A Taxonomy of Snow Crystal Growth Behaviors: 1. Using c-axis Ice Needles as Seed Crystals Kenneth G. Libbrecht Department of Physics, California Institute of Technology Pasadena, California 91125, [email protected] Abstract. I describe a new approach to the classification of snow crystal morphologies that focuses on the most common growth behaviors that appear in normal air under conditions of constant applied temperature and water-vapor supersaturation. The resulting morphological structures are generally robust with respect to small environmental changes and thus should be especially amenable to computational modeling. Because spontaneous structure formation depends on initial conditions, the choice of seed crystal can be an important consideration, and I have found that slender c-axis ice needles provide an exceptionally good starting point for this series of investigations. A sharp needle tip exposes a single basal surface that often simplifies subsequent morphological development, and the absence of a nearby substrate allows for the exploration of a broad range of supersaturations with well-controlled boundary conditions. The overarching goal of this endeavor is to facilitate detailed quantitative comparisons between laboratory ice-growth experiments and corresponding computational models, which will should greatly improve our understanding of the ice/vapor molecular attachment kinetics as well as our ability to model diffusion-limited growth dynamics in the ice/vapor system. This specific case-study of water ice connects broadly to many areas in aqueous chemistry, cryobiology, and environmental science, while the physical principles of molecular attachment kinetics and diffusion-limited growth apply more generally to other systems in crystal growth and materials science. along with capped columns and numerous ❅ Classification of Snow platelike forms during his Arctic voyages Crystal Morphologies recounted in 1820 [1820Sco]. Soon after the development of optical With the advent of photomicroscopy in the magnifiers, numerous observers began late 1800s, it became possible to document documenting the remarkable diversity of snow crystal morphologies in substantially natural snow crystal forms [1982Fra, greater detail, thus revealing a surprisingly large 2002Wan]. For example, René Descartes made menagerie of complex growth forms [1931Ben, an especially detailed account of snow crystal 1954Nak]. To provide a consistent structures in his famous Les Météores in 1637, nomenclature for discussing these structures, including several varieties of stellar plates and Ukichiro Nakaya developed the early the first recorded observations of capped classification system shown in Figure 1, columns [1637Des, 1982Fra, 2021Lib]. Robert consisting of 41 distinct forms. Nakaya’s chart Hooke sketched numerous snowflake forms in was later expanded by Magono and Lee to 80 Micrographia [1665Hoo], and William categories [1966Mag] and recently expanded Scoresby documented triangular snow crystals further by Kikuchi and Kajikawa to include 121 1 Figure 1: (left) The first morphological classification system for natural snow crystals, developed by Ukichiro Nakaya in the 1930s [1954Nak]. Figure 2: (below) This large snow-crystal classification system contains 121 separate morphological categories [2011Kik, 2013Kik]. individual snowflake types [2013Kik, 2011Kik], as shown in Figure 2. Descriptive names have also been ascribed to many growth forms, and Figure 3 shows a selection of names that are in relatively common use [2006Lib1]. It should be noted that none of these charts provides a complete listing, and some snow crystals have more than one common name. Although classification systems such as these are valuable in providing a language for discussing observations of natural snow crystals, they are not especially useful for revealing the physical dynamics that guide the formation of the different structures. Because the morphology of an individual snow crystal depends on the entire growth history it experienced, the number of possible permutations is extremely large, so no reasonable classification chart can encompass all observed forms. If our overarching goal is to understand the physical origins of the observed menagerie of snow crystal morphologies, a better approach is to focus on the fundamental processes involved in ice crystal growth and how these spontaneously generate such a rich variety of complex structures. 2 Figure 4: The above example illustrates a split star that broke apart when it fell on a collection board and was soon photographed (photo by Patricia Rasmussen [2003Lib2]). Figure 5 shows additional possible topologies for split stars. Figure 3: In addition to the alphanumeric nomenclature initiated by Nakaya, descriptive names have also been adopted for numerous snow crystal types, and this chart Figure 5: Several different split-star topologies can shows a selection that have become popular over the arise, depending on which branches grow out on the years [2006Lib1]. two capped-column plates, as larger branches on one plate stunt the growth of corresponding branches on the other plate. An Example – Split Stars To better see why classification systems are not sibling in each of the six cases. Depending on well suited for describing snow crystal growth which branches dominate on which plane, dynamics, consider the case of split stars several different split-star topologies are shown in Figures 4 and 5. The formation of possible, as illustrated in Figure 5. these crystals begins with a simple prism that Comparing Figure 5 with Figure 2, we see soon develops two end plates, essentially a that the types P5a, P5b, and P5c in Figure 2 capped column where the column is short, so describe some of the possibilities for broken the two end plates grow quite close to one split stars, but additional variations are not another. If the hexagonal tips subsequently included in this classification scheme. develop branches, corresponding branches on Moreover, the P3 and P4 series do not the two plates are so closely spaced that they distinguish unbroken split stars from single- interfere with one another’s development. As a plane stars, even though these are topologically result, one of each pair quickly overshadows its different morphologies. The Kikuchi-Kajikawa classification system could be expanded further 3 to include the full spectrum of split stars and important clues for understanding the physics split-star fragments, but a rather large number underlying snow-crystal growth. Plotting the of specific cases must be added to describe all basic crystal morphology as a function of air the different permutations of a relatively temperature and water-vapor supersaturation simple growth process. Not only does the yielded the result shown in Figure 6, now classification system increase rapidly in size to known as the Nakaya diagram. These include additional possible growth outcomes, it observations were soon confirmed by other also tends to obscure the essential physics researchers, yielding the similar diagrams underlying the different structures. shown in Figure 7. The cartoon version in Our take-away conclusion from this Figure 8 is especially useful for expressing the example is that there is an inevitable degree of essential features of the Nakaya diagram to a futility in any attempt to classify and catalog all non-scientific audience. the different forms that can develop during the growth of natural snow crystals. Clearly some form of nomenclature is necessary when speaking or writing about natural snow crystals, but it provides little guidance for understanding how these complex structures emerge as ice grows from water vapor in air. The Nakaya Diagram In addition to developing the first classification system for natural snow crystals, Nakaya also made pioneering observations of laboratory- grown specimens, and these quickly provided Figure 6: The first Nakaya diagram plotting snow crystal growth morphology as a function of Figure 7: Additional published versions of the Nayaka temperature and water-vapor supersaturation diagram described in: [1958Hal] (top), [1961Kob] [1954Nak]. (middle), and [1990Yok] (bottom). 4 explain the rich menagerie of snow crystal types, the origin of this organization requires a deeper dive into the underlying molecular physics of crystal growth. This is a nontrivial subject about which quite a lot has been written over the years [1971Mas, 1987Kob, 2001Nel, 2017Lib, 2019Lib, 2021Lib], with [2021Lib] providing the most comprehensive review to date. The basic physics of diffusion- limited growth, notably the Mullins- Sekerka instability [1964Mul], is responsible for the formation of complex dendritic structures, while the molecular attachment kinetics produces the Figure 8: This cartoon version of the Nayaka diagram shows the key variations in snow-crystal morphology with temperature and transitions between platelike and supersaturation. columnar growth as function of temperature [2019Lib1, 2020Lib3]. While One important feature of the Nakaya our recent model of the attachment kinetics diagram is that it describes synthetic snow provides a compelling semi-quantitative crystals grown under constant environmental explanation of the overall temperature conditions, as opposed to the varying structure seen in the Nakaya diagram conditions experienced by natural snow [2019Lib2, 2020Lib1, 2020Lib2], the overall crystals in the atmosphere. These laboratory state of our understanding of the molecular results nicely explain