Scoop up a handful of snowflakes and you hold a miniature universe. Scientists believe there are one trillion, trillion, trillion different kinds of snowflakes, all from three basic ingredients; water vapor, ice crystals, and microscopic particles. Each flake begins as a microscopic crystal that forms on a bit of dust or maybe a fragment of bacteria suspended in the atmosphere. It begins to fall, picking up water vapor that adds to the shape and size.
The shape of a snow crystal is based on the shape of the molecules. This was hypothesized in 1611 by Joseph Kepler, who observed that flakes always have 6 sides, and said this must be from being formed by tightly packed spheres. Although the technology did not yet exist to confirm the existence of atoms and molecules, Kepler inferred the presence of spheres smaller than the human eye can detect.
It wasn’t until the late 19th century that anyone realized how intricate and beautiful a snow crystal can be, and that no two are alike. Wilson “Snowflake” Bentley spent years trying to photograph snowflakes by putting together a microscope and a bellows camera. He finally succeeded in 1885 and went on in 1931 to publish Snow Crystals, which is still printed in soft cover form by Dover Publications.
Bentley was struck by the intricacy and transience of snowflakes, and wanted to capture that loveliness for other people to see. He called snowflakes “miracles of beauty” and said, “Every crystal was a masterpiece of design and no one design was ever repeated. When a snowflake melted, that design was forever lost. “ (Wilson Bentley, 1925)
It would be impossible to study that many shapes without classifying them in some kind of system. In 1951, the International Classification System for Snow and Ice was developed. It defines seven main snow crystal types; plates, stellar crystals, columns, needles, spatial dendrites, capped columns and irregular forms. It also identifies 3 other forms of frozen precipitation; graupel, ice pellets, and hail.
The first person to classify snow crystal forms scientifically was a frustrated nuclear physicist, Ukichiro Nakaya. In 1932, Nakaya was assigned as a professor in Hokkaido, the Northern Island of Japan. No facilities existed there for nuclear research, but there was an abundance of snow. So the scientist identified and classified all the major snow crystal types. He also grew snow crystals in the laboratory and described how they formed under varying conditions.
Nakaya identified 41 distinct morphological types of snowflakes and in 1954 published his beautiful book, Snow Crystals, Natural and Artificial. His work gave us the foundation for understanding the formation of these lovely natural phenomena.
The scope of this article is not wide enough to deal with all 41 of Nakaya’s classifications, so I will confine discussion to the seven types recognized in the international system, plus graupel. Each snow crystal starts with the same three elements, but the particular form it takes is largely determined by the amount of water vapor in the air and the temperature.
The most common snowflake is the irregular crystal. These are small and clumped together so that the symmetry of the crystal is not easily seen.
Stellar plates are the next most common type of snow crystal. They form at 5-28 degrees Fahrenheit (-2 to -15 Celsius). The form is flat and has 6 sides. Markings on stellar plates may be very intricate. They usually show branches that extend from the sides. Stellar plates are further subdivided into sectored plates, stellar dendrites (the branches are longer and thinner and have branches of their own), and fernlike stellar dendrites, which have so many divisions they resemble ferns. The fluffiest snow is composed of stellar dendrites.
Another common snowflake is the double plate. This is basically a short hollow column sealed on both ends. The cap grows outward to form the plates. One plate is always much larger than then other.
Split plates and stars are a form of double plates. Part of one plate grows large along with part of the other plate.
Needles look like tiny bits of white hair to the naked eye. They form when the temperature is at 23 degrees Fahrenheit (-5 Celsius).
Hollow columns are very small. They are six-sided columns and are sometimes closed on the end.
Triangular crystals form at 28 degrees Fahrenheit (-2 Celsius), proving that for every rule there is an exception. No one knows why these triangular plates form at this temperature. Sometimes the three corners sprout branches.
Twelve sided snowflakes form when a double plate snowflake twists so that one plate is lined up at a 30 degree angle from the other. These are quite rare.
Graupel is basically a snowflake that has been covered with tiny droplets of water that have frozen to the surface of the crystal. They look like miniature snowballs.
Observing the many types of snowflakes can be fascinating, especially if you have a microscope to reveal the beauty contained in these small art treasures of nature.
References and further information:
http://www.its.caltech.edu/~atomic/snowcrystals/ (Snow Crystals.com)