FEW SIGHTS are more captivating than a landscape shrouded by a layer of freshly fallen snow. Familiar jagged irregularities like houses, fences and trees are smoothed to a gently sculpted curve, and this visual beauty is often enhanced by an eerie quietness, a tranquillity that stems from the ability of snow to absorb, like an acoustic tile, every obtrusive sound that impinges on its surface.
Snow is a more complex entity than merely frozen rain. Individual ice crystals form in the high atmosphere when water vapour freezes around tiny particles of dust or salt. The crystals grow as they drift downwards, attracting additional molecules of frozen water, almost one by one, and maintaining, for reasons that are still not fully understood, a shape whose cross section is almost invariably a hexagon.
The commonly accepted view is that this shape can be traced to the basic molecular structure of water, which one scientist has described as "two little hydrogen atoms stuck onto a big oxygen atom, like the ears on Mickey Mouse's head" - it is believed that the angle at which the hydrogen atoms protrude from the oxygen, being about 120 results in snow crystals with the familiar six sided symmetry.
Within the limits of their genetichexagonality, however, crystals come in many shapes and sizes. The most common is the regular hexagonal prism, with six rectangular sides and a top and bottom that are flat hexagons. But their height varies considerably - some are so shallow that they look like a six sided 50p piece, while others are elongated, and resemble a length from a six sided pencil.
And there are also more complex forms a frequent one, described as stellar, consists of a hexagonal hub with six spokes, resembling a wheel without a rim, while on the very beautiful dendritic crystal, the six spokes each have their separate branches, making it similar in shape to the traditional Christmas card representation of a snowflake.
As the individual crystals drift earthwards into relatively warmer air, they congregate into snowflakes, each of which may contain 1,000 or more crystals. If the temperature is near or perhaps even slightly above the melting point, snow flakes become wet and adhere to other flakes, and may form flakes as large as several inches in diameter. On very rare occasions, they may grow larger still on January 18th, 1887, snow flakes "larger than milk pans", 15 inches in diameter, fell over several square miles near a place called Fort Keogh in Montana, US.