Weather: Never mind the snow - admire its geometry

The six-way symmetry of snowflakes is a consequence of arrangement of molecules in ice crystals. Contrary to popular belief, two snowflakes can be identical.

As everyone knows, no two snowflakes are identical. In fact, that particular old weather myth proved to be a load of snowballs in the late Eighties. The story had gained much credence thanks to the antics of Wilson "Snowflake" Bentley, one of the greatest snow-men of the century, who took photographs of more than 6,000 individual snowflakes over a period of almost 50 years until his death in 1931. And no two of them were identical. In 1986, however, two identical snowflakes were discovered for the first time by scientists on a collection plate aboard an aircraft, and later sightings of identical flakes have also been reported.

The first hint of the crystalline structure of snowflakes appeared in the 16th century, when the great astronomer Johannes Kepler wrote a little book, The Six-Cornered Snowflake, as a New Year's present for his sponsor. Reasoning by analogy with the six-fold symmetry of discs packed together on a plane surface, he argued that snowflakes must be a packing of minuscule, identical units.

Those units were discovered much later to be the crystals of ice formed when water freezes in the air. Under most atmospheric conditions (from about zero to -50C) ice crystals have a hexagonal structure formed by the arrangement of water molecules in the ice lattice. Crystals are plate- like between zero and -3C and between -8 and -25, but are column-like between -3 and -8, and below -25.

As individual crystals fall through the air, they grow and gather speed, and - thanks to the high moisture available in the supercooled clouds in which they are created - they sprout at the corners to form needle or "dendrite skeletal" crystals. In fact, seven different shapes of ice crystal are recognised: prism, plate, dendrite, needle, Hopper-type (with a cut-glass effect), hollow prism and scroll.

As a crystal drops and grows, it adds different forms to its shape according to the moisture content and temperature of its surroundings. The final shape of the snowflake is a record of its speed, temperature and moisture on its journey. For two snowflakes to be identical, they would have had to experience identical conditions at all stages of their descent.

Like fingerprints, snowflakes have a potential for billions of different patterns, which is why it was assumed that no two are identical. Yet there are far more snowflakes than people. Indeed, if we were to do the sums, we might well find that it is very likely that two are identical. What is extremely unlikely, however, is that anyone could find them and compare them before they melted.

Since most rain begins its descent as ice and melts on the way down, it is easy to see that snow tends to fall from low cloud, and does not have enough time to melt. The sort of powdery snow we have been getting in most of the country is typical of very cold weather. In such conditions, the ice crystals are too dry to stick together easily, so the snowflakes remain small. The really chunky snowflakes we sometimes see are really hundreds of smaller ones, stuck together.

They say that the largest snowflake ever found was 8in by 12in, and fell in Siberia in 1971. And as all children know, it's the big snowflakes that make the best snowmen and snowballs.