[ THE OUTSIDE STORY ]
Catch a Falling Snowflake
Who hasn’t marveled at a lacy snowflake coming to rest on a jacket sleeve?
Do you wonder how it could survive the fall to earth in one piece, or if it’s
really true that no two snowflakes can look exactly alike?
A snowflake begins high up in the clouds, not as a snowflake but as a
small particle of dust, salt, or ash. When a cloud cools below 32 degrees,
some specks of water vapor freeze onto the particle. As it moves through the
cloud, the particle absorbs additional water vapor, building up microscopic
layers of ice. When water molecules freeze, they bond together in a way that
forms a six-sided ice crystal.
Eventually, the ice crystal becomes too heavy to stay aloft in the cloud,
and it begins the descent to earth, usually with many others like itself. This
is the precipitation we call snow. A snowflake can be a single ice crystal,
but it’s usually a cluster of crystals, often 200 or more, all symmetrically
connected. The common feature of all ice crystals, and thus all snowflakes, is
the hexagonal structure of branches, short stub-like arms, or sides.
Snow crystals are categorized by their general shape. There is the familiar
intricate, star-shaped snowflake, called a stellar dendrite. Spatial dendrites
also have branches, but they are more untidy and dissimilar. Plates have
just the suggestion of arms growing at the six corners of the hexagon. Other
common types are: needles, columns (which look like short, unsharpened
pencils), and capped columns (which look like columns with graduation
caps attached to the ends). Eighty snow crystal varieties have been formally
identified, catalogued, and labelled by scientists who study the world of
winter. Two books with exquisite photographs are The Snowflake: Winter’s
Secret Beauty (Libbrecht and Rasmussen) and Snow Crystals (Bentley and
Different cloud temperatures and humidity levels determine the snow
crystal’s initial size and shape. Thin, plate-like crystals grow when temperatures
are close to the freezing point. At cooler temperatures, needles, columns,
and dendrites appear. Additionally, higher humidity produces more intricate
designs, while simpler forms of plates and columns occur in drier air.
Finding a perfectly intact snowflake is a treat. In a 30-minute free-fall,
snow crystals are tossed about by the wind, encounter different temperatures
and levels of humidity, and get in each other’s way. Pieces may break off or
melt, or crystals that collide might stick together. So, can two snowflakes be
exactly alike? Only if they attract water vapor in a cloud in the exactly same
way, fall through exactly the same temperature and moisture conditions, and
swirl in identical aerial dances. Perhaps a statistician could give the odds, but
I rather enjoy conducting my own research during a snowstorm.
It is easy to collect snowflakes for your own enjoyment and study. Keep
a sheet of stiff black paper in the freezer, ready to grab when flakes start
falling. Stand out of the wind and let snowflakes land on the paper. Then
retreat to a protected area that is cold, such as a garage or covered deck.
Use a magnifying glass to study the individual flakes. The hardest part is not
breathing warm air on them!
You can also preserve snowflakes on a glass microscope slide. Keep the
slide in the freezer and a can of clear acrylic spray in the refrigerator. When
you are ready to go outside, put on gloves (you do not want the slide to warm
up in your hands), spray the acrylic onto the slide, and dash outside. You can
hold the slide in your outstretched hand, or place it on a level spot. Let a few
flakes fall onto it, then place it in a protected spot outside for a few hours.
The snowflakes will freeze into the plastic spray, which also freezes, creating
a permanent impression of your snowflakes. Under the microscope you will
be able to see tiny parts of snowflakes that you missed with your own eye.
The Outside Story is sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: firstname.lastname@example.org