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Winter is here with excess snow closing airports, stopping trains, and stranding motorists. However, skiers and snowboarders are among the few who are happy to greet the first flakes of 2010. But not all who are on the slopes are enjoying natural powder.

While meteorologists may have difficulty divining Mother Nature's schedule, chemists have increasingly found ways to replicate her results. Some slopes are lubricated with mixtures of vegetable oil (CAS Registry Number (RN): 57-11-4), glycerin (CAS RN: 56-81-5), cellulose (CAS RN: 9004-34-6), and paraffin wax such as tetracontane (CAS RN: 4181-95-7) to help keep the skiers happy until natural snow falls.

Snow is an accumulation of ice crystals formed from water vapor. In 1611, Johannes Kepler first described the high degree of symmetry and patterning in snowflakes. Shortly thereafter, Rene Descartes expanded appreciation for the multiple forms of snow crystals in his 1637 work, Les Meteores. William Bentley's 1931 catalogue immortalized the snowflake as winters icon and fueled the notion that no two snowflakes are alike.

Snow crystals are sensitive enough to temperature and moisture that slight variations in either can change snowflake shapes.^{1, 2} Supersaturation causes a vapor to exceed the normal saturation point at a given temperature. Snow crystals form when large numbers of liquid water droplets collect on minute dust particles and supersaturate the enclosed air. As the temperature drops, the droplets freeze while quickly accumulating water vapor. Eventually, they fall out of solution and out of the sky as snowflakes. Because ice has a hexagonal crystal structure, snow crystals often form six symmetrical arms, reflecting the similar variations they undergo during expansion.

On March 12, 1936, Ukichiro Nakaya grew the first artificial snow crystal on a single hair from rabbit's fur by circulating water vapor in a cooled chamber.³ In the 1930s, Nakaya found himself with limited laboratory resources and an abundance of snow, so he documented the multiple shapes of snowflakes and their dependence on temperature and supersaturation.^4,5

An entire decade passed before Vincent Schaefer added dry ice (CAS RN: 124-38-9) from a chilled chamber to a cloud and induced the first artificial snowfall. The snow melted after falling 3,000 feet and before hitting the ground. Among the millions and billions of different crystals created by Mother Nature or snow guns and water, be sure to stay warm and enjoy the chemistry this season.

Contributed by
Kathryn J. Meloche, Ph.D.
Public Relations Representative

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References

1. Libbrecht, K. G. Snowflakes and Snow Crystals. SnowCrystals.com. http://www.its.caltech.edu/~atomic/snowcrystals/ (accessed July 28, 2010).
2. Hallet J. and Mason, B. J. The Influence of Temperature and Supersaturation on the Habit of Ice Crystals Grown from the Vapor. Proc. R. Soc. Lond. A. 1958, 247, 440-453. SciFinder user link.
3. Nakaya, U.; Sato, I.; Sekido, Y. Preliminary Experiments on the Artificial Production of Snow Crystals. J. Faculty Sci., Hokkaido Imp. Univ. 1938, Ser. II, 1. SciFinder user link.
4. Nakaya U. Snow Crystals: Natural and Artificial (Cambridge: Harvard University Press) 1954. SciFinder user link.
5. Nakaya, U.; Toda, Y.; Maruyama, S. Further Experiments on the Artificial Production of Snow Crystals. J. Faculty Sci., Hokkaido Imp. Univ. 1938, Ser. II, 2. SciFinder user link.