Snow crystals form when tiny supercooled cloud droplets
(about 10 μm in diameter) freeze. These droplets are able to remain liquid at
temperatures lower than −18 °C (−0 °F), because to freeze, a few molecules in
the droplet need to get together by chance to form an arrangement similar to
that in an ice lattice, then the droplet freezes around this
"nucleus." Experiments show that this "homogeneous"
nucleation of cloud droplets only occurs at temperatures lower than −35 °C (−31
°F). In warmer clouds an aerosol particle or "ice nucleus" must be
present in (or in contact with) the droplet to act as a nucleus. The particles
that make ice nuclei are very rare compared to nuclei upon which liquid cloud
droplets form, however it is not understood what makes them efficient. Clays,
desert dust and biological particles may be effective, although to what extent
is unclear. Artificial nuclei include particles of silver iodide and dry ice,
and these are used to stimulate precipitation in cloud seeding.
Symmetry
A snowflake often exhibits six-fold radial symmetry. The
initial symmetry can occur because the crystalline structure of ice is
six-fold. The six "arms" of the snowflake, or dendrites, then grow
independently, and each side of each arm grows independently. Most snowflakes
are not completely symmetric. The micro-environment in which the snowflake
grows changes dynamically as the snowflake falls through the cloud, and tiny
changes in temperature and humidity affect the way in which water molecules
attach to the snowflake. Since the micro-environment (and its changes) are very
nearly identical around the snowflake, each arm can grow in nearly the same
way. However, being in the same micro-environment does not guarantee that each
arm grows the same; indeed, for some crystal forms it does not because the
underlying crystal growth mechanism also affects how fast each surface region
of a crystal grow.
Uniqueness
Snowflakes form in a wide variety of intricate shapes,
leading to the popular expression that "no two are alike". Although
statistically possible, it is very unlikely for any two snowflakes to appear
exactly alike. Initial attempts to find identical snowflakes by photographing
thousands of them with a microscope from 1885 onward by Wilson Alwyn Bentley
found the wide variety of snowflakes we know about today.
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