Bentley’s Struggle to Photograph Snow Crystals
by Duncan C. Blanchard

At about age fourteen, Wilson Bentley’s interest and curiosity about the world of nature had so overwhelmed him that he no longer was interested only in reading about it in books. He had to see this world for himself. On his fifteenth birthday on February 7, 1885, he received the gift of an old microscope that had been used by his mother during her school-teaching days. From that time on, he was not the same. He had found a focus in life. He became obsessed with the fragile beauty of common things from the world around him as seen through the microscope, especially the delicate   beauty of the snow crystals. Years later, Bentley had this to say about the beginning of this life-long obsession.

 “When the other boys of my age were playing with popguns and slingshots, I was absorbed in studying things under this microscope: drops of water, tiny fragments of stone a feather from a bird’s wing. But always, from the very beginning, it was the snowflakes that fascinated me most. . . . Under the microscope I found that snowflakes were miracles of beauty, and it seemed a shame that this beauty should not be seen and appreciated by others. I became possessed with a great desire to show people something of this wonderful loveliness, an ambition to become, in some measure, its preserver.”

But how do you preserve this beauty and show it to others? He took the microscope to an unheated woodshed attached to the back of the house. There he spent countless hours peering through his microscope at snow crystals on a microscope slide.

As if Bentley did not have enough difficulty finding a snow crystal and getting this tiny sliver of ice safely under the microscope, he had to be extremely careful not to breathe out in the direction of the crystal; otherwise it would melt instantly before his eyes. Holding his breath while looking through the microscope, he tried to sketch on a piece of paper exactly what his eyes were seeing. When he could hold his breath no longer, he turned his head away, rapidly exhaled, gulped in another lungful of cold air, and returned to his drawing. This went on for snow crystal after snow crystal, all different, and for storm after storm, not for one but for three years.

By this time, he had about four hundred drawings, but he still had problems. Even though he was willing to sit there in the bone-chilling temperatures of the woodshed and patiently continue to draw, nature placed a severe limit on the time he had to work. Snow crystals are made of ice, and water molecules evaporate or sublimate from ice, even at temperatures far below thirty-two degrees Fahrenheit, though at a much slower rate than they do from liquid water at a far higher temperature. Another problem facing Bentley was that the evaporation does not occur at the same rate from all places on the crystal. It occurs much more rapidly from the tips or sharp edges to flatten or smooth it and thus distort the crystal as it shrinks in size. Bentley probably had less than five minutes to get the drawings done. Anyone who has seen the beautiful but complex structure of snow crystals knows that even fifty minutes would not be enough to draw them. Faced with this problem, Bentley knew he would never be able to capture these “miracles of beauty” in his drawings. Somehow, somewhere, he would have to find another way.

Increasingly frustrated in making drawings of snow crystals seen under the microscope, his thoughts turned to photography. He wondered if it would be possible to use what was then a relatively new technology barely out of its infancy, and adapt it to make photographs of snow crystals. Before 1880 wet plate photography was commonly used. But in the late 1870s dry plate photography was developed in England, and by the early 1880s was available in America. Dry plates, unlike wet plates, could be exposed, put aside, and developed days or weeks later. This new, easier method of photography arrived just in time for Wilson Bentley.

In the early 1880s, cameras were still a novelty and not used by the average person, much less by farmers’ sons in the Mill Brook Valley of Jericho. Indeed, George Eastman, who was later to revolutionize photography by making his Kodak cameras easily available to amateurs, was just then inventing the flexible paper films which were used for the next 80 to 100 years. So who or what gave Bentley the idea it might be possible to use  photomicrography to capture on photographic plates all the intricate details of a snow crystal? Perhaps he read about it in a magazine or book, or maybe an older person, a friend of the family knowledgeable in photography, told him about it and helped him get started. Bentley, in his many articles published years later, never mentioned anyone helping him in his early struggles with photography, but there is one person who may have been influential.

Henry Seeley, about twenty years older than Bentley, was born in the valley not far from the Bentley farm. He served in the Civil War, and in 1872 opened a photographic studio in Bridgeport, Connecticut. During frequent visits to Jericho, he often talked to Bentley about photography. After Bentley’s death, Seeley said:

“He was a genius, known the world over. No question about that. … I got him his first camera, and showed him the making of plates, and whenever there was a new book published on photography I used to send it to him, but it was not many years before he could send me information. “

Seeley does not mention a microscope or photomicrography, so the camera he got for Bentley may have been a small one for photographing scenes around the valley a year or two before Bentley decided to try photomicrography. Nevertheless, even if Seeley was not directly involved in Bentley’s pioneering experiments in the photomicrography of snow crystals, it seems clear that he introduced Bentley to photography. This was all Bentley needed to go where no one had gone before in the photography of the natural world.

