Physics Photo of the Week

April 13, 2007

Snow Stars
I took these close-up photos of snow crystals on March 4, 2007.  The weather obviously was cold and the snow was highly scattered - excellent for getting isolated snowflakes to examine their crystal shapes.  I was hesitant to feature snow after the warm weather of March and early April, but last weekend's devasting freeze and several inches of snow reminded us that winter still lingers - even when dogwood bloom - and can return with a vengeance.

The amazing feature of individual snow flakes or "snow stars" is the six-fold symmetry throughout the crystal.  How are the intricate details of the branches replicated on all six major branches of the snow stars?  This question has entranced physicists and naturalists for several hundred years.  The best answer comes from and article by Kenneth G. Libbrecht (American Scientist 95, pp 52-59 (January-February 2007)).  All snow crystals begin as a microscopic "seed" - a hexagonal plate about 10-3 mm across.  In the environment
where ice crystals grow directly from the water vapor in the air around it - a process called sublimation - the water vapor will more likely condense or subliminate on the corners of the crystal more quickly than on the face of a crystal.  Thus the crystals will grow more quickly from a corner.  Because all six corners of the crystal experience the same concentration of water vapor, all six corners will grow at the same rate.  As the corner grows faster than the faces of the hexagon, the corners sprout into the needles that form the points of the stars.  As the points protrude further from the seed, they receive more water vapor, and thus grow faster.  Thus we can explain why snow flakes tend to grow as symmetrical stars. 

There is more to it however.  Notice that each main arm of the star has sub branches and these patterns are almost identical among all six main branches.  Libbrecht attributes this to the large dependence of growth morphology with temperature and humidity.  Suppose the snow star reaches a region of temperature and humidity where the growth favors the needles to branch.  All six of the branches experience the same humidity and conditions and thus will branch at the same time - thus preserving and enhancing the six-fold symmetry.

Notice that free-falling snow flakes exhibit the full six-fold symmetry whereas frost crystals show needles or plates, but not the star-like forms.  See PPOW for March 2, 2007.  Frost crystals form by precipitating on imperfections and protrusions on an inanimate surface.  The frost bonding surface prevents the full six-fold symmetry for "frost flakes". 



Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu.


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