Wilson Bentley's photographs of snow crystals strike a chord with us all.


Since its debut in Johannes Kepler's book On the Six-Cornered Snowflake in 1611 (see Nature 432, 953; 2004), the snowflake shape has become as familiar as nature's other geometrical designs, such as the spider's web, the honeycomb and mollusc shells. The intricate and remarkably varied structures of snow crystals were shown for the first time in Robert Hooke's Micrographia in 1665.

The most unlikely hero in the later story of snowflake structure is a Vermont farmer with no orthodox scientific credentials, Wilson Bentley (1865–1931). Born into a farming family, the young Bentley developed an intense fascination with the structure of snow. At the age of 20, using a specially constructed rig of bellows camera and microscope, he took the first photograph of a single snow crystal.

One of the great eccentrics of science, 'Snowflake' Bentley (as he was known in his local community) resided steadfastly in the family farmhouse for his whole life. But this did not stop him publishing a series of well-regarded books and articles in scientific and popular journals.

His 1931 book Snow Crystals, which contains photographs (like that shown above) of almost 2,500 crystals, stands alongside Ernst Haeckel's illustrated volumes on the radiolaria as a classic on the geometrical sciences of nature. Bentley's systematic procedures, delicate manual dexterity, incredible patience and tolerance of working in freezing conditions were motivated by a sense of poetic wonder.

Some of his enraptured tone is conveyed in a piece he wrote for Harper's Monthly Magazine: “Quick, the first flakes are coming; the couriers of the coming snow storm. Open the skylight, and directly under it place the carefully prepared blackboard, on whose ebony surface the most minute form of frozen beauty may be welcome from cloud-land. The mysteries of the upper air are about to reveal themselves, if our hands are deft and our eyes quick enough.”

It was Bentley who told the world that no two snowflakes are precisely alike. This variety, we now know, has a number of causes: the construction of the crystals from about 1018 water molecules; their formation under varying temperatures as they swirl with their cloud; and the unpredictability of the processes of aggregation from one crystal to another.

Bentley also knew that, in contrast to popular belief, most crystals are not wholly symmetrical — it's just that the symmetrical ones are generally selected for illustration.

The complexity of the infinite variations on the hexagonal structure continues to attract serious scientific attention, for example from physicist Kenneth Libbrecht at the California Institute of Technology. Libbrecht's lively website, which includes animated images of crystal growth (http://www.its.caltech.edu/~atomic/snowcrystals/movies/movies.htm), is both informative and appropriately entertaining for the festive season. The image shown below is from Ken Libbrecht's Field Guide to Snowflakes (Voyageur, 2006).


Mathematically, the snowflake has also been enlisted in the cause of fractals. In 1904 the Swedish mathematician Niels Fabian Helge von Koch published his 'snowflake curve', generated from an equilateral triangle. Each side is trisected and the centre segment replaced by two sides of a smaller equilateral triangle projecting outward. This process is repeated ad infinitum. The total length increases with each step, but the length of each side approaches zero. In this sense it is unlike a snowflake in nature, the lower limit of which is set by the size of a water molecule.

The hexagonal configurations familiar from Christmas lights in streets around the world and glittering on countless Christmas trees testify that the passion shown by these snowflake pioneers is shared by everyone who responds intuitively to the geometry of nature. Perhaps that is all of us.