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Nature Baroque
Snowflakes & Crystals
by Jerry Dennis
drawings by Glenn Wolff

Jerry Dennis's work can be found in other magazines including the Smithsonian and Outdoor Life. He is the author of A Place on the Water as well. Glenn Wolff's illustrations appear regularly in the New York Times and many other publications.This selection appeared in their 1993 book, It's Raining Frogs and Fishes, which offers answers to the questions countless children (and adults) ask of the world around them. The pair recently released the follow-up book, The Bird in the Waterfall: A Natural History of Oceans, Rivers and Lakes.

Child & Snowflakes
What child can resist tasting snowflakes? Head back, mouth open, tounge stretched to catch that bit of coldness--for children, snowflakes on the tongue are as essential to a northern winter as sleds and snowmen. We take it for granted that snow will fall, that every flake, its constituent crystals arranged in elaborate symmetry, is a wonderous and unique creation. But is it? And how are they formed? What chemistry is at work in those dense, dark clouds of winter?

There is more to the birth of a snowflake than Aristotle's assertation that "when a cloud freezes there is snow." Snow is not merely frozen rain. Rain occasionally freezes, falling to the ground as sleet or freezing rain, but snow originates independent of atomospheric drops of water. Individual ice crystals for high in the atmosphere when water vapor freezes around dust or other particulates. Without particles to serve as condensation nuclei, water vapor can be cooled to -40 degrees Farenheit before freezing occurs. A supercooled cloud of this sort seeded with a few particles often escalates into a snowstorm. The individual crystals collect additional molecules of water vapor one at a time, building on one another symmetrically in a rapidly growing, widening circle. Temperature, wind, humidity, and even barometric pressure will determine the growth and ultimate form of the crystal. Large and elaborate crystals for at higher temperatures and humidity while, while the small, basic crystals such as thos common in polar regions form when temperature and humidity are very low. As the crystals fall they bump against each other, breaking off pieces of ice that in turn serve as nuclei for new crystals. As they pass through warmer layers of air they adhere to one another, congregating into snowflakes that may contain a thousand or more crystals.

Snowflakes, then, are aggregates of snow crystals. When the temperature is near or slightly above freezing, snowflakes become wet, adhere to other flakes, and grow to two or three inches in diameter. On very rare occasions, they can grow larger yet. According to a report in a 1915 issue of Monthly Weather Review, a snowfall on January 28, 1887 dropped flakes "larger than milk pans," measuring fifteen inches in diameter by eight inches thick across several square miles near Fort Keogh, Montana.

Only when the temperature remains consistently below freezing will complete, individual crystals fall to the ground. If the temperature of the cloud they form in and the air they descend through is warmer than 27 degrees Farenheit, the crystals tend to be flat and hexagonal. Between 27 and 23 degrees, they tend to be needle-shaped. Between 23 and 18 they are likely to be hollow and columnar, with prismatic sides. At temperatures below 18 they can be columnar, hexagonal, or fernlike. Virtually all have six sides. That hexagonal tendency is something of a mystery, although some scientists suggest it is produced by electrical charges in the crystals, while others say it is basic to the molecular structure of water molecules. The atoms in a molecule of H20 are arranged, in physicist Hans C. von Baeyer's graphic description, "with two little hydrogens stuck onto a big oxygen like ears on a Mickey Mouse's head." Scientists like von Baeyer believe that the angle at which the hydrogen molecules protrude from the oxygen atom--about 120 degrees--causes snow crystals to grow to a six-pointed symmetry that repeats the molecular structure of water.
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The scientifically untestable notion that no two snow crystals are alike is probably true. There are too many variables involved: Did the crystal for around a nucleus of volcanic ash, or a bit of sea salt, or a fleck of industrial waste? At what altitude did it form? What temperature was the air it passed through? How much humidity did it contain? For two crystals to be alike, they would have to form in exactly the same conditions, collect the same number of molecules of water vapor in the same order and bump into the same number of other crystals during their long descent to the ground.

No two snow crystals may be identical, but general categories or types have been identified for years. Beginning in the 1880s, a Vermont farmer and amateur photographer named Wilson Bentley began examining ice crystals and photographing them under magnification. Armed with apparently unlimited patience, a microscope, and a box camera, Bentley produced over 6,000 photographs, 2,000 of which appeared in his 1931 book Snow Crystals, and identified hundreds of types of crystals.

Serious study of snow crystals was performed in 1910 by a Russian meteorologist who identified 246 types in 176 days of observation. In the 1930s Japanese meteorologist Ukichiro Nakaya consolidated the list to seventy-nine categories of crystals plus anomalies and oddballs he called "mavericks."

In 1951 the International Commission on Snow and Ice simplified things immensely by devising a classification system recognizing seven basic forms of snow crystals: plate crystal, stellar crystal, column, needle, spatial dendrite, capped column, and irregular crystals.

Stellar crystals (or simply "stars") are the classic, most familiar form of snow crystals, and the basis of the "no two alike" myth. They are not as common as aggregate flakes, irregular crystals, or asymmetrical crystals but we are familiar with them because of countless artists' renditions.

