Bachelor, OR. Another electron microscope image of a plate and column snow crystals adjoined. Section through a cloud showing where the freezing process takes place.
Illustration showing how ice crystals grow. An incredible microscope photo of a Stellar snow crystal with well grown dendrites. Posted by Bevan Waite November 19, Get our top 3 articles emailed to you weekly!
Got an opinion? This shows essentially what would happen if two snow crystals traveled side-by-side as they fell from the clouds. What synchronizes the growth of the arms? The six arms of a snow crystal all grow independently, as described in the previous section. But since they grow under the same randomly changing conditions, all six end up with similar shapes. There are no mysterious forces -- quantum-mechanical, acoustical, or anything else you might have heard about -- that provide communication between the arms to ensure they all grow alike.
One analogy I like is what happens on a rainy day. You look outside, see that it is raining, so you grab your umbrella on your way out. Then you find out that your neighbors are all carrying umbrellas too. You didn't communicate with one another about this, yet everyone's umbrella usage was synchronized. The same goes for the six branches of a snow crystal. When the temperature or humidity changes around the crystal, the six branches all change their growth in snychrony, even though the branches do not communicate with one another.
Now, let me assure you that the vast majority of snow crystals are not very symmetrical. Don't be fooled by the pictures -- irregular crystals see the Guide to Snowflakes are by far the most common type. Just take a look for yourself next time it snows. Near-perfect, symmetrical snow crystals are fun to look at, and sought after by photographers, but they are not common.
Why six? The six-fold symmetry you see in a snow crystal arises from the arrangement of water molecules in the ice crystal lattice. As this ice crystal model spins around, you can see the hexagons in the structure. But a crystal is a three-dimensional structure, and snowflakes are also three-dimensional. Stellar plates are thin and flat see the Guide to Snowflakes , but other snow crystals are not. The simplest snowflakes When snow crystals first begin growing, they are shaped like the simple hexagonal prisms shown here.
Each prism has two basal facets and six prism facets. Hexagonal prisms can be long, slender, hexagonal columns, or thin, flat, hexagonal plates, or anything in between. Hexagonal prisms display the simple, perfect order of the molecular lattice. These crystals result from slow growth, and they are usually small in size. Snowflake Asymmetry If you take a close look at the snow crystal on the left, you will see that it is not very symmetrical.
Sure it has six similar branches, but the sidebranches are randomly positioned on the each of the branches. This is a fernlike stellar dendrite see the Guide to Snowflakes , and each branch grows independently of the others.
Going beyond the morphology diagram, much progress in understanding snow crystals has come from work in crystallography and metallurgy done by many scientists over several decades, as the foundations of modern materials science were being laid throughout the 20th century.
The semiconductor industry provided considerable impetus in these fields, as suddenly the ability to produce large crystals—which required an understanding of their growth dynamics—was a business necessity.
Figure 4. The temperature dependence of snow-crystal forms shown in the morphology diagram can be brought to life with lab-grown snow. These small crystals were all grown as they fell freely in a chamber held at an intermediate level of supersaturation, but at varying temperatures. The left panel shows a montage of crystals grown at -2 degrees Celsius, the middle at -5 degrees and the right at degrees. Photographs courtesy of Kenneth G.
The formation of facets—flat crystalline surfaces—is a nearly ubiquitous phenomenon in crystal growth. Faceting plays a major role in guiding the growth of snow crystals. Once a cloud droplet freezes, the expanding crystal develops facets because some crystalline surfaces accumulate material more slowly than others. Condensing molecules are especially attracted to rounded surfaces that are rough on atomic scales, because such areas present greater available molecular binding.
Molecularly flat regions—the facet surfaces—have fewer dangling chemical bonds and are thus less favorable attachment sites. After a crystal grows for a while, only the slow-growing facet surfaces remain.
