Our Window on Nature

. . . exploring the world around us

A Closer Look at Snow

Filed under: Sky,Weather — Lowell Christie -- December 24, 2016 @ 11:26 am

snowflake_study_5243837232Love it or hate it, when snow season appears much of the country is 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.

It may be hard to appreciate, looking from the operational end of a snow shovel, but ice crystals and the snowflakes they form are things of beauty, often perfectly symmetrical, sometimes simple, and sometimes wonderfully complex. An ice crystal, as you might expect, is a single crystal made of ice. When two or more ice crystals stick together you have a snowflake. (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, by 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 will also explain why the six points on a single ice crystal usually look the same. (Those that say identical snowflakes have been found are talking about early stages of single ice crystals.)
The structure of an ice crystal (and therefore of a snowflake) is determined by, in addition to its hexagonal shape, a combination of temperature and humidity. At a temperature of around 28 degrees Fahrenheit (F) ice crystals form as thin plates and stars. As the temperature nears 23 degrees, columns and slender needle shapes appear instead. Continuing down to 5 degrees, plates and stars reappear, and if you drop 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 take on different characteristics.

Varying humidity will also result in changes in the ice crystal. They will form simpler shapes at relatively low humidity and become more complex when the humidity is extremely high. 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 eighty different snow crystal types.

The interior of clouds have varying areas of humidity and temperature, the components with the most control 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, different for each new crystal. That means that each will have a unique structure, depending upon where it was located when each addition of water vapor occurred.

But 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.

Over the centuries many 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 Descarte wrote detailed descriptions of snowflakes saying:

“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 life-long 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 over 5,000 snowflakes, and shortly before his death published a book (still in print) containing over 2,400 of these images. You can see some of his black and white pictures on the Internet at http://snowflakebentley.com/.

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 snow storms, he would step out the door holding a black-painted board on which to catch the falling flakes. Using a bird 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 expensive 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 sweater. A small pocket magnifying glass is adequate for taking a close-up view of the unique snowflakes that land on your sleeve. But it’s habit-forming, and you may get lost in a miniature world.

If you are interested in photographing snowflakes, you can find equipment information, suggestions, and some amazing color photographs at the website http://www.snowcrystals.com.

P1216

1 Comment

  1. Jackie Solomon:

    As usual, thanks for your insight.

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