Object lesson

8th December 2000 at 00:00
Wilson A Bentley was a man obsessed. The object of his passion was a "miracle of beauty", a "masterpiece of design". He spent a lifetime trying to capture the fleeting loveliness before it melted away.

That's right, he was the first person to photograph a snowflake. In fact, he snapped more than 5,000 between 1880 and 1931, when he died. It is down to this self-educated farmer from Vermont, USA, that we know that no two flakes are the same.

What most people think of as snowflakes (those fancy six-pointed stars) are actually snow crystals. Flakes are fluffy clumps of crystals which can be several centimetres across; the biggest, at 20cm by 30cm, was reported to have fallen in Siberia in 1971.

Snow crystals are formed from super-cooled water droplets. These tiny drops, which are still liquid despite sub-zero temperatures, freeze around particles of wind-carried dust. The resulting crystals are hexagonal prisms - because water molecules link up in a six-sided shape. Apart from the classic star design, the crystals come shaped like oak leaves, needles and even frisbees. It all depends, not surprisingly, on the weather. For example, star-shaped hexagons are produced in high clouds at temperatres of about minus 15, while six-sided needles prefer the cosy minus 10 found at lower altitudes.

Bentley was really only at the tip of an iceberg in terms of snow science. Even in the 21st century many aspects of crystals and their formation continue to puzzle scientists. But they agree with Bentley on one thing: every year about 10 24 snowflakes are thought to be produced (that's 10 with 24 zeroes, which is a BIG number). And the chances of there ever being two identical snowflakes in the lifetime of the universe are reckoned to be zero.

To start with, not all water molecules are the same. Then, as the molecules start to freeze, they don't always stick together perfectly. Finally, as the crystals whirl through the clouds on their journey to Earth they each encounter different temperatures and concentrations of water vapour which influence their intricate shapes. They bud and branch in marvellous dendritic, or tree-like, patterns which are also found in mineral crystals.

And, of course, in the patterns of the human nervous system. That's the system that tells the brain: "Something very cold has just gone down my shirt. Who threw that f***ing snowball?"

Stephanie Northen


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