Snow science addresses how snow forms, its distribution, and processes affecting how snowpacks change over time. Scientists improve storm forecasting, study global snow cover and its effect on climate, glaciers, and water supplies around the world. The study includes physical properties of the material as it changes, bulk properties of in-place snow packs, and the aggregate properties of regions with snow cover. In doing so, they employ on-the-ground physical measurement techniques to establish ground truth and remote sensing techniques to develop understanding of snow-related processes over large areas.
History
Snow was described in China, as early as 135 BCE in Han Ying's book Moral Discourses Illustrating the Han Text of the Book of Odes, which contrasted the pentagonal symmetry of flowers with the hexagonal symmetry of snow. Albertus Magnus proved what may be the earliest detailed European description of snow in 1250. Johannes Kepler attempted to explain why snow crystals are hexagonal in his 1611 book, Strena seu De Nive Sexangula. In 1675 Friedrich Martens, a German physician, catalogued 24 types of snow crystal. In 1865, Frances E. Chickering published Cloud Crystals - a Snow-Flake Album.Cloud Crystals - a Snow-Flake Album, Author: Chickering, Frances E., Year: 1865 In 1894, A. A. Sigson photographed snowflakes under a microscope, preceding Wilson Bentley's series of photographs of individual snowflakes in the Monthly Weather Review.
Ukichiro Nakaya began an extensive study on snowflakes in 1932. From 1936 to 1949, Nakaya created the first artificial snow crystals and charted the relationship between temperature and water vapor saturation, later called the Nakaya Diagram and other works of research in snow, which were published in 1954 by Harvard University Press publishes as Snow Crystals: Natural and Artificial. Teisaku Kobayashi, verified and improves the Nakaya Diagram with the 1960 Kobayashi Diagram, later refined in 1962.
Further interest in artificial snowflake genesis continued in 1982 with Toshio Kuroda and Rolf Lacmann, of the Braunschweig University of Technology, publishing Growth Kinetics of Ice from the Vapour Phase and its Growth Forms. In August 1983, Astronauts synthesized snow crystals in orbit on the Space Shuttle Challenger during mission STS-8. By 1988 Norihiko Fukuta et al. confirmed the Nakaya Diagram with artificial snow crystals, made in an updraftJapanese Utility model No.3106836 and Yoshinori Furukawa demonstrated snow crystal growth in space.
Measurement Snow scientists typically excavate a snow pit within which to make basic measurements and observations. Observations can describe features caused by wind, water percolation, or snow unloading from trees. Water percolation into a snowpack can create flow fingers and ponding or flow along capillary barriers, which can refreeze into horizontal and vertical solid ice formations within the snowpack. Among the measurements of the properties of snowpacks (together with their codes) that the International Classification for Seasonal Snow on the Ground presents are: Height (H) is measured vertically from the ground surface, usually in centimeters. Thickness (D) is snow depth measured at right angles to the slope on inclined snow covers, usually in centimeters. Height of snowpack (HS) is the total depth of the snowpack, measured vertically in centimetres from base to snow surface. Height of new snow (HN) is the depth in centimeters of freshly fallen snow that accumulated on a snow board during a period of 24 hours or some other, specified period. Snow water equivalent (SWE) is the depth of water that would result if the snow mass melted completely, whether over a given region or a confined snow plot, calculated as the product of the snow height in meters times the vertically-integrated density in kilograms per cubic meter. Water equivalent of snowfall (HNW) is the snow water equivalent of snowfall, measured for a standard observing period of 24 hours or other period. Snow strength (Σ) whether compressive, tensile, or shear, snow strength can be regarded as the maximum stress snow can withstand without failing or fracturing, expressed in pascals per second, squared. Penetrability of snow surface (P) is the depth that an object penetrates into the snow from the surface, usually measured with a Swiss rammsonde, or more crudely by a person standing or on skis, in centimeters. Surface features (SF) describes the general appearance of the snow surface, owing to deposition, redistribution and erosion by wind, melting and refreezing, sublimation and evaporation, and rain. The following processes have the corresponding results: smooth—deposition without wind; wavy—wind deposited snow; concave furrows—melt and sublimation; convex furrows—rain or melt; random furrows—erosion. Snow covered area (SCA) describes the extent of snow-covered ground, usually expressed as a fraction (%) of the total. Slope angle (Φ) is the angle measured from the horizontal to the plane of a slope with a clinometer. Aspect of slope (AS) is the compass direction towards which a slope faces, normal to the contours of elevation, given either degrees from true North N = 0° = 360° or as N, NE, E, SE, S, SW, W, NW. Time (t) is usually given in seconds for a measurement duration or in longer units to describe the age of snow deposits and layers.
