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Natural Snowflakes
  --Photo Gallery I
  --Photo Gallery II
  --Photo Gallery III
  --Guide to Snowflakes
  --Snowflake Books
  --Historic Snowflakes
  --Ice Crystal Halos
  --Snowflake Store
Designer Snowflakes
  --I: First Attempts
  --II: Better Snowflakes
  --III: Precision Snow
  --Snowflake Movies
  --Free-falling Snow
  --Designer's Page
Frost Crystals
  --Guide to Frost
  --Frost Photos
Snowflake Physics
  --Snowflake Primer
  --Snow Crystal FAQs
  --No Two Alike?
  --Crystal Faceting
  --Snowflake Branching
  --Electric Growth
  --Ice Properties
  --Myths and Nonsense
Snow Activities
  --Snowflake Watching
  --Photographing Snow
  --Make Your Own
  --Snowflake Fossils
  --Ice Spikes
  --Activities for Kids
Snowflake Touring
  --Snowflake Hot Spots
  --Northern Ontario
  --Hokkaido, Japan (2) (3)
  --Michigan U. P.
  --California Mountains
Copyright Issues
Frequently Asked Questions
   ... Things you always wanted to know about snow crystals ...
Why do snow crystals form in such complex and symmetrical shapes?
   To see why snowflakes look like they do, consider the life history of a single snow crystal, as shown in the diagram at right.  (Click on the picture for a larger view.)
   The story begins up in a cloud, when a minute cloud droplet first freezes into a tiny particle of ice.  As water vapor starts condensing on its surface, the ice particle quickly develops facets, thus becoming a small hexagonal prism.  For a while it keeps this simple faceted shape as it grows.
   As the crystal becomes larger, however, branches begin to sprout from the six corners of the hexagon (this is the third stage in the diagram at right).  Since the atmospheric conditions (e. g. temperature and humidity) are nearly constant across the small crystal, the six budding arms all grow out at roughly the same rate.
   While it grows, the crystal is blown to and fro inside the clouds, so the temperature it sees changes randomly with time.  But the crystal growth depends strongly on temperature (as is seen in the morphology diagram).  Thus the six arms of the snow crystal each change their growth with time.  And because all six arms see the same conditions at the same times, they all grow about the same way.  The end result is a complex, branched structure that is also six-fold symmetric. 
And note also that since snow crystals all follow slightly different paths through the clouds, individual crystals all tend to look different.

   The story is pretty simple, really, nicely encapsulated in the diagram above.  And it's even a bit amazing, when you stop to ponder it -- the whole complex, beautiful, symmetrical structure of a snow crystal simply arises spontaneously, quite literally out of thin air, as it tumbles through the clouds. 

What synchronizes the growth of the six arms?
   Nothing.  The six arms of a snow crystal all grow independently, as described in the previous section.  But since they grow under the same randomly changing conditions, all six end up with similar shapes.
   If you think this is hard to swallow, let me assure you that the vast majority of snow crystals are not very symmetrical.  Don't be fooled by the pictures -- irregular crystals (see the Guide to Snowflakes) are by far the most common type.  If you don't believe me, just take a look for yourself next time it snows.  Near-perfect, symmetrical snow crystals are fun to look at, but they are not common.
Why do snow crystals have six arms?
   The six-fold symmetry of a snow crystal ultimately derives from the hexagonal geometry of the ice crystal lattice.  But the lattice has molecular dimensions, so it's not trivial how this nano-scale symmetry is transferred to the structure of a large snow crystal.
   The way it works is through faceting.  No long-range forces are necessary to form facets; they appear simply because of how the molecules hook up locally in the lattice (see Crystal Faceting for how this works).  From faceting we get hexagonal prisms, which are large structures with six-fold symmetry.  Eventually arms sprout from the corners of a prism, and six corners means six arms.
   Faceting is how the geometry of the water molecule is transferred to the geometry of a large snow crystal.
Why is snow white?
   No, it's not a white dye.  Snow is made of ice crystals, and up close the individual crystals look clear, like glass.  A large pile of snow crystals looks white for the same reason a pile of crushed glass looks white.  Incident light is partially reflected by an ice surface, again just as it is from a glass surface.  When you have a lot of partially reflecting surfaces, which you do in a snow bank, then incident light bounces around and eventually scatters back out.  Since all colors are scattered roughly equally well, the snow bank appears white.
   In fact, the ice does absorbs some light while it's bouncing around, and red light is absorbed more readily than blue light.  Thus, if you look inside a snow bank you can sometimes see a blue color.  I took a few pictures of this once in the California mountains.
Is it ever too cold to snow?
   In principle it can snow at any temperature below freezing.  It snows at the South Pole even though the temperature is rarely above -40 C (-40 F). 
   In more hospitable climates, however, it doesn't snow so much when the temperature is below around -20 C (-4 F).  When a parcel of moist air cools, it starts producing snow before it gets that cold.  By the time the temperature drops to -20 C, the snow has already fallen and the air is pretty dry.  The clouds that remain are made of ice crystals, and these don't produce much snow (see the Snowflake Primer for how clouds make snow).
Why study the physics of snowflakes?
There are several good reasons for studying how snowflakes form.
   First of all, crystals are useful in all sorts of applications, and we would like to know how to grow them better. Computers are carved out of silicon wafers, which in turn are cut from large silicon crystals.  Many other semiconductor crystals are used for other electronics applications.  Lasers are also made from crystals, and a variety of optical crystals are used extensively in telecommunications. Artificial diamond crystals are used in machining and grinding.  The list of industrial crystals is actually quite long.
   By studying the physics of snowflakes, we learn about how molecules condense to form crystals.  This basic knowledge applies to other materials as well.  As we learn more about the physics and chemistry of how crystals grow, maybe someday we can use that knowledge to help fabricate new and better types of crystalline materials.
   This is the way that basic science becomes useful -- figure out how things work the best you can, and later on use that knowledge in unforeseen applications. 

