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: http://www.its.caltech.edu/~atomic/snowcrystals/
. 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 dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
individual use or educational use. Any commercial use without
written permission of the photoprovider is forbidden.
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