Category: Weather

Snowflakes – or to use a more scientific term, snow crystals – come in a variety of different shapes including long, thin needles, flat hexagonal plates, columns, and irregularly-shaped pellets called graupel. The International Snow Classification System recognizes ten different shapes in all, only one of which is the traditional snowflake image. The classic six-armed snowflake shape is called a ‘stellar dendrite’ by scientists.

When teaching programs about snow, someone inevitably asks me, “Is it really true that no two snowflakes are alike?” As far as I can tell, the answer is, well, ‘maybe’, and here’s why.

Three things are needed to form these intricate crystals, and the first two are fairly obvious: water, and temperatures below freezing. The third item is a little more inconspicuous. Water cannot condense and freeze all on its own. Every snowflake needs a piece of atmospheric dust or salt at its core. This particle is referred to as a ‘nucleating agent,’ and it attracts water molecules which then condense and begin to freeze. From there, a snowflake’s overall shape is determined by a number of other variables including the atmospheric temperature, the amount of available moisture, wind speed, and mid-air collisions with other snowflakes.

To add more complexity, consider that each individual snowflake contains somewhere on the order of 10 quintillion water molecules. That’s ten with eighteen zeros behind it. While the way these molecules bind to each other is dictated by the laws of physics, the sheer number of ways in which 10 quintillion water molecules can arrange themselves as they freeze into place is mind boggling. But then again, how many snowflakes do you think fall in the typical March snowstorm in Utah? A lot. One scientist has estimated that the number of individual snowflakes that have fallen on Earth in the planet’s history is ten with 34 zeros behind it. In all of those snowflakes is it possible that two are exactly alike? Yeah, maybe… but good luck finding them!

For more information and some beautiful snowflake photographs, please visit our website at www.wildaboututah.org. Thank you to the Rocky Mountain Power Foundation for supporting the research and development of this Wild About Utah topic.

For the Stokes Nature Center and Wild About Utah, this is Andrea Liberatore.

Credits:

Photos: Courtesy Tim Garrett, University of Utah,
Kenneth Libbrecht, Caltech University
Text: Andrea Liberatore, Stokes Nature Center

Additional Reading:

Halfpenny, J.C and Ozanne, R.D. 1989. Winter: An Ecological Handbook. Boulder, CO: Johnson Books, http://www.amazon.com/Winter-Ecological-Handbook-James-Halfpenny/dp/1555660363

Gosnell, Mariana. 2007. Ice: the Nature, the History, and the Uses of an Astonishing Substance. Chicago, IL: The University of Chicago Press, http://www.amazon.com/Ice-Nature-History-Astonishing-Substance/dp/0679426086

Libbrecht, Kenneth .1999. A Snowflake Primer: the basic facts about snowflakes and snow crystals. http://www.its.caltech.edu/~atomic/snowcrystals/primer
/primer.htm

Snow graces the winter sky in many different forms. We have large lazy flakes drifting down, sharp needles driven by harsh winds and thick curtains that swiftly blanket the landscape. Late winter storms offer good chances to observe one of our more unusual and distinctive kinds of snowfall; “graupel”. Graupel – that sounds like some kind of respiratory malady, doesn’t it? Also known as soft hail or tapioca snow, graupel consists of tender round snow pellets no bigger than a pea. The name comes from the German word for hulled grain, “graupe”.

Graupel accompanies warmer winter storms, the kind we often have in March, as well as during summer showers high in the mountains. The pellets form when snow crystals fall through a low cloud of super-cooled liquid droplets. The foggy droplets readily coalesce and freeze around the falling ice crystals, accumulating to form soft graupel pellets. The process is somewhat akin to making rock candy from a concentrated hot sugar syrup, or the method used to generate artificial snow. In contrast, sleet forms when raindrops fall through a cold air layer and freeze.

Our big snowfalls are spawned by storms that generate sprawling unbroken cloud decks. Graupel snow, on the other hand, tumbles down from the bellies of fluffy cumulus clouds. As a consequence, squalls of graupel are brief, the pellets accumulating in a thin white bumpy layer, hence the other common name, “tapioca snow”. A buried layer of tapioca snow is prone to avalanche for the first day or two, after which the pellets anneal and stabilize. Any connoisseur of Utah snow should have graupel in their lexicon of wintry terminology, at the ready to impress any Sun Belt visitor met on the slopes.

This is Linda Kervin for Bridgerland Audubon Society.
Credits:

Photos: Courtesy & Copyright Jim Cane

Text: Jim Cane & Linda Kervin, Bridgerland Audubon Society
Additional Reading:

Riehl, Herbert: Introduction to the Atmosphere, McGraw-Hill Book Company, 1972 http://www.amazon.com/Introduction-Atmosphere-Herbert-Riehl/dp/0070526567

Rime and Graupel http://emu.arsusda.gov/snowsite/rimegraupel/rg.html

American Meteorological Society, Glossary of Meteorology http://amsglossary.allenpress.com/glossary/search?id=graupel1

Graupel – What is Graupel? http://weather.about.com/od/g/g/graupel.htm