   Another good reason to study snowflakes is to better understand structure formation and self-assembly. Humans usually make a thing by starting with a block of material and carving from it.  Computers, for example, are made by patterning intricate circuits on silicon wafers.
   Nature uses a completely different approach to manufacturing. In nature, things simply assemble themselves. Cells grow and divide, forming complex organisms. Even extremely sophisticated computers (such as your brain) arise from self-assembly.  Your DNA does not contain nearly enough information to guide the placement of every cell in your body.  Most of that structure simply arises spontaneously as you grow, following poorly understood rules.  Biological self-assembly is an extremely complex process, and we do not understand much about how it works at a fundamental level.
   The snowflake is an very simple example of self-assembly. There is no blueprint or genetic code that guides the growth of a snowflake, yet marvelously complex structures appear, quite literally out of thin air. As we understand better how snowflakes form, we learn about self-assembly.  As the electronics industry pushes toward ever smaller devices, it is likely that self-assembly will play an increasingly important role in manufacturing.  Learning about self-assembly from the ground up will probably by useful in this context also.  Again, in the study of basic science we try to solve the easy problems first (like snowflakes), and later use that knowledge to develop engineering applications we cannot yet foresee.

   History has shown over and over that the fundamental knowledge gained by doing basic science (without worrying about what it's good for) often leads to useful engineering applications.  There is a great deal of interesting physics, chemistry, and materials science wrapped up in snowflake growth, and studying the lowly snowflake may indeed teach us something useful.

   Now, all that being said, my personal motivation is not from potential practical applications.  I am not trying to make better artificial snow, better ice for Olympic skating, bigger diamonds, faster computers, or anything like that.  I believe that basic science can and should be pursued for its own sake.  Scientists try to understand everything they can about how nature works, on the premise that all knowledge is potentially useful.  Einstein didn't worry about the practical applications of relativity -- he just wanted to understand how nature worked.  Maxwell didn't think about cell phone technology when he worked out the laws of electromagnetism -- he just wanted to understand how nature worked.
   I want to figure out the underlying physics of snowflake growth because this is an interesting puzzle in molecular dynamics.  I would like to understand the fundamental physics of how molecules jostle into place to form a crystal.  How fast does this happen?  How does it change with temperature? What happens if there are chemical impurities on the ice surface?  There are many such questions, and ice is an interesting case study in crystal growth.  These remarkable structures simply fall from the sky -- we ought to understand how they are formed!  With over six billion people on the planet, surely a few of us can be spared to ponder the subtle mysteries of snowflakes. 

Who else is working on the science of snow crystals?
   Not many people are thinking about why snow crystals look like they do, and most of them are meteorologists looking at how snow crystal formation affects the properties of cold clouds.  Here is a partial list (in no particular order), along with contact information (in the links):
Charles Knight, National Center for Atmospheric Research
Dennis Lamb, Jerry Harrington, Penn State
Brian Swanson, Marcia Baker, University of Washington
John Hallett, Desert Research Institute
Raymond Shaw, Michigan Technological University
Norihiko Fukuta, University of Utah
Yoshinori Furukawa, Hokkaido University (Japan)
John Wettlaufer, Yale University
   I have also been collaborating with David Griffeath and Janko Gravner, who are mathematicians working on cellular automata methods for computer modeling of snow crystal formation.
   My scientific focus is somewhat different from the meteorologists in that I am not trying to connect snow crystal formation to clouds or climate issues.  Rather, I am a physicist looking at the statistical mechanics and molecular dynamics of snow crystal growth.  I am tackling the problem by making precision measurements of crystal growth rates, by comparing measurements with dynamical theories of crystal growth, and by examining the mathematics of pattern formation and growth instabilities that occur during solidification.

Return to was created by Kenneth G. Libbrecht, Caltech
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