Each feathery flake that falls from the sky is unique, thanks to a variety of conditions.
Window On Nature
By Lowell & Kaye Christie, F47246
Love it or hate it, during the winter season, good portions of the United States and Canada are covered with a blanket of white. In the Northern Hemisphere, about half of the land area sees some snowfall each year, and every state has experienced at least an occasional dusting of the white powder.
Although it may be difficult to appreciate the concept while you’re at the operating end of a snow shovel, ice crystals and the snowflakes they form are things of beauty, often perfectly symmetrical “” sometimes simple, other times wonderfully complex. An ice crystal, as you might expect, is a single crystal made of ice. When two or more ice crystals stick together, a snowflake is born. (Some definitions consider a single large crystal a snowflake.)
Ice crystals develop around tiny dust particles in clouds. But unlike raindrops, they bypass the liquid stage. Water vapor, through a process called sublimation, passes directly from the vapor state to a crystal form, taking on the hexagonal (six-sided) shape determined by its atomic structure. As the ice crystal grows, this original shape partially controls the final result.
The original crystal becomes larger as nearby water vapor condenses and freezes on its surface. The six-pointed ends of the hexagon grow faster than the flat surfaces, often creating a star-shaped object with six almost identical projections or arms.
Understanding why it is highly unlikely there will ever be two identical snowflakes also explains why the six points on a single ice crystal usually look the same. (Those who say identical snowflakes have been found are talking about single ice crystals in the early stages of snowflake development.)
The structure of an ice crystal (and, therefore, of a snowflake) is determined by its hexagonal shape, along with the temperature and humidity. At a temperature of approximately 28 degrees Fahrenheit, ice crystals form as thin plates and stars. As the temperature nears 23 degrees, columns and slender needle shapes appear. Continuing down to 5 degrees, plates and stars reappear, and if the temperature drops to minus-22 degrees, the result is a combination of plates and columns. If a developing ice crystal moves from one temperature zone to another, any newly added condensation will give it different characteristics.
Varying humidity also results in changes to the ice crystal. Crystals form simpler shapes at relatively low humidity and become more complex as the humidity increases. Combining these two variables creates the potential for an unlimited number of shapes. They range from simple prisms to hollow columns, capped bullets, 12-branched stars, arrowhead twins, and fernlike stellar dendrites. One classification scheme lists 80 different snow crystal types.
Clouds have varying areas of humidity and temperature, the components that have the greatest influence over the final shape of an ice crystal. As ice crystals develop inside these clouds, local turbulence constantly moves the particles around as they increase in size. Each growing crystal will be subjected to an always-changing, never-repeating environment that is different for each new crystal. That means each snowflake will have a unique structure, depending upon where it was located when each addition of water vapor occurred.
Although each ice crystal grows in a different complex set of circumstances, the six developing points of any single hexagon move together through the cloud, experiencing the same environment at the same time. That causes them to develop in a similar manner, usually into symmetrical shapes. The result is symmetrical beauty in unlimited variety.
For centuries people have been fascinated by the mystery of snowflakes. In 1611, Johannes Kepler (who developed the laws of planetary motion) wrote the first scientific treatise on snowflakes, called “On the Six-Cornered Snowflake,” where he questioned the reasons for their symmetry. In 1635 René Descartes wrote this detailed description of snowflakes: “These were little plates of ice, very flat, very polished, very transparent, about the thickness of a sheet of rather thick paper . . . but so perfectly formed in hexagons, and of which the six sides were so straight, and the six angles so equal, that it is impossible for men to make anything so exact.”
But the ultimate observer of snowflakes had to be Wilson “Snowflake” Bentley, a Vermont farmer who, upon seeing a snowflake through a microscope at age 11, began a lifelong exploration of these winter visitors. Starting in 1885 at the age of 20, he began photographing snowflakes through a microscope, continuing until his death in 1931. Bentley captured and photographed more than 5,000 snowflakes, and shortly before his death he published a book (still in print) that contains more than 2,400 of these images. You can see some of his black-and-white pictures online at www.snowflakebentley.com.
Mr. Bentley was very selective in choosing snowflakes, since very few reach the ground in good condition. Battered by the wind, bouncing off each other, perfect snowflakes are hard to find. Working in an unheated shed on his farm during local snowstorms, he would step out the door holding a black-painted board on which to catch the falling flakes. Using a feather, he brushed off the many damaged ones, looking for the specimen he was willing to transfer to a microscope slide.
Most of us don’t have the patience or the photographic equipment required to make a serious hobby of chasing snowflakes, but it can be almost as much fun taking a walk in lightly falling snow while wearing a dark-colored coat or sweater. A small pocket magnifying glass is adequate for a close-up view of the unique snowflakes that land on your sleeve. But it’s habit-forming, and you may get lost in this miniature world.
If you are interested in photographing snowflakes, you can find equipment information, suggestions, and some amazing color photographs at www.snowcrystals.com.