Bentley probably read all he could find on photography, especially photomicrography, and concluded he could progress no further in his quest until he had the proper kind of microscope and a camera. But where would he get the money to buy such technical equipment? He was still a teenager. Besides, his regular chores on the farm left him with little time to work elsewhere to earn the money he needed. It was his mother who convinced a reluctant father that they just had to find the money that young Wilson needed. Years later, Bentley told what happened.

“When I was seventeen years old, my mother persuaded my father to buy for me the camera and microscope which I developed into the apparatus I am still using. It cost, even then, one hundred dollars! You can imagine, or perhaps you cannot, unless you know what the average farmer is like, how my father hated to spend all that money on what seemed to him a boy’s ridiculous whim.

“Somehow my mother got him to spend the money, but he never came to believe it had been worthwhile. He and my elder brother always thought I was fooling away my time, fussing with snowflakes.”

When the camera and microscope arrived, Bentley must have spent many weeks experimenting, trying different ways to connect them together and to rapidly focus on the ground glass screen at the rear of the camera whatever was on the stage of the microscope. His new compound microscope was the lowest-priced model of about ten made by the Bausch and Lomb Optical Company. It came with a three-quarter inch focal length objective of wide aperture.  “Because of its simplicity,” the Bausch and Lomb catalog said, “It may be used by inexperienced hands without detriment or injury to it.” The bellows camera, about seven inches square, could be extended nearly three feet. At the back was provision for focusing on a ground or clear glass screen and for the attachment of a photographic plate holder.

But before he could do any experimentation with his new microscope and camera, he had to have something to place them upon. He obtained a wood table about three feet high, a bit over two feet in length, and slightly more than a foot wide. The carpentry skills displayed by the use of screws rather than nails in the table, were in marked contrast to the jury-rigged frame that was built and placed on the table to hold the camera and microscope. The wooden frame, held together only by nails, had the appearance of being slapped together hastily in order to get on with the experiment. This suggests that Bentley had obtained the table elsewhere and had himself made the frame which, though crude and by no means elegant, served his purposes admirably for over fifty years. In this and in other things, Bentley showed that he believed in that old bit of Yankee philosophy: “If it ain’t broke, don’t fix it.”

The camera took up so much room on the table that, in order to accommodate the microscope, Bentley had to make the frame long enough to stick out about a foot beyond the end of the table. He placed the microscope on the frame extension and bent the upper part backwards ninety degrees around a hinged joint. He removed both the eyepiece of the microscope and the lens of the camera, then pushed the microscope toward the camera until the tube or main body had seated itself in a light-tight sleeve attached to the front of the camera. He next used either the fine or coarse focusing knob to get a sharp, magnified image on the camera’s ground-glass screen of whatever was in front on the microscope stage.

For many people, the focusing would have posed no problem, but for Bentley, slight of build and with short arms, it was nearly insurmountable. Standing at the back of the camera under a large black cloth to keep out stray light, he strained to reach the focusing knob to sharpen the fuzzy image he saw on the ground-glass screen. He could not reach it, so he had to take a few steps to the front of the camera to make a slight focusing adjustment. Often he had to repeat this, and soon realized that when he worked with a snow crystal, he would have to move much more rapidly. But how would he do it?

He did this in a very clever way. He had noticed that when the microscope was in its bent-back position, the axis of rotation of the fine-focus knob was horizontal, parallel to, and below the optical axis of the microscope. On  both sides of the frame he attached vertically a short piece of wood with a  metal eyelet atop each piece. Near the back of the camera and on both sides of the frame he attached a wooden wheel. He next took a long piece of string and wound it a couple times around the focusing knob, then led it through both eyelets and back to the wooden wheels where he attached it. With this arrangement Bentley could look at the fuzzy image on the ground-glass plate, and turn the wooden wheels until the focusing knob brought the image on the plate into a sharp focus. The beauty of this was that he could do it in just a few seconds.

