Physics Photo of the Week

January 22, 2010

Nature's Symmetry - Snow Stars
We had plenty of these during the recent winter break in at Warren Wilson College.  The cold weather sometimes brings "sprinkles" of single snowflakes to the cold surfaces.  These snow stars range in size between 3 and 8 mm across.  The most elaborate stars are the largest. 

Scientists have long wondered what makes the nearly perfect symmetry in snow stars - especially why each main stem resembles all five other stems - complete with the branches upon branches.  Cal Tech Professor Kenneth Libbrecht
has studied snowflakes extensively and has posted very interesting discussions at his web site: .  He even  has amazing time-lapse movies of snow flakes growing in special chambers.

The 6-fold symmetry of snow flakes arises from the hexagonal crystal structure of ice.  Each snow star begins as a tiny (about 1 x 10-6 m diameter) hexagonal prism.  The tiny crystal begins to form in a cold cloud of saturated or supersaturated water vapor.  The vapor is below the freezing point, so that the vapor can sublimate directly as a solid as it freezes out of the vapor - without going through the liquid state.  In the presence of the saturated water vapor the vapor condenses preferably on the corner edges of the hexagons.  Because the crystals are so small, each edge "sprouts" a needle at the same rate.  When the needle points become too long, they sprout side branches.  Because of the locally uniform enviroment in temperature and humidity, the side branches all sprout at the same time and position, so all branches grow in an identical fashion.  All snowflakes are different - as far as we know.  Each snowflake experiences slightly different variations in temperature and humidity for which the dendrite growth is extremely sensitive.  Since no two snowflakes arise from idenical temperature and humidity conditions, no two snowflakes are alike.

Watch for more physics photos of winter phenomena in the near future!

Photographic notes:  These photos were made with a digital camera (Canon XSi) at high resolution, with the stock zoom lens set at 18 mm focal length.  The camera was held as close as possible to the snow stars to still remain in focus.  The images were then highly cropped around single snow stars.  Close-up lens attachments or a special macro lens
would produce superior images.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature 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 

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