Atmosphere And Weather

How Snow is Formed



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"How Snow is Formed"
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Snowflakes are amazing things. You can stare at these light, delicate crystals for hours, marvelling at their delicate beauty. No machine-produced snow even comes close.

It's even more amazing that these delicate snowflakes come from the same source as every other type of precipitation, high in the atmosphere where the temperature can be far below the freezing point of water. Even though it's so cold, every snowflake starts as a single liquid water molecule suspended in midair. This supercooled water molecule will stay in a liquid state until it is jostled.

The instant the supercooled molecules of water encounter anything solid, even a tiny speck of dust, they will crystallize around it. Only supercooled water can freeze quickly enough to crystallize into a delicate, six-sided snowflake. Its initial temperature determines what form that structure will take.

As each of these frozen crystals bumps into more water molecules, it joins in and the snowflake grows bigger and heavier. Eventually, these delicate ice crystals grow heavy enough for gravity to take over. At this point, they start to fall.

There are a lot of things that can happen to a snowflake on the way down. Very few snowflakes keep a perfect crystal structure all the way down. Some split apart completely, but still remain connected to each other. Many snowflakes partly melt and refreeze as they pass through warmer and colder parts of the atmosphere. These changes in temperature are the second thing which determines a snowflake's final shape.

You'd think the most solid shapes, the solid prisms and columns, would have the best chance of surviving. However, it's the delicate stellar dendrites which we know best. These beautiful ice stars are the largest snow crystals which reach the ground. Many of them are one or two centimetres across! That's millions upon millions of tiny water molecules, all joined together in a delicate beauty which has survived a kilometres-long fall. When these snowflakes land on your coat, you can clearly see their structure just by looking at them.

A snowflake can also die at this stage. If it completely melts, it will turn into rain. If it completely melts and then refreezes, it loses all its delicate structure and turns into sleet. However, if only the edges melt together a little as rime builds up on them, the resulting graupel will look a lot like a fuzzy snowflake. Graupel is also sometimes called 'soft hail' because the ice builds up gently on something delicate instead of something hard.

When it lands on the ground, the snowflake begins to change again. The delicate edges are worn away. The molecules start to shift around to make a different type of crystal. In well-bonded snow, the molecules link electromagnetically with the previous layer of snow, but it takes time for newly-fallen snowflakes to adjust and bond.

However, if there's a lot of snow coming all at once, or if there's a lot of freezing and thawing, the snowflakes can't bond with the previous layer. Instead, they become granular. Snow which falls on top of that layer can't bond with it either, but it can form a large slab on top of it. This type of snow is never stable. When the deep granular layer gives way on a mountain slope, it becomes an avalanche.

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