But how do you get a snow crystal on the glass slide in front of the microscope? Bentley had solved this problem during the long months of earlier winters when he tried to draw the crystals he saw under his microscope. He collected the crystals on a small black board. Standing outside the woodshed, he watched the snowflakes and crystals falling on the board and occasionally looked at them with a magnifying glass. If he did not see one he wanted to photograph, he would brush the snow off with a turkey feather and start anew. When he found a crystal he wanted to photograph, he would rush inside the woodshed to transfer it to a microscope slide.

Here began the most delicate part of the whole process. Using a broom splint from one of his mother’s brooms, he gently touched it to the middle of the snow crystal, picked it up and placed it in the center of the microscope slide. He needed a steady hand to do this. Some of the crystals were only a sixteenth of an inch in diameter. Commenting on this in later years, Bentley said, “My hand is perfectly steady. . . .  have never used liquor, tobacco, or any other stimulants that affect the nerves.”

The delicate part of the process over, Bentley began a race against time. Quickly he placed the slide under an observation microscope to make sure the snow crystal was unbroken. If it was, he placed the slide on the microscope stage and hastened to the back of the camera. He rapidly obtained a focus on the ground glass, removed it, replaced it with a photographic plate and exposed it to the magnified image of the snow crystal for anywhere from about thirty to three hundred seconds. For light he simply pointed his camera toward a window in the side of the woodshed and allowed the light to pass through the snow crystal on its way to the photographic plate. Using this technique of transmitted light, Bentley expected his photomicrographs to show not only the surface but the internal structure of the snow crystal. Since he had no use for the camera lens and had removed it, he had no shutter to open and close for the proper exposure time. He made his own shutter with a small black card mounted in front of the objective. He removed the card for the required exposure time, then replaced it. The plate was then ready for developing.

Bentley probably completed the camera-microscope assembly in †he late fall of 1883 and eagerly looked forward to the first snows of winter. Did he obtain instant success in his quest to capture the image of a snow crystal? He did not. He struggled for weeks on end with the apparatus but nothing seemed to work.

“Here I was with this expensive apparatus which had been given to me so reluctantly. I had been sure I could do wonderful things with it, but I failed over and over again! . . . If there had only been someone to explain what was wrong! But away off here on a farm there was nobody to help me. Again and again I failed. The winter slipped away, and I was almost heartbroken. But by the next season I had found the secret of my trouble. I began to use a very small ‘stop’--- a thin plate, with a tiny opening to shut out most of the light. With this and a longer exposure, I got a clear image. . . .”

The “tiny opening “ was probably the smallest of four medium-sized holes in a metal disc mounted just beneath the microscope stage. The disc --- a revolving diaphragm --- could be rotated to bring any of the holes directly beneath the stage and thus control the amount of light passing through the snow crystal and the microscope. The smallest hole had a diameter about one sixteenth of an inch, the largest was five-eights of an inch. It appears that during that first winter Bentley used the largest opening in an attempt to get a lot of light rapidly onto his film and thus lowering the time required to get the picture. This was fatal. With a large opening, the depth of field was very small, and it was extremely difficult to get a sharp focus. When he finally used the smallest opening on the disc, the resultant increase in the depth of field made it possible to quickly obtain a sharp, crisp focus. A longer exposure was now necessary, but it gave the “clear image” he was seeking.

Bentley later said: “The day I developed the first negative made by this method, and found it good, I felt almost like falling on my knees beside that apparatus and worshipping it! I knew then that what I had dreamed of doing was possible. It was the greatest moment of my life.”

This was on January 15, 1885. It was a dark, overcast day during which about five inches of snow fell outside the woodshed. With eager anticipation Bentley probably developed the first of five negatives taken that day, long before he had to turn his attention to the evening chores. Though he did not know it, Bentley’s success that day in the woodshed had produced the world’s first photomicrographs of snow crystals. He was still only nineteen years old!

From that moment on, his magnificent obsession with the snow crystals never waned, and he faithfully returned to his studies every winter of the forty-six that remained in his life.

On Sunday, March 1, 1931, a “lovely day” as Bentley described it,  with an early morning temperature of twelve degrees rising to thirty-four later in the day, he photographed a single snow crystal. That brought his total collection of photographs to 5,381! There would be no more. After forty-seven successive winters of snow crystal photography, he had taken his last picture. His old battered camera would never be used again.

