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That’s snow business

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Snowflakes (Callie Jones)

Enough snow fell in Britain on 2 February 2009 for us all to make a quarter of a million snowballs each, according to mathematician Carol Vorderman. That is 3,840 billion kilos of the white stuff.

And, on the basis that no two snow crystals are the same, that represents an enormous variety of crystal shapes.

It was Wilson “Snowflake” Bentley, a self-educated farmer from Vermont, who discovered that snow crystals come in an infinite variety of forms. He was the first person to photograph a single snow crystal in 1885, and went on to capture more than 5,000 snow crystals during his lifetime, finding that no two were the same.

His pioneering photomicrography was published in 1931 in ‘Snow crystals’, a collection of more than 2,400 crystal images.

Snow crystals form when tiny supercooled cloud droplets, about 10µm in diameter, freeze. As the crystals fall through clouds, they aggregate to form snowflakes. The basic form of a snow crystal is a hexagonal prism, which can be plate-like or columnar. As the crystals get bigger, branches sprout from the corners to make more the complex snowflake shape.

According to the ‘Guinness Book of Records’, the largest snowflake on record measured 38cm wide and 20cm thick. It landed in Montana, US, in 1887.

The size and shape of snow crystals depends on the temperature and humidity at which they are formed. Thin plates and stars form at around –2C, while columns and slender needles appear near –5C. Plates and stars again form near –15C, and at around –30C both plates and columns are formed.

It is never too cold to snow, as long as there is a source of moisture and some way to lift or cool the air. But most heavy snowfalls occur at relatively warm temperatures, typically –9C or warmer, because air can hold more water vapour when it is warmer.

Snow appears white because it reflects most of the sunlight, in contrast to most other natural materials, which absorb some sunlight to give them their colour. The complex structure of snow crystals creates numerous tiny surfaces that reflect visible light. What little sunlight is absorbed by snow is absorbed uniformly over the wavelengths of visible light, to give snow its white appearance.

The fluffiest snow has a water-to-snow ratio of 0.01 to 0.05 and typically falls with light winds and temperatures around –9C. Fresh, undisturbed snow contains a high proportion of trapped air, making it an effective insulator. Fresh, uncompacted snow is typically 90–95 per cent trapped air.

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