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Snowflakes: Up Close and Personal
by Kenneth Libbrecht

Have you ever stopped to look closely at the snowflakes that fall on your sleeve during a cold, calm snowfall? If not, then you’ve been missing something quite extraordinary. A miniature work of art, each snowflake is carefully crafted by the clouds into its own unique, symmetrical pattern. Best of all, by the millions these diminutive ice sculptures fall from the sky, littering the landscape, waiting to be admired.

Because snowflakes are small, the eye alone cannot appreciate their full character. Magnification details their form and structure. Whenever I go skiing, I always carry a magnifying glass hoping for an opportunity to do some good snowflake watching. Or, if I forget to pack one in a pocket, I always keep another in the glove compartment of my car. A cold windshield makes a wonderful snowflake observatory; it has a comfortable slope at just the right height, and the smooth surface makes it easy to brush the snow aside to make room for new flakes.

When packing a magnifier, you don’t need the bulky Sherlock Holmes variety; go instead with a small, fold-up model you can find at most drugstores and hardware stores. If I really want to travel light, I cut off the mounting, so I’m left with just a plain glass lens, which is thin and nicely slides into any pocket. Although we can’t predict when a dazzling display of snowflakes will fall, we can easy to be prepared for it.

Depending mostly on temperature, snowflakes create a wonderful variety of shapes and patterns, with different types appearing in different snowfalls. On cold days, with the thermometer reading close to zero, small, flat symmetrical snow stars with well-defined crystal facets look like tiny ice mirrors as they catch the light. Cold-weather snowflakes can be quite photogenic, since they have well-formed, symmetrical shapes, and they don’t melt away so quickly when you put them under the microscope. Canonical snowflakes, cold-weather flakes make excellent photographic subjects and are copied in paper cut-outs and winter holiday decorations.

Most snowflakes of this type are six-fold symmetric to some degree, reflecting the underlying symmetry of the crystalline structure of ice. Occasionally, a rare 12-branched snowflake results when two six-branched crystals form and grow together. Nature will not produce eight-fold symmetry, or likewise four-, five-, or seven-fold symmetry. Water molecules just don’t hook up that way.

Because snowflake’s intricate structures quickly evaporate away on the ground, it’s best to observe them right after they fall. The sharp edges and corners slowly deteriorate, leaving a snow bank made of little blocks of ice, without the original elaborate patterns--temporary works of art.

When the temperature drops to 10 degrees, look for large, leafy stellar dendrites floating gently to earth. The branches of these crystals contain many side branches, giving them a delicate, fern-like appearance. A single crystal of ice in spite of its complex shape, a stellar dendrite lines up water molecules from one end to the other. Branches and side branches orient with 60-degree angles relative to one another, even on opposite sides of the flake, a structure that reflects the precise ordering of the water molecules in the crystal. The complex geometry of a snow crystal still reflects the simple composition of its molecules.

With high humidity and low clouds, stellar dendrites can grow quite large. One Christmas Eve in Canada, I saw some monster stellar dendrite crystals nearly a half-inch across. Individual snow crystals—complex structures that are still single ice crystals—rarely get to this size in nature. Because they become heavier as they grow, after 10 – 20 minutes they drop out of their cloudy nurseries and cease growing larger.

Snow crystals frequently collide and stick together in mid-air as they fall, forming puff balls that seem to explode when they hit clothing. Made of thousands of linked snow crystals, these snowy puff balls are also called snowflakes. The leafy stellar dendrites make the lightest puff balls and hence the fluffiest snow when they hit the ground, since the arms of the crystals keep the snow loosely packed. On colder days, pick up a handful of freshly fallen snow to spy the interlocking dendrites before they lose too much of their shape.

In warmer snowfalls, with temperatures reaching up into the 20s, snowflakes change their appearance completely. Instead of the thin, plate-like stellar crystals found at lower temperatures, high-temperature crystals appear blocky—small hexagonal columns of various lengths, something like the shape of a wooden pencil, often with hollow ends. In just the right conditions, copious extremely thin columns look like ice needles. Although quite abundant, columnar and needle-like snow crystals lack the photogenic popularity of their plate-like cousins. Mall decorations don’t feature columnar crystals. Sometimes nature mixes up a bit. Quite commonly, a snow crystal can start out growing like a column, but decide later in its life to grow like a plate. The end result is a column with a plate on each end, called a capped column. Snowfall producing weather fronts often push air to higher altitudes and cool it steadily with time, which is just the condition needed to make capped columns.

Remarkably, stellar snow crystals can have extremely complex symmetrical shapes. Some are almost plant-like in appearance, like little flowers, although there is certainly no genetic code that guides its development. It’s just made of ice, after all.

So how do the six branches of a stellar snow crystal all manage to grow in the same intricate pattern? How does one arm know what the others are doing? Snow crystal growth is exceedingly sensitive to the air temperature and humidity. Imagine the life of an individual snow crystal. It begins when a tiny cloud droplet, one of a vast number in a typical cloud, freezes into a small particle of ice. The ice immediately begins to grow by condensing water vapor directly from the air around it. In a short time, facets form, and the crystal may grow into a small, simple hexagonal plate.

As the crystal grows, it blows to and fro inside the cloud, where it experiences regions with varying temperatures and humidities. At some point during its travels, conditions may prompt the plate to sprout arms from its corners. Because each corner sees the same conditions, each corner shoots an arm up at the same time. As the arms grow, their shape varies with cold and humidity, developing a complex pattern that reflects growth conditions. And since each arm sees the same conditions at the same time, each arm grows identical. The result is a snowflake that is both complex and symmetrical.

Of course, nature doesn’t always work that way. Most snow crystals do not form with six beautifully symmetrical arms. If you watch snowflakes, you’ll soon discover many lopsided, asymmetrical shapes. Frequently, an entire snowfall brings nothing but formless ice globs. The really stunning crystals are rare.

Because each growing crystal follows its own path through the clouds, an endless variety of unique snow crystal patterns arises. Different paths give different snowflake patterns, but all are variations on the same theme--ice crystals growing from water vapor in the air.

So is it really true that no two snowflakes are alike? Certainly the large, ornate stellar crystals are so complex in shape that there is essentially no chance for twins. In this sense snowflakes are as unique as fingerprints; random complex patterns of any kind never repeat exactly.

But don’t take my word for it: get a magnifying glass, go out in the snow, and you be the judge. Watch snowflakes; you will soon catch sight of your own stellar dendrites, capped columns, 12-sided snowflakes, or specimens even more exotic. You never know what you might find floating down from the clouds.

For More Information: Visit www.snowcrystals.com on the web, or you can read the whole snowflake story in “The Snowflake: Winter’s Secret Beauty,” by Kenneth Libbrecht and Patricia Rasmussen, which includes nearly two hundred snowflake photographs.

Photos by Patricia Rasmussen and Kenneth Libbrecht



© Cross Country Skier: November 2004, Vol. 24 Issue 1


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