World Famous Photographers Tip Hat to Bentley
Wayne Howe

Doug and Mike Starn are known for their manipulations of technique and materials that create arresting photographs. A small exhibit of their work can be seen through April 19^th at the Middlebury College Museum of Art in Middlebury, Vermont. The museum recently acquired photomicrographs of snowflakes, and other works of these well regarded photographers.

The identical twin brothers first began photographing objects at age thirteen. Now, at 48, the brothers’ work is exhibited throughout the world including the Metropolitan Museum of Art and the Whitney in New York, The Los Angeles County Museum of Art, the Art Institute of Chicago and the High Museum in Atlanta. Overseas exhibits include Sweden, Britain, the Netherlands and Korea. The exhibit at Middlebury includes snowflakes, as well as works depicting trees similar to an installation being completed for New York’s South Ferry subway terminal.

At the entry to the gallery room stands a large print that combined inkjet with varnish and acrylic paint, and measures almost four feet by five. The blend of photography, handmade materials and varnish gave the image an extra dimension. A gun metal blue cast gave the edges of the snowflake a hard and cold edge. In a tip of the hat to Bentley, Emmie Danadio, curator of the exhibit wrote, “The tree and the snowflake images in view are defining subjects for the Starns. This body of work was conceived in Vermont, where the pioneering excursions in photographic microscopy by Wilson Snowflake Bentley (1865-1931) first demonstrated that no two snowflakes are alike. “  

A larger work on display is composed of 24 snowflake images. The installation has a sixties modernist ascetic. The images were reproduced using inkjets and then sandwiched between lightweight aluminum and Plexiglas using liquid glue. The snowflakes are all of the same type: delicate lacy dendrite forms characteristic of high humidity. The grid of photos brought to mind a page from Bentley’s book. However, they differed in that they were all of one type and none of the flakes were whole. They all look damaged. None of these photographs would have been retained by Bentley. Produced during snowstorms over the course of three winters in Jay, Vermont, Doug and Mike Starn must have been mindful of the challenges Bentley faced in his work. 

Bentley’s appreciation for both the science and beauty of snowflakes is found here, though his works are absent.  A bit of text on the wall of the gallery captures a common spirit. Doug and Mike Starn found snowflakes, “transient, unique and instantly recognizable.” Bentley recorded similar sentiments.

For more information on this exhibit go to http://museum.middlebury.edu/exhibitions

Wayne Howe
12 February 2009

The Bentley length scale and snow crystal variety
by Jon Nelson

“… by means of these wonderfully delicate and exquisite figures, much may be learned of the history of each crystal and the changes through which  it has passed in its journey through cloudland.”

                                                                                                      W. A. Bentley, 1898

“… the crystals will in all probability be greatly modified by passing through atmospheric strata varying so greatly in density, temperature, humidity, etc. That they are greatly modified by these flights in the cloud is clearly shown by the interior structure of many of the crystals. ”

                                                                                                      W. A. Bentley, 1901

Bentley claimed that the various layers (‘strata’) of air that a snow crystal falls through largely determine the appearance of the crystal. The first direct support of this claim came about 30 years later, from laboratory experiments by Ukichiro Nakaya, a Japanese physicist. Now, 100 years later, probably all cloud scientists would agree with Bentley. So, with all the crystal and cloud research that has been done since Bentley and Nakaya, can we determine the history of a crystal if we see its form?  Can we finally do what Bentley said we should be able to do?

    Hardly. Laboratory experiments have certainly advanced our knowledge of snow, though large, crucial gaps remain and many experiments appear to contradict other experiments. Greater advances have occurred with cloud measurements. For example, researchers recently measured differences in temperature between points less than one centimeter apart in clouds. (Which is not so easy when one is flying at over a hundred meters per second!) Nearly the same sensitivity measurements have been made of cloud humidities and air motions. Although I was not involved in any of these cloud measurements, the researchers gave me data that I used to estimate the size of the layers that Bentley mentioned. Now, knowing the size of the layers does not allow us to know the exact history of a crystal, but it does help us understand something else dear to Bentley: the wonderful variety of snow crystal forms. As far as I know, nobody else has estimated this size, so I took the liberty of naming it the ‘Bentley length’.

