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A Closer Look: Why Do Snowflakes Have Six Sides?
How does snow form?
When temperature and humidity conditions are conducive,
individual water molecules from small droplets of liquid water in the
atmosphere, particularly
in clouds, can condense slowly to form solid water or ice.
What is
the shape of a water molecule?
A snowflake is built up molecule by molecule.
Each time a growing snowflake moves past water droplets, several molecules
of water are added to
it. As we mentioned in the video for Session 6, in order to explain
this behavior, we must refine our model of the water “particle” from
a simple sphere to a small “V” shape, as shown in the
following picture:
The central atom is an oxygen atom, which has hydrogen
atoms bound to it on either side. The angle between the two arms of
the molecule
is
104.5
degrees, first determined around 1930 by x-ray
diffraction techniques.
How does
this lead to six-sided snowflakes?
The oxygen atom has a particularly
strong attraction to the electron clouds of the two hydrogen atoms and
pulls them closer. This
leaves the two hydrogen
ends more positively charged, and the center of the “V” more
negatively charged. When other water molecules “brush
up” against
this growing snowflake, strong forces between the negatively
charged and positively charged parts of different particles
cause them to join together
in a very specific three-dimensional pattern with a six-sided
symmetry. Each water molecule that joins the snowflake
reflects this pattern until
eventually we can see its macroscopic six-sided shape.
If
snowflakes are built from a particular pattern, why aren’t
all snowflakes identical?
As a snowflake moves up and down in the atmosphere, slight
changes in temperature and humidity cause the exact pattern
to change
as it is built.
The overall
shape of each flake, however, remains six-sided.
Several snowflake formations.
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