Every snowflake carries in its structure a record of the atmospheric conditions responsible for its formation and growth. Because only one flake at a time can fall through a given path in the air the structure of each flake is unique. Of course gross similarities are observed in all crystalline substances, snowflakes not excepted. In snowflakes such similarities are explained by the fact that water habitually forms hexagonal crystals and by prominent meteorological factors such as the depth of the air through which the crystals fall, the temperature at various elevations and related differences in the ratio of water vapor to other gases in the atmosphere.

Investigators who specialize in deciphering the history of snow crystals must sometimes wish it were possible to make permanent collections of actual flakes. Unfortunately snow crystals are as perishable as they are beautiful. Usually the crystals either melt or sublime (evaporate from solid to vapor) promptly on reaching the ground. For centuries students of snowflakes had to content themselves with drawings of the characteristic forms. A partial solution to the problem of recording the geometric patterns of flakes came in the l9th century with the invention of photomicrography. One zealous amateur, Wilson A. Bentley of Jericho, Vt., took up the hobby in 1885 and eventually published a book of more than 5,000 pictures of snowflakes; the book is still prized as the classic reference in its field.

The most significant advance, however, came in 1940 with the replication of snowflakes in plastic. This technique was developed by Vincent J. Schaefer of Schenectady, N.Y., then a research associate at the General Electric Research Laboratories and now director of research at the Atmospheric Sciences Research Center of the State University of New York. "About 25 years ago," writes Schaefer, "while I was waiting for a bus on my way home from work, a snowfall occurred that consisted of unusually beautiful snow crystals of the dendritic, or branching, type. I wondered if it might not be possible to devise a way of preserving such beauty. I had recently been making castings of crystal patterns in aluminum, and the thought came to me that perhaps a modification of the procedure could successfully capture the impression of snow crystals and that of frost patterns.

"When I reached home, I placed in a bottle a small amount of a new plastic solution with which I had been experimenting and put it in the freezing compartment of the refrigerator. This substance was polyvinyl formal. It is obtainable under the brand name Formvar 15-95 from the Monsanto Company, Bircham Bend Plant, P.O. Box 2130, Springfield, Mass. 01101. The solution I refrigerated consisted of one gram of Formvar 15-95 in 100 milliliters of ethylene dichloride, a solvent.

"When the material had cooled below the freezing point of water, I spread a thin layer of it on an equally cold glass microscope slide, scraped a few frost crystals from the walls of the freezing compartment onto the wet slide and then-leaving the slide in the refrigerator-waited for the solvent to evaporate. After a few hours, when the slide was dry, I took it out of the compartment and examined the frost crystals with a hand lens. They were perfectly preserved and remained so long after the slide had become warm. My excitement can be imagined.

"During the rest of the winter I was out in many snowstorms making replicas of snow crystals by this method and investigating other methods. Naturally I learned that there are a few tricks to the technique. Not all my early replicas were equally good. Even so, the art of replicating snowflakes in plastic is fairly easy to master after the experimenter becomes familiar with the behavior of the plastic.

"The replica consists of a film of plastic with several properties. The film is thick enough (and hence strong enough) to retain its shape when the ice melts. Yet it is thin enough to make a faithful impression of sharp corners and other crystalline details and to permit rapid evaporation of the solvent.

"One minor difficulty that can plague the beginner is the sublimation of the crystal as the solvent evaporates. Another is 'blooming,' the formation of a white, powder-like surface on the normally transparent plastic film. Blooming occurs during evaporation if the surface of the film cools below the dew point of the surrounding air so that moisture condenses on the drying surface in the form of tiny water droplets or frost. Ways to prevent blooming include maintaining the replica solution a bit colder than 0 degrees centigrade, reducing the rate at which the solvent evaporates (such as by immersing the replica in an atmosphere charged with enough solvent vapor to retard evaporatiO21) or lowering the relative humidity.

"For optimum results the concentration of Formvar in solution must also be adjusted with respect to the size of the particles to be replicated. In the case of 'diamond dust'-ice crystals that rarely exceed .2 millimeter in diameter-and other crystals up to approximately one millimeter in diameter, the concentration of Formvar in solution should range between .1 and .5 percent when ethylene dichloride is used as the solvent. Such a proportion, for example, would be .1 to .5 gram of Formvar in 100 milliliters of ethylene dichloride. The concentration can be increased to 1 percent in the case of ordinary snow and frost crystals. For replicating snow on the ground, as well as rime and graupel (also known as soft hail), the concentration should be increased to 2 percent or more.

"Evidently luck was with me on the occasion of that first experiment. I have never hit on a more effective replicating material. Still, materials other than Formvar dissolved in ethylene dichloride can be used. Any solvent can be substituted that is not soluble in water. For example, chloroform can be used if more rapid evaporation is desired. Accelerated evaporation induces quicker cooling, however, thereby encouraging the condensation of moisture and the possibility of blooming. The experimenter should avoid such common solvents as acetone, amyl acetate and alcohol because they are soluble in water. Similarly, benzene is unsatisfactory as a solvent because it solidifies before it reaches the freezing point of water.