   Before describing what this length is, I should clarify two things. One, the length is not fixed: it depends on the cloud conditions, particularly the temperature. Two, these ‘layers’ are not something one can easily discern by looking at a patch of cloud: the size of a layer depends not only on the properties of the cloud, but also on how fast the crystal can respond to changes in the cloud, particularly temperature changes. Specifically, the length is the average distance a crystal falls through the cloudy air before some fluctuation in air temperature or humidity causes the crystal to make a noticeable change in shape. About this change of shape, one of Nakaya’s great achievements was to demonstrate that snow crystal shape and size is remarkably sensitive to temperature: grow a snow crystal at –11 °C for 30 minutes and it ends up less than half a millimeter across, but grow it at –15 °C, only 4 degrees colder, for the same length of time and the crystal is more than 5 mm across. Using similar data on snow crystals and the above-mentioned data on clouds, I estimated that the Bentley length is about 8 meters at –11 °C, and only 0.8 meters at –15 °C. To a snow crystal, even four degrees is a huge change.

    To understand the role of the Bentley length in snow crystal variety, it helps to look into all sources of crystal variety. First, consider an analogy. The uniqueness of snow is sometimes compared to the uniqueness of individuals. That we are all unique is well known to be a result of our genes and our environment. Even identical twins who grow up in the same general environment will, despite having the same genes, develop differently due to small differences in their environment. The genes do not determine the person, but rather determine how the development of the person responds to the environment. Similarly, the structure of the initial embryo of ice, the seed that starts a snow crystal, influences how the crystal will respond to the cloudy environment. Concerning the environment, we can divide the crystal environment into that which would occur if all cloud fluctuations were removed, and the small differences in environment caused by the fluctuations. For example, consider a crystal that starts at –15 °C the middle of a cloud that everywhere has an updraft speed of 12 cm/s. At first the crystal slowly cools as it rises. But soon it reaches a size at which it falls faster than the updraft and from then on, the crystal slowly, and continuously, warms as it descends towards cloud base at –8 °C.  This history would produce a crystal that may look something like that on the left in the illustration below. Conversely, the crystal would look different if it started at –16 °C or some other temperature. But real clouds have fluctuations in the updraft speed, fluctuations in temperature, and fluctuations in other things. These fluctuations also affect the crystal form, just as the small differences in environment affected the twins. One such fluctuation on the right side of the illustration led to a slightly different crystal. So, that which is unique to a given crystal can be said to have originated from a combination of three sources. The third source, the fluctuations, depends on the Bentley length.

   To recap, two crystals could be different because they started with different characteristics that affected their final appearance. That’s the first source. The two crystals could instead be different because they started at different temperatures or grew in different clouds. Or, as the third source, the two crystals could have passed through different cloud layers. Bentley also discussed these three sources. He originally assumed that the first one was negligible:

Of the tens of thousands now filling the air, an infinitesimal proportion fall on this board; nor is there any good reason to doubt that when they started from equal heights on their journey earthward, many of the snow crystals were exactly alike in shape, and size, and probably in density.

But later, through a “fortunate accident” involving a broken water pipe that flooded a cold room in his house, he decided that crystals are also born with individual characteristics that can affect their appearance.

   Even with our incomplete knowledge of snow-crystal growth, it is possible to estimate the crystal variety resulting from these sources. Here I define ‘variety’ to mean the number of possible distinguishable crystals that could result from crystals falling through the layers. According to the data, the third source of variety greatly exceeds the second, and the second source greatly exceeds the first. So, the atmospheric ‘layers’, the third source, is likely the main source of crystal individuality. And if one would like to estimate the crystal variety for some set of conditions, one needs to know how many layers a crystal passes through. Here is where we use the Bentley length.

   To see just how much the Bentley-length source dominates snow crystal variety, we apply a little mathematics to the example in the illustration. According to experiments on ice-crystal growth rates and fall speeds, by the time the crystal falls below cloudbase, it would have fallen through about 1850 meters of air and grown to about 7.6 millimeters across. (Yes, this is a big crystal.)  

    With a Bentley length of 0.8 meters, which is appropriate for the crystal’s starting temperature, the number of layers the crystal passed through would be the 1850 meters divided by the 0.8-meter length, and assuming that half of the layers produced changes to the crystal, the crystal experienced 1156 changes. One example of a change is shown in the illustration. However, the Bentley length depends on temperature, and by the time the crystal gets to cloud base, the length is more than four times larger. If we instead use the average Bentley length for the crystal’s journey, the crystal experienced 468 changes. This might seem like a small number, but we have not yet calculated the variety.