The crystal eventually becomes faceted, regardless of its initial shape. The molecular bonding to the crystal lattice determines which surfaces grow slowly, and thus which lattice planes become facets. The process of faceting is how the geometry of the water molecule is transferred to the geometry of a large crystal. Different mineral crystals have different facet structures, depending on the details of their molecular lattices.
When faceting dominates snow crystal growth, the resulting hexagonal crystal has six side faces, called prism facets, capped by top and bottom surfaces, called basal facets. This is the basic shape of small or slow-growing snow crystals.
Visible remnants of this form can often be seen at the centers of larger, more complex snow crystals, revealing their simpler initial shapes. Under some conditions, water molecules will attach more readily to the prism surfaces than the basal surfaces, producing thin plates of ice.
In other circumstances, the molecules attach to the basal facets, resulting in columns. In either case, faceting is one of the most important mechanisms for producing different shapes and patterns.
Faceting cannot be the whole story, however, or all snow crystals would be shaped like simple hexagonal prisms, which is certainly far from the case. Something else happens when the crystal size is large—typically about half a millimeter across—or when the growth is rapid.
Then a crystal may sprout branches because of a well-known growth effect called the Mullins-Sekerka instability , or simply the branching instability. This process largely explains how complex, flowerlike snow-crystal structures can arise spontaneously from nothing more than freezing water vapor.
As snow crystals grow, they use up the water vapor in their immediate surroundings, and it takes a certain amount of time for additional molecules to diffuse through the air to reach the crystal. Snow-crystal growth is therefore said to be diffusion limited , and different regions on a crystal effectively compete for available resources.
If a spot on a crystal—for example, one of the points on a hexagonal plate—sticks out farther into the air, then water molecules will preferentially collect on that point, simply because the diffusion distance is slightly shorter. With a slightly greater source of material, the point will grow a bit more rapidly, which in turn causes the point to become more pronounced. The result is a positive feedback that reinforces the effect, so large branches eventually sprout from the six points of a hexagonal snow crystal.
With time, numerous side branches may in turn sprout from random bumps or faceted tips on the main arms. Instabilities like this are at the heart of pattern formation, and nature is one unstable system heaped on top of another. The sun heats the air near the ground and the warm air rises—a convective instability that gives rise to the wind, clouds and the rest of Earth's weather.
The resulting wind blows on the surface of the ocean, making the ocean surface unstable, which generates waves and drives moisture into the atmosphere. As these waves run into shallow beaches, they become unstable and break. Faceting and branching are two dominant forces in snow-crystal growth, pulling in opposite directions.
Faceting is a stabilizing process that drives the formation of flat surfaces and simple shapes. With faceting alone, snow crystals would all look like hexagonal prisms. In contrast, branching is an unstable process that takes simple shapes and makes them complex. Branching alone would produce crystals with much structure but no symmetry, looking a bit like miniature ice tumbleweeds.
The constant interplay of both these forces is necessary to sculpt the shapes and patterns seen in snow crystals. Although branching and faceting explain many properties of snow-crystal growth, they do not immediately explain the overall organization of the morphology diagram. For the past several decades, the assumption was that the growth rates of the prism and basal surfaces were simply temperature dependent.
If the prism surfaces accumulated material more quickly than the basal surfaces, plates resulted. If the opposite occurred, columns and needles were formed.
The relative growth rates would have to change dramatically with temperature—by at least two orders of magnitude over a few degrees—in order to be consistent with the morphology diagram, but no other explanation seemed possible. For the past several years, I have been investigating snow-crystal formation by making precise measurements of crystal growth rates under well-controlled conditions. The idea is essentially the same as Nakaya's, except I can use the benefits of 21st-century technology: laser interferometry, digital data recording, exquisite temperature control and a host of other advances Nakaya could hardly have imagined.
I expected to see the growth vary strongly with temperature in my measurements, in order to agree with the morphology diagram. If thin plates and slender columns grew just a few degrees apart, then it stood to reason that the growth rates of the basal and prism facets would differ greatly from one another and change rapidly with temperature. Figure 5. Laboratory-grown snow crystals offer the opportunity to explore in detail the combination of growth mechanisms and the different conditions under which they occur.
