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Water molecules
at first attach to the hexagonal faces because the hydrogen
bonding is more accessible there than at the intersection or
edges of the faces. But the edges stick out farther into the
moist air and can claim the water molecules before they reach
the faces, so a dynamic equilibrium is established, particularly
at the temperatures at which Ice Puppy 112 grew, low
humidity favoring slow plate growth and high humidity favoring
fast dendrite, fern shaped growth. At other temperatures, the
available humidity affects ice crystal growth, with low humidity
causing the flat faces of the “c” axes to grow as needles and
columns. The structure of an ice crystal is a record of its growth in well defined temperature and humidity ranges, with the crystal form changing as the temperature and humidity change. (Gosnell, op. cit., pgs 28 and 422) In an article on the electron micrographs of ice crystals of Eric Erbe and William Wergin, Ivan Amato notes that the six armed snow crystals that form from 3ºF to 10ºF are technically named dendrites. (Discover, February 2004, pp 56-61) Kenneth Libbrecht, whose microphotographs of snowflakes recently appeared on the 39 cent US stamp, further defined the humidity regimes that allowed me to start to understand the growth history of the Flat Lake Ice Puppies. From -7ºF to 14ºF, the range within which the Ice Puppies were forming, at less than saturated humidity ice forms first solid plates, then, as humidity increases, thin plates, followed past the saturation point by sectored plates and, finally, dendrites. (Ken Libbrecht’s Guide to Snowflakes, Voyager Press, 2006) It was Erbe and Wergin’s electron micrograph of hexagonal plates attached at odd angles and not the usual 60º angle defined by hexagonal close-packed |
crystal structure that hinted at a
possible mechanism for the complex initiation of Ice Puppy
112. It seems that the explanation of the odd angles is that
hexagonal plates formed independently on birch seed husks, which
had been blown uniformly over the fresh ice surface of the lake.
The birch seed husks had taken the place of Nakaya’s rabbit hair
as nucleation centers, just as minute particulate matter, or
dust, initiate snow flakes.
I had
been thinking that the similarity between the different dendrite
branches, or feathers, of Ice Puppy 112 must mean that
their formation was part of the same crystal structure.
Libbrecht notes that in some cases when an ice crystal is
forming and another is initiated attached to it, the
thermodynamic path of least resistance is for the new crystal to
match the structure of the first. The odd angles of Ice Puppy
112’s feathers, however, can be explained by separate
nucleation events, and the likeness of its feathered dendritic
arms to each other is explained by their each being exposed to
the same fluctuations of temperature and humidity as they grew.
Libbrecht calls these initiating plates radiating plates, and
also has a microphotograph of a spatial dendrite.
The
temperature rose that morning from -5ºF to 10ºF. The increasing
humidity driving the ice crystal formation came from the surface
of the frozen lake as the sun rose, starting with the formation
of separate hexagonal plates from -5ºF to 3ºF, changing to
dendrites as the temperature rose through 3ºF and humidity
increased with its sublime influence.
This
manner of explanation is suggested by Libbrecht, who says there
is often no answer to the question “What kind of snowflake is
that?” because many do not fit into a category. The question to
answer, he says, is “How did that snow flake grow into that
shape?” This, he says, can be answered.
By late
afternoon the Ice Puppies had all evaporated.
For
more on snow crystals, particularly on how to collect and
preserve snowflakes, visit Kenneth Libbrecht’s site:
www.snowcrystals.com.
Further
Excerpts:
In the Dead of Winter
The nocturnal northern flying
squirrel
© 2005 - 2010 Larry Taylor