    When the crystal passed through a given layer, the growth could have sped up or it could have slowed down. In the case of the illustration, the colder region caused the growth to speed up, and this produced the narrower tip and sidebranches. If instead, the fluctuation had been a region of warmer air, the growth would have slowed and the crystal tip would have widened. However, the Bentley length tells us only how many changes occurred during the crystal’s fall to earth; it does not tell us where (or when) each change occurred. Indeed, each change could have occurred at any one of many stages in the crystal’s history. The larger a crystal is, or the closer we choose to inspect the crystal features, the greater the number of stages. Assuming that we can use a good-quality microscope to examine the crystal to 1/1000 of a millimeter, the number of distinct stages for our example is about 3800. At this point, we have everything needed to calculate the variety.

    Each of the 468 changes were one of two, equally likely types, and could have occurred in any one of the 3800 stages of the crystal’s growth. The resulting number of possibilities is a staggeringly large number: a one followed by 768 zeros or 10⁷⁶⁸. (For math-inclined readers, this is “3800 choose 468 times 2 to the 468^th power.) To get an idea of the immense size of this number, consider that the number of atoms in the entire universe has been estimated to be a 1 followed by only 70 zeros or 10⁷⁰. The number of snow crystals that have ever formed on Earth is much smaller - 1 followed by only about 35 zeros or 10³⁵. If every crystal that grows under such cloud conditions has equal chance to be any one of these 10⁷⁶⁸ possible crystal forms, we can be sure that no two of them will be alike. Of course, Bentley instuitively knew this result. But it turns out that we cannot be so sure if we instead examined crystals that were born much closer to the bottom of the cloud. Crystals born near cloud base pass through fewer Bentley lengths, particularly so if the temperature is much warmer than –15 °C. In fact, one can argue that there probably have been two such crystals that appear the same.

Variety numbers  For the crystal in the illustration, the number of Bentley lengths passed through is  468,  and, for the resolution we chose, the number of growth stages is 3800.  The resulting number of possible crystal forms is about    1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,  000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,  000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,  000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,  000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,  000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,  000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,  000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,  000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,  000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,  000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,  000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000.  In comparison, the number of atoms in the known universe is about           10,000, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000. Whereas, the number of crystals that have ever formed on Earth is “only” 100,000,000,000,000,000,000,000,000,000,000,000.

The Bentley length depends on fundamental properties of snow crystals and clouds. The values I used here were calculated from a few sets of cloud measurements and one set of snow-crystal growth measurements. As we learn more about both, the values will undoubtedly be refined. But they are still fundamental. The famous physicists Neils Bohr, Arthur Compton, and Max Planck have atomic length scales named after them, lengths learned by most students of atomic physics. Karl Schwarzschild has a size named after him, a size in black-hole physics that depends on the mass of the star, and Peter Debye has a material-dependent length named after him. Wilson A. Bentley now has one for snow. The Bentley length depends on temperature, but nevertheless represents a well-defined property of nature. Will meteorology and physics students in the future learn about the Bentley length? Whether they do or not, it is nice to know that Bentley’s name has been honored with a scientific term, an honor given only to a handful of scientists. 

W. A. Bentley “Forty years’ study of snow crystals” Monthly Weather Review, Nov. 1924, page 532. He studied the crystals in the room, assuming they all started their growth under the same conditions. However, the editor pointed out in the article that the conditions throughout the room were not necessarily the same. Also, some of the crystals Bentley examined may have been born on the glass slide upon which he was photographing. The form of such crystals can be affected by the glass surface.

More details are in J. Nelson “Origin of diversity in falling snow” Atmos. Chem. Phys., 8 (2008), pages 5669-5682. Can be accessed at http://www.atmos-chem-phys.net/8/5669/2008/acp-8-5669-2008.html
A more careful calculation, in the above article, results in a slightly smaller number. But this difference does not affect the main points given here.
See the last page of C. Knight and N. Knight “Snow crystals”, Scientific American 228 (1973), pages 100-107.
See the final section of the article in reference 2.

News From The Gift Shop

The 2009 Snowflake Bentley Pewter Ornament has been designed  (see below) and modeled.  Casting will begin in March and will be ready for sale at the end of April.  Also in the Collection are our Scatter Pin, Necklace, Earrings and Zipper Pull using this beautiful Bentley snowflake design.