Since experimental chambers cannot provide the same free-fall time as the atmosphere, it is desirable to grow supported crystals. One technique is to grow snow crystals on the tips of thin ice needles that sprout from an electrified wire left. The strong electric field attracts water molecules in the air, greatly accelerating the needle's growth. When the electric charge is removed, crystal growth returns to normal.
By controlling temperature and humidity, designer snowflakes can be created. For instance, a dendrite can be made to grow hollow columns at its points, then plates can be grown to cap each column, resulting in a shape that resembles a fancy chandelier right. The series of temperature shifts required to produce such a form would be highly unlikely within a natural cloud bank. My measurements showed otherwise.
There were some differences among facets, but they were not nearly as great as expected. Likewise, the growth rates changed somewhat with temperature, but not nearly enough to explain the features seen in the morphology diagram.
The situation became even more intriguing when I began making careful computer models of the diffusion-limited growth of crystals, especially the growth of thin plates. The usual tenets of the branching instability suggested that the humidity should be highest at the edge of a plate, since this sticks out farthest into the air surrounding the crystal.
Instead, my models indicated that the fast growth of the plate edges tends to deplete the water vapor there, so the humidity near the edge is actually lower than at other regions. This counterintuitive result meant that the branching instability could not promote the growth of thin plates and would actually drive the growth to blockier forms. Putting the measurements and the modeling together made it impossible to explain the morphology diagram with what was known about ice-crystal growth.
The modeling showed that growth rates had to change by a factor of or even 1, over just a few degrees in temperature, which seemed implausible from what is known about crystal-growth dynamics. And indeed, the measurements showed that such rapid changes did not happen. Something new was needed to explain the dramatic changes in crystal shape with temperature. I've become convinced that the solution lies in an effect I call structure-dependent attachment kinetics.
The basic hypothesis is that the intrinsic growth rate of a faceted surface depends on its structure. In particular, I have proposed that an extremely narrow facet surface, as exists at the very edge of a thin, platelike crystal, will grow much faster than a broader facet.
The molecular dynamics of how this happens are still unclear. One possibility stems from the fact that the molecules at the edge of a thin plate are more loosely bound than molecules on a broader surface, simply because they have fewer neighboring molecules to provide binding. This may cause the edge to become intrinsically rougher, as the surface molecules have more freedom to jostle about.
A rougher surface is more accepting to condensing molecules, which could make the growth rates increase. The ice surface is known to have a rather complex molecular structure, which is not completely understood even in simpler circumstances, so it is not currently possible to prove or disprove this hypothesis. But I maintain that this phenomenon, or something quite similar, must exist to explain the data.
Figure 6. Computer models of water-vapor levels around the edges of thicker top left and thinner center snow-crystal plates, shown from the side, demonstrate that there is less water vapor available around the edges of thinner, faster-growing plates.
Colored bands indicate water-vapor levels, increasing from blue to red. Such models support a new growth mechanism, called the knife-edge instability, which can amplify small, intrinsic changes in crystal growth. This instability accelerates the growth of thin plates and can help explain the formation of capped columns, where the crystal seems to instantaneously change from columnar to platelike growth bottom.
Photograph courtesy of Kenneth G. The phenomenon of structure-dependent attachment kinetics leads to a new kind of growth behavior that I call the knife-edge instability. If the edge of a plate-like crystal, which starts out with some initial thickness, is made a bit thinner, then structure-dependent attachment kinetics dictates that the intrinsic growth rate of the edge will increase.
The increased growth at the edge causes it to sharpen, which makes the edge still thinner, increasing the growth rate even more. Here again, there is a positive feedback that enhances the crystal growth and drives the formation of thin, platelike structures.
The instability progresses until other mechanisms intercede to halt further sharpening of the edge. Not only does the knife-edge instability reconcile the crystal-growth data in a reasonable way, it also explains many aspects of natural snow crystals.
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