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Explainer: The making of a snowflake

Author: Matthew Cappucci / Source: Science News for Students

a girl playing in a snowdrift
Snowflakes take a long journey down from the clouds in which they form. But it takes more than cold air and moisture to create them.

Snowflakes come in an infinite range of shapes and sizes. Many appear to be two-dimensional works of art. Others look like a matted cluster of fraying ice strands.

Most come as individuals, although some can fall as multi-flake clumps. What all have in common is their source: clouds that usually hover at least a kilometer (0.6 mile) above the ground.

a composite image showing how snowflakes clump together when they collide to create much larger compound snowflakes
When snowflakes collide, their branches can tangle. This can create a compound flake. This often leads to whoppers (like those in first and third rows) by the time the flakes land.

In winter, the air up there can be very cold — and will get chillier the higher you go. To form snowflakes, those clouds need to be below freezing. But not too cold. Snowflakes form from the moisture in a cloud. If the air gets too cold, a cloud won’t hold enough water for anything to precipitate out. So there has to be a balance. That’s why most flakes develop at or just below freezing — 0º Celsius (32º Fahrenheit). Snow can form in cooler environments, but the colder it gets, the less moisture will be available to make a snowflake.

In fact, a cloud’s air has to be supersaturated with moisture for a flake to form. That means there is more water in the air than would normally be possible. (The relative humidity can reach 101 percent during supersaturation. That means there is 1 percent more water in the air than it should be able to hold.)

When there is too much liquid water in the air, a cloud will try to rid itself of the excess. Some of that excess can flash freeze into crystals, which then lazily meander to the ground.

Or that’s the simple answer. The details aren’t quite that straightforward.

Cold water alone won’t a snowflake make

One more thing is needed to turn cloud moisture into a flake. Scientists call it a nucleus (NOO-klee-uhs). Without something to glom onto, water droplets can’t freeze. Even when the air temperature is well below freezing, water droplets will remain liquid — at least until they have a solid object onto which they can attach.

Usually, that will be something like a pollen grain, dust particle or some other airborne bit. It could be smoglike aerosols or the volatile organic compounds released by plants. Even tiny soot particles or microscopic metal bits spewed in a car’s exhaust could become the nuclei around which snowflakes crystalize.

Indeed, when the air is very clean, it can be very difficult for a cloud’s moisture to find a nucleus.

Near the ground, any object can prove a suitable freeze-onto zone. That’s how we get rime ice to form on the branches of trees,…

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