NEW THIS YEAR!  Vermont Snowflakes offering a Snowflake Bentley Charm Bracelet featuring five snowflake charms.  Details on the Bracelet will follow when the new Collection is introduced.

Mr Bentley’s Electric Crystals by Jon Nelson

Wilson A. Bentley was captivated by the beauty of snow crystals for most of his life. Perhaps then it should not be too surprising that he pondered the physical cause of their symmetry and intricacy. In doing so, he turned his mind to electricity. He considered that the crystalline surfaces had electric charges, with more charges concentrated at branch tips¹. When the tips of the branches overflowed with charge, the charges dribbled down the sides to produce ‘growth nuclei’ for the sidebranches. This process, he argued, could explain the symmetry of snow crystals:

"That the crystals, when permitted, attain to such a marvelous degree of symmetry and complicity, shows that the alignment of the growth nuclei, presumably tiny electric charges, is symmetrically regular to an almost unbelievable degree."
 

Here he connects electricity to the formation of sidebranches and the branch symmetry. In other writings, he connects snow electricity to growth rate, and snow electricity to lightning. In the specifics he was wrong, but in general he was surprisingly prescient. Snow crystals are indeed electric crystals, and the electricity itself is captivating. To see why, consider some of the amazing things that Mr Bentley’s electric crystals can do.

    It has been known, at least since Benjamin Franklin's experiments, that lightning in clouds is an electrical discharge. As for the charging, in 1860, William Thomson (later Lord Kelvin) suggested that the falling rain in the thundercloud was the charging current. However, Thomson did not specify how the drops got charged.  For a long time, nobody even suspected ice as a player. But in 1904, Mr Bentley asserted that, one, much of the rain in thunderstorms is, in fact, melted snow, and two, the snow is the source of the electricity. Other suggested mechanisms came and went. Yet it wasn’t for another 75 years before experiments and observations made it clear that indeed the electricity in ice is the main source of cloud electricity. Although the details of Bentley’s idea are inconsistent with current knowledge of crystal growth and thunderstorms, we should still give him credit for nominating electric crystals as the key player in thunderstorm electricity.  

     The current knowledge is this. When left by themselves, ice crystals apparently have negative charges on their surfaces and equal but opposite charges somewhere inside. As Bentley guessed, the amount of charge on the surface seems to increase when the crystals grow, with the greatest charging possibly being on the delicately branched crystals. When these crystals drift upward in a thundercloud, some of them ricochet off their larger, and blobbier, glazed-ice brethren, the hail particles. On average, they shed a little of this surface negative charge in the collision, leaving them with net positive charge and the hail with net negative charge. In each collision, about 10 to 100 thousand negative charges may be transferred. The large, negatively charged hail particles fall lower, and the small, positively charged ice crystals continue to drift upward until they can go no higher (about 6 miles above ground). Here, in the cloud’s upper reaches, a fist-sized volume of air may have tens of thousands of the crystals, so even if each crystal has only a little bit of excess charge, their cumulative charge can be huge. The ice collisions thus separate massive amounts of charge, putting negative charges near cloud bottom and positive charges near cloud top. This is analogous to rubbing a rubber balloon on your head: Each time a hair scrapes against rubber, the hair looses a little negative charge to the rubber. It may not be much, but the cumulative effect of many hairs and many rubs can transfer enough negative charge that, upon removing the balloon, you can hear the crackle of tiny sparks. In the thundercloud, it takes less than 30 minutes for these crystals to accumulate enough charge in the upper reaches for a spark to start. From the initial spark, one or more lightning bolts follow. Mr Bentley’s crystals can put on quite a show!

So, think about these little crystals for a moment – not only do they dazzle us with their beauty when they fall as snow, not only do they shape the land when they stack up in glaciers, but they also create the largest sparks in the world, sparks that have temperatures higher than anywhere else on Earth. Not bad for a tiny flake of frozen water. Not bad at all.

But that is not the only way the ice flakes show their electrical colors. The same charge exchange that happens with hail also happens whenever snow blows against something. In the windy plains, snow blowing against wire fences can deposit so much charge that the poor creature that gets too close to the fence may get knocked down by a spark².  Elsewhere, tent-bound persons in a night-time blizzard have been known to see an electrical glow appear when they place their hand near the tent wall. In this case, each snow crystal that hits the tent charges the fabric a little bit more. The fact that a diffuse glow occurs instead of a spark is likely due to the tent fabric being a poor electrical conductor. This means that the current to the hand is slower and occurs over a broader path – a glow instead of a spark. A similar thing happens when an airplane flies through an icy cloud. The electrical discharge in this case is called St. Elmo’s fire, and it occurs on the outside of the plane.  And sometimes, we just hear snow’s electrical effects, as when snow blowing against antennas create radio static. These are just a few of the ways to experience Mr Bentley’s electrical crystal show.

