Snow is made up of billions of white snowflakes and, while it’s common knowledge that no two snowflakes are alike, it’s less commonly understood exactly why this is.

Snowflakes form when an extremely cold water droplet freezes onto a pollen or dust particle in the sky, creating an ice crystal. As the snowflake falls to the ground, water vapour freezes onto the first crystal, building new crystals. These new crystals are the six arms of the snowflake, which give it its characteristic hexagonal shape; the arms are symmetrical because they reflect the internal arrangement of the crystal’s water molecules. This process is called crystallisation.

The exact shape of the crystals is based on the temperature and humidity at which the snowflake forms. For example, long, needle-like crystals form at -5℃, and flat, plate-like crystals form at -15℃.

Each arm of every snowflake is intricate and detailed, and determined by external conditions. A crystal may begin to grow arms in one way, but a sudden shift in humidity will suddenly change the direction of growth. However, each arm will be identical to the other five because they all experience the same atmospheric conditions. Importantly, the water molecule arrangement means there are always six arms on a snowflake – they are always hexagonal.

Snowflakes photographed by Wilson Bentley in 1902.

Snowflakes fall because they are heavier than the surrounding air. When they fall through more humid air, water vapour freezes to the crystal. This means that it grows progressively larger as it takes on its expanding hexagonal shape. There are unlimited variations within the hexagonal, geometric shape because no crystals experience identical conditions. 

However, just because snowflakes form, it does not mean that there will be snow on the ground. The temperature increases closer to the ground, meaning that in most conditions snowflakes melt before the have the chance to settle. We only experience snowfall if the air temperature is below freezing all the way to the Earth’s surface.

Sometimes, these snowflakes pass through a very thin layer of warm air and experience only partial melting. When they exit, they refreeze as small pellets of ice, which we recognise as sleet. If the layer of warm air is thicker, the crystals are completely melted, and we experience freezing rain.

Meteorologists can predict when it will snow by identifying a moisture-laden air mass which might pass over a particular area. If the highest temperature within this mass is still below freezing, the snowflakes will not melt on their journey. Provided that lower elevations also maintain a low temperature, the snowflakes will make it to the ground as snow and stick without melting.

Featured image: George Tuli

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