Instruments
Depth – Depth of snow is measured with a snowboard (typically a piece of plywood painted white) observed during a six-hour period. At the end of the six-hour period, all snow is cleared from the measuring surface. For a daily total snowfall, four six-hour snowfall measurements are summed. Snowfall can be very difficult to measure due to melting, compacting, blowing and drifting.
Liquid equivalent by snow gauge – The liquid equivalent of snowfall may be evaluated using a snow gauge or with a standard rain gauge having a diameter of 100 mm (4 in; plastic) or 200 mm (8 in; metal). Rain gauges are adjusted to winter by removing the funnel and inner cylinder and allowing the snow/freezing rain to collect inside the outer cylinder. Antifreeze liquid may be added to melt the snow or ice that falls into the gauge. In both types of gauges once the snowfall/ice is finished accumulating, or as its height in the gauge approaches , the snow is melted and the water amount recorded.National Weather Service Office Binghamton, New York (2009). Raingauge Information. Retrieved on 2009-01-02.
Classification The International Classification for Seasonal Snow on the Ground has a more extensive classification of deposited snow than those that pertain to airborne snow. A list of the main categories (quoted together with their codes) comprises: Precipitation particles (PP) (See below) Machine-made snow (MM) – May be round polycrystalline particles from freezing of very small water droplets from the surface inward or crushed ice particles from crushing and forced distribution Decomposing and fragmented precipitation particles (DF) — Decomposition is caused by a decrease of surface area to reduce surface free energy initial break up by light winds. Wind causes fragmentation, packing and rounding of particles. Rounded Grains (RG) – Vary from rounded, usually elongated particles of size around 0.25 mm, which are highly sintered. They may be wind packed or faceted rounded, as well. Faceted Crystals (FC) – Grow with grain-to-grain vapour diffusion driven by a large temperature gradient is the primary driver of faceted crystals within the dry snowpack. Depth Hoar (DH) – Grain-to-grain vapour diffusion driven by large temperature gradient is the primary driver of depth hoar within the dry snowpack. Surface Hoar (SH) – Rapid growth of crystals at the snow surface by transfer of water vapor from the atmosphere toward the snow surface, which is cooled by radiative cooling below ambient temperature. Melt Forms (MF) – Range from clustered round grains of wet snow through melt-freeze rounded polycrystals when water in veins freezes to loosely bonded, fully rounded single crystals and polycrystals.to polycrystals from a surface layer of wet snow that refroze after having been wetted by melt or rainfall. Ice Formations (IF) – Encompass the following features: Horizontal layers, resulting from rain or meltwater from the surface percolating into cold snow and refreezing along layer barriers. Vertical fingers of frozen drained water. A basal crust resurgent from melt water ponding above a substrate and freezes. A glaze of ice on the snow surface, resulting from freezing rain on snow. A sun crust from melt water at the surface snow refreezes at the surface due to radiative cooling.
Precipitation particles
The classification of frozen particulates extends the prior classifications of Nakaya and his successors and are quoted in the following table: + Precipitation particles Subclass Shape Physical process Columns Prismatic crystal, solid or hollow Growth from water vapour at −8 °C and below–30 °C Needles Needle-like, approximately cylindrical Growth from water vapour at super-saturation at −3 to −5 °C below −60 °C Plates Plate-like, mostly hexagonal Growth from water vapour at 0 to −3 °C and −8 to −70 °C Stellars, Dendrites Six-fold star-like, planar or spatial Growth from water vapour at supersaturation at 0 to −3 °C and at −12 to −16 °C Irregular crystals Clusters of very small crystals Polycrystals growing in varying environmental conditions Graupel Heavily rimed particles, spherical, conical, hexagonal or irregular in shape Heavy riming of particles by accretion of supercooled water droplets Hail Laminar internal structure, translucent or milky glazed surface Growth by accretion of supercooled water, size: >5 mm Ice pellets Transparent, mostly small spheroids Freezing of raindrops or refreezing of largely melted snow crystals or snowflakes (sleet). Graupel or snow pellets encased in thin ice layer (small hail). Size: both 5 mm Rime Irregular deposits or longer cones and needles pointing into the wind Accretion of small, supercooled fog droplets frozen in place. Thin breakable crust forms on snow surface if process continues