The key process in all of this is the surface charges on the ice. So let’s look a little deeper into the matter and see where these charges come from. If we magnify a region of an ice crystal, such as the top view shown in (a) and (b) below, we see that the positions of the water molecules are nicely arranged in a regular order. This ordering has a honeycomb pattern. But if we look even closer, as in (c), the regularity gets a little messed up. The oxygens (black) in each water molecule sit nice and orderly in a hexagon, but the hydrogens (red) can be arranged many different ways. The only restriction is the Bernal-Fowler bond rule, which says that one hydrogen sits between every pair of adjacent oxygens. But it turns out that the hydrogens are notorious rule breakers, and it is a darn good thing too, as we will see later. 

      In many materials, the charges are carried by electrons. But the electrons in the H₂O molecule are essentially stuck, unable to leave the molecule. Nevertheless, the hydrogens, which stick out like Mickey Mouse ears, have excess positive charge, whereas Mickey’s ‘chin’ has a corresponding excess of negative charge, as shown in (d). This +/– asymmetry makes H₂O a polar molecule. Now, there are two ways that the polarity leads to a flow of electrical charge. Consider the charge flow as a dance; the oxygen and hydrogen are the partners, and the partners have two dance steps: the ‘hand-off’ and the ‘twist’. 

   In the hand-off, a hydrogen from one molecule is “handed-off” to the next molecule. If both molecules were originally electrically neutral, as in (e) below, the resulting hand-off (green-black arrow) creates a negative charge (OH^-) and a positive charge (H₃O⁺). Subsequently, a hand-off to OH^- (or from H₃O⁺) effectively moves the charge through the ice. When two crystals collide, OH^- on the surface move from one crystal to another. The reason for the abundance of OH^- on the surface is still a mystery, but the twist is probably crucial.

 In the twist, a water molecule rotates, effectively removing a hydrogen from one bond and transferring it to another. In the first twist in (f) below, the Bernal-Fowler bond rule gets broken twice: an ‘empty’ bond and a ‘double’ bond are created. In the second twist, the double-bond moves. The empty and double bonds are viewed as two types of charges, but personally, I have trouble viewing them this way. Instead, I think of the charges as arising from a coordinated dance move: When a chain of molecules do the twist, the end result is that the hydrogen positions have become a little more lined up. If we straighten out such a chain, as in (g) below, then the “+” in one Mickey-Mouse ear matches up with a “-” in a chin and the two cancel each other out. Only the “+”s on top and the “–”s on the bottom remain. In reality, the cancellations aren’t perfect, but the effect is the same: the “+”s on top pull on the OH^-charges, whereas the “–“s on the bottom push (remember that opposites attract and likes repel). Here is the last important fact: the electrical dance has far more twists than hand-offs – about 10,000 times more. In other words, Bentley’s electric crystals prefer the twist to the hand-off. From what we can tell, all this twisting attracts the OH^- to the surface. Once at the surface, these negative charges can move to another surface during a rebounding collision. And that is basically what we now know about the inner workings of the ice-crystal electric show. Enjoy.

  In a way, the electricity inside snow crystals is just as captivating as their beauty  outside. As we still do not know much about the electrical processes on the inside and the crystallization processes on the outside, the two may someday be found to be closely related, as Wilson Bentley first supposed. It is perhaps not too surprising that he thought about crystal electricity – at that time, electricity was portrayed as a kind of magical fluid. Magazine advertisements boldly asserted the wonders of electrical contraptions like J. Moses’s Electro-Galvanic Spectacles, Heidelberg’s Alternating Current Electric Belt, and the many strange-sounding electrical devices of Dr. Scott. They are all gone now, but we will always have Mr Bentley’s Electric Crystals. 

References

1. This information and the quote are from Bentley’s unpublished notes, discovered and typed up by Duncan Blanchard.
2. R. L. Ives, Weather Phenomena of the Colorado Rockies. J. Franklin Institute 226, 691 (1938).