Snowflakes
You’ve probably
heard that no two snowflakes are alike. Of course, nobody has ever confirmed
that statement by examining every one of the estimated one septillion snowflakes
that drift to Earth each year. Still, Kenneth Libbrecht, a professor at the
California Institute of Technology, is confident that the statement is
true.
Snowflakes aren’t flaky, says Libbrecht. At their basic
level, they’re crystalline. The lattice of every snowflake is six-sided in
shape. The simplest snow crystals are six-sided flat plates and six-sided
columns. Such crystals are common in places where the air is extremely cold and
dry. Snow crystals acquire their special beauty when their simple six-sided
symmetry blossoms. Under the right conditions, each of the six comers of a
crystal sprouts what is called an arm. In a matter of minutes, the arms can
become highly ornate and give the crystal a star like appearance.
Several factors in the environment affect the shape and growth rate of a
snow crystal. One factor is humidity. Crystals grow faster and in more intricate
shape as humidity increases. A second factor is air temperature. A snowflake is
born when several molecules of water vapor in a cloud land on a speck of dust
and freeze to form a simple crystal. As the young crystal bops around in the
cloud, it passes through air pockets of varying temperatures. If the crystal
passes through a pocket of air that is, say, -15 degrees Celsius, it will grow
quickly and sprout six arms, says Libbrecht. If the crystal is then tossed into
a warmer pocket, one about -10℃, the arms’ tips will stop growing quickly and
form six-sided plates. If the crystal then drifts into an even warmer pocket of
about -5℃, its top and bottom will grow more quickly than its sides and become
more column like in shape.
In the course of its life span, a
snow-crystal might flutter through many warmer and colder pockets, acquiring a
complicated and unique growth history. Such a history will give rise to a
snowflake that is unlike any other. Each arm on the snowflake will look exactly
like every other one, but the crystal itself will be one of a kind.
Using his cooling tanks, Libbrecht has learned how to create snow crystals
of different shapes -- plates, columns, needles, etc. Libbrecht has even refined
his techniques so that he can make crystals that look highly similar to one
another. Still, he lacks the control to manufacture identical twin snowflakes. A
slight difference in humidity and temperature can upset the growth profile of a
crystal. |