In addition to the basic crystals, snow can form into ice pellets when it is buffeted by strong winds that break off the points of the crystals and pack them into tiny balls. Graupel is formed by crystals falling through layers of supercooled droplets of water vapor which remain liquid as long as they are suspended in the air, but freeze the instant they come in contact with anything solid and coat it in a dense covering of rime frost.

In places where snow is a frequent companion, it is sometimes personified as a living thing. In Japanese folktales it is Yuki-onne, the Snow Woman, who appears before men wandering in snowstorms and lures them to sleep and death. In Nordic mythology snow is the Old Man, an eged king of Finland named Snaer, whose daughters are Thick Snow, Thin Snow, and Snowstorm. To the Inuits of the far north snow appears in so many forms and shapes it requires an advanced vocabulary to describe it. To them, api is snow not yet touched by wind; upsik is snow changed by wind into a firm mass; siqoq is smoky snow blowing along the surface of the ground; annui is falling snow; quali is snow that sticks to the branches of trees; saluma roaq is a snow surface of very smooth and fine particles; natatgonaq is a snow surface of rough and coarse particles; and det-thlok is snow so deep snowshoes are required to walk in it. Dozens of variations--as many as 200, by some accounts--make it possible for Inuits and Eskimos to speak more precisely about snow than anyone on earth.

The winter vocabulary of the English language is growing. We have adopted the Russian word sastrugi to describe windblown drifts, common in the Artic and Antarctica, that look like waves of water. Cross-country and alpine skiers have adapted a litany of descriptive slang expressions to identify the conditions they encounter on their skis. Among them are such colorful terms as windslab, glop, fluff, neve, breakable crust, crud, sugar, corn, boilerplate, and cement.

Explorers in Antarctica found to their dismay that in extremely cold temperatures (-50 is not uncommon) snow can become unskiable. At those temperatures, the tiny ice crystals that fall almost continuously, even from clear skies, create a dry, harsh surface more like sand than snow. Skis and sled runners, instead of melting the points of the crystals to make them slippery, merely roll the crystals over and over.

Snow changes continuously as it falls and after it has landed. Once on the ground, snowflakes trap tiny air pockets and form an excellent natural insulation. Temperatures on the surface can be more than 50 degrees colder that temperatures beneath seven inches of loose, fresh snow. As the snow settles is metamorphoses. "Old snow" is settled and dense, resulting from the altering of loose, pointed crystals into small, round grains. Later it becomes firn, with spaces between the grains shrinking, resulting in compacting and hardening of snow. If metamorphosis continues long enough, firn can become glacial ice.

During any ordinary snowfall in New England or North Dakota or British Columbia or Siberia or Finland, about 1 million crystals of snow fall to cover each two-foot square area with ten inches of snow. Snow covers about half the land on the earth's surface, at least for part of each year, as well as about ten percent of the oceans. About 48 million square miles of the earth are covered year round with snow or ice.

The greatest snowfall in a twenty-four-hour period recorded in North America occurred on April 14 and 15, 1921, when seventy-six inches of snow fell on Silver Lake, Colorado. More recently, on April 5 and 6, 1969, Bessans, France, was buried beneath sixty-eight inches of snow in nineteen hours. During a snowstorm from February 13 to 19, 1959, 189 inches fell on the Mount Shasta Ski Bowl in northern California. The snowiest place on record in North America is Ranier Paradise Ranger Station in Washington, where in the winter of 1971-72 a total of 1,122 inches of snow fell. The greatest depth of snow ever measured on the ground at one time in North America was 451 inches--over 37 1/2 feet--at Tamarack, California on March 11, 1911. Oddly enough, the interior of Antarctica recieves very little snow. Most of the precipitation at that coldest spot on earth falls in the form of ice crystals, with an annual precipitation equal to less than two inches of water--only slightly more than falls on the Sahara Desert each year. The vast ice cap at the center of the continent grows, but only slowly, over millions of years.
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"As pure as the snow," may not be the purest of metaphors. Snow, it seems, contains much more than just frozen moisture and air. In fact it contains enough nitrates, calcium, sulphate, and potassium picked up from dust and atmospheric gases to make it an important source of agricultural nutrients in many parts of the world. It also contains less savory traces of industrial pollution. When snow crystals form in air contaminated with sulphur dioxide the result is acid snow, which accumulates on the ground in winter and releases highly acidic meltwater into rivers and lakes in the spring.

The best time to catch and observe snow crystals is when the temperature is moderately cold (about 25 degrees Farenheit is ideal), with no wind to throw the flakes against each other and break their points. The crystals fall individually, or more often, sticking together in loose clusters that fall apart into separate crystals when they land. Wear a dark jacket or carry a piece of dark-colored fabric stretched over cardboard, and after it has been acclimated to the outside temperature, it will preserve even the finest, most delicate crystals until you have had time to examine them.

Most crystals are an eighth of an inch or less in diameter, much smaller than we are led to expect from the representation on Christmas card. Mixed with those eighth inchers are occasional midgets hardly larger than the dot on this letter i. Occasionally comes a behemoth, perfectly symmetrical and ornate as baroque jewelry, measuring as big around as a pencil eraser. Those giants spiral slowly downward, their flat surfaces horizontal, and are especially satisfying to catch on sleeve or tongue.

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