where to see it in New Zealand

In winter, snow often falls on the South Island's mountains and the volcanoes of the central North Island, where there are ski areas around Queenstown and on Ruapehu. Now and then, a bitterly cold southerly brings snow right down to sea level in southern cities such as Dunedin.

Try it together

When it snows, or on a trip to the snow, chill a piece of dark cloth or card in the freezer, then use it to catch falling flakes. Look quickly with a magnifying glass to find the six points. No snow? On a frosty morning, look at the ice crystals on the grass.