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.
Click
here to see the Physics Photo
of
the Week Archive.
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