"Just as it is possible to use solvents other than ethylene dichloride, so one can use plastics other than polyvinyl formal. Among the alternatives are polystyrene and methyl methacrylate. Polystyrene is readily available as Styrofoam. Polymerized methyl methacrylate is commonly known as Plexiglas. Scraps of Plexiglas reduced to granules by means of a wood rasp dissolve readily. Both materials are soluble in either ethylene dichloride or chloroform.

"A number of procedures have been devised for making replicas. One of the simplest calls for the plastic solution to be stored in a squeeze bottle made of polyethylene and kept below the freezing temperature of water. Enough solution is squirted onto a cooled glass slide to wet the upper surface. The slide is exposed to falling snow in order to collect specimens on the wet surface and is then placed in a cold, ventilated area for drying. The characteristic odor of the solvent becomes less apparent as the slide dries. When the odor can no longer be detected, the replica is ready for use. The best results are obtained when the slide is kept well below 0 degrees C. until the plastic hardens. Hardening may require some hours even in a well-ventilated area. Water molecules of the specimen migrate through the plastic film without difficulty.

"The most beautiful snowflakes occur in three basic forms: hexagonal plates imprinted with symmetrical designs, triangular plates and six-pointed stars with lacelike branches. Surprisingly, few storms produce perfect crystals of these types in substantial quantity. The dendritic stellar forms usually appear in large numbers during gentle snowfalls on cold, still nights.

"The platelike crystals, both hexagonal and triangular, appear to originate in high clouds. Six-pointed stars of simple design often form at altitudes of 1,000 feet or less. Elaborately ornamented crystals of the stellar type most often fall from clouds of intermediate height-the crystals of greatest complexity appearing when the weather is mild. The crystals comprising diamond dust are so small that they require many hours to fall a few thousand feet. Most of them sublime on the way. For this reason it appears that these minute particles form close to the ground. Snowstorms of the most common type produce clusters of flakes in random array that contain few symmetrical crystals and range in diameter from a few millimeters to several centimeters.

"Nicely formed crystals can be caught on a sheet of black velvet (black for ease of visibility and velvet for ease of handling) that is exposed to the fall for a few seconds and then examined in a cold, sheltered location such as an unheated shed, garage or tent. Specimens can be lifted from the velvet easily with a toothpick that has a small drop of 1 percent replica solution on the tip. The drop of solution can be collected on the tip by dipping the wood into the solution and withdrawing it quickly. The bottom of the suspended drop is then quickly but gently applied to the center of the specimen. Surface tension will lift the crystal from the velvet. Care must be taken not to press the wooden tip against the specimen or the crystal may be broken.

"The crystal is then transferred to the wet slide. Surface tension exerted by the large area of fluid now in contact with the bottom of the crystal will pull the specimen away from the wooden point. It will promptly sink into the film on the slide. As many as 20 crystals can be so arranged on a two-inch cover glass of the type used for making 35-millimeter projection slides. Once prepared, replicas can be covered by a second glass fastened in place with binding tape for optical projection, photographic enlargement or microscopic examination.

"Base materials other than glass can be used. Replicas show up nicely on black cardboard, for example. I have also used sheets of polyethylene, which are easy to cut into strips of the desired size. The thin transparent plastics such as Saran Wrap can serve as flexible mountings. They are easy to handle when they are supported by a frame such as: an embroidery hoop.

"To speed up the drying of replicas I have frequently made precoated slides. The base material is dipped into or otherwise coated with a replica solution of the required concentration and dried. The precoated surface is then moistened uniformly with solvent and charged with specimens. The coating dissolves in the immediate vicinity of the snowflakes. Much of the excess solvent is soaked up by the surrounding film. Because of surface attraction every nook and cranny of the specimen acquires a thin plastic coating. Some of my most successful replicas have been made by this procedure.

"Crystals smaller than one millimeter in diameter, such as 'diamond dust,' can be replicated most successfully by a variation of the precoated-slide technique. After the crystals have come to rest on the dry coating the slide is exposed to the vapor of the solvent, either ethylene dichloride or chloroform. (A vapor chamber for exposing slides can be improvised from a small wooden box equipped with an airtight lid. A shallow dish of solvent placed on the bottom of the chamber provides the vapor; a slit in the side of the box that slide.) The vapor condenses as a thin film on the coating but tends to accumulate around each specimen. There it dissolves the plastic locally and by a combination of surface tension and capillary effects coats the snowflakes in three dimensions.

