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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
Snowflake Branching
   ... The origin of the complex structure of snowflakes ...
   One thing you notice right away about snow crystals is that they form some elaborate and complex shapes -- often displaying lacy, branching structures.  Where does this complexity come from?  After all, snow crystals are nothing more than ice which has condensed from water vapor.  How does the simple act of water vapor freezing into ice produce such intricate designs?
mullinsx.jpg (3493 bytes)   The answers to these questions lie in just how water molecules travel through the air to condense onto a growing snow crystal.   The water molecules have to diffuse through the air to reach the crystal, and this diffusion slows their growth.  The farther water molecules have to diffuse through the air, the longer it takes them to reach the growing crystal.
   So consider a flat ice surface that is growing in the air.  If a small bump happens to appear on the surface, then the bump sticks out a bit farther than the rest of the crystal.  This means water molecules from afar can reach the bump a bit quicker than they can reach the rest of the crystal, because they don't have to diffuse quite as far.
   With more water molecules reaching the bump, the bump grows faster.  In a short time, the bump sticks out even farther than it did before, and so it grows even faster.  We call this a branching instability -- small bumps develop into large branches, and bumps on the branches become sidebranches.  Complexity is born.   This instability is a major player in producing the complex shapes of snow crystals.

dendritex.jpg (4903 bytes)   When the branching instability applies itself over and over again to a growing snow crystal, the result is called an ice dendrite.   The word dendrite means "tree-like," and stellar dendrite snow crystals are common (see the Guide to Snowflakes).  
   We can change diffusion in the lab and see how dendrites change.  If one grows snow crystals in air below atmospheric pressure, they have fewer branches.   This is because diffusion doesn't limit the growth so much at lower air pressures, so the branching instability is not so strong.  At higher pressures, more branches appear.

   The growth of snow crystals depends on a balance between faceting (see Crystal Faceting) and branching.  Faceting tends to make simple flat surfaces, while branching tends to make more complex structures.  The interplay between faceting and branching is a delicate one, depending strongly on things like temperature and humidity.  This means snow crystals can grow in many different ways, resulting in the great diversity we see in snow crystal forms.

Return to was created by Kenneth G. Libbrecht, Caltech
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