"Another convenient technique makes use of the clear lacquer solutions now commercially available in aerosol-spray cans. Snowflakes that have collected on a base material are simply sprayed with the solution. The method is particularly useful for replicating frost on windowpanes. For best results the spray can should be held far enough away so that the plastic is almost dry when it makes contact with the crystals. The optimum distance varies with the drying time of the spray and must be determined by experiment. Krylon, an aerosol lacquer available from dealers in paint and art supplies, is particularly effective for this technique.

"Replication experiments need not be limited to small crystals and frost. For example, interesting and useful records of snow accumulations on the ground can be compiled by making daily replicas during a period of a week or more. For the base I use a sheet of window glass as wide as the snow is deep and from 15 to 20 inches or more in length, depending on the desired number of replicas. I apply a vertical strip of 5 percent solution about two inches wide at one end of the glass and press the glass gently against a vertical cut in the snow layer. The glass is then removed, along with the adhering film of snow, and placed with the replica side up in a cold, sheltered place. The plastic will dry within 24 hours and the film of snow will have sublimated. At approximately the same hour on succeeding days additional strips, each adjacent to its predecessor, are similarly applied to the glass and the replicating procedure is repeated. The series provides a sequential account of the gradual consolidation of snow by recrystallization even when the temperature remains well below freezing through the entire period of observation. Such effects as the gradual disappearance of layering that is always evident in fresh falls and the conversion of light flakes into 'corn' snow of relatively high density become strikingly evident in the record.

"Replicas can also be made of the structure of bulk ice, much as the crystalline pattern in metals is revealed by polishing and etching techniques. Experiments of this kind can be made even in summer in locations where hailstones fall. In fact, any kind of ice can be used: an icicle from the edge of a roof, a small piece of ice from a pond or stream or even an ice cube from the refrigerator. Whatever the specimen, the ice should be kept well below freezing temperature. If necessary, place the specimen in the freezing compartment of a refrigerator for a few hours. Begin the procedure by cutting a piece of plastic foam about the size of the specimen. This will be used as a handle to insulate the specimen from heat. To attach the handle, make a reasonably flat surface on the specimen by sawing or otherwise cutting off any roughness. Then spread a bit of water on the plastic foam and apply the wet surface to the flattened ice. The water will promptly freeze, cementing the handle to the specimen.

"Next, heat an electric iron or other flat metal object until it is warm to the touch. Press the specimen against the warm iron until the ice melts enough to form a flat surface. Immediately wipe the surface dry with a soft cloth. The temperature of the specimen will have risen substantially. Return it to the freezing compartment for an hour or so.

"Cover one surface of a flat board about 10 inches square with a piece of velvet stretched tight and securely tacked at the edges. The velvet will serve as a polishing buff. Remove the cooled ice from the freezing compartment and, manipulating the specimen by the handle, rub the flattened surface of the ice briskly across the velvet until it acquires a polished surface. The polishing operation requires only a minute or so. Then store the specimen in a cold, dry atmosphere for etching. The crystalline pattern will appear in a matter of minutes. Etch pits, marking crystal boundaries in the ice, are formed by sublimation. They stand out clearly as differences in the intensity of reflected light. Etching by sublimation requires that both the ice and the atmosphere be dry. Otherwise frost may form on the specimen and spoil the experiment.

"To replicate the etched surface, cover the polished area with a film of 2 percent Formvar solution and store the specimen in a freezing compartment until the plastic is dry. Then transfer the ice to a bowl of water. The replica will float free when the ice melts. It can then be maneuvered onto a glass slide and, when dry, covered by a second slide to make a permanent mount.

"Beautiful ice crystals can also be grown synthetically and replicated. For this experiment I use a water-soluble material such as polyvinyl alcohol or gelatin in concentrations ranging from 1 to 3 percent-one to three grams of alcohol or unflavored Knox gelatin in 100 milliliters of distilled water. A sheet of well-cleaned glass or other base material is flooded with a film of the solution and placed level in a cold, protected location until the film is supercooled- that is, has chilled below its normal freezing point. If a minute ice crystal or other appropriate freezing nucleus is now touched to the film, beautiful crystals will form immediately. As the crystals grow the alcohol or gelatin will precipitate out of the ice, marking the outlines of each crystalline feature. If the assembly is maintained at subzero temperature for several days, the ice will sublime, leaving a replica of great beauty.

"By careful seeding it is possible to grow huge single crystals of ice by this method. I have made some that measure up to 10 inches in diameter by restricting nucleation to a single site. Similar effects can be induced in soap bubbles that are either free-floating or held in a ring. They are difficult to replicate but useful for the detection of ice-forming nuclei in the air."

Bibliography

PREPARATION OF PERMANENT REPLICAS OF SNOW, FROST, AND ICE. Vincent J. Schaefer in Weatherwise, Vol. 17, No. 6, pages 279-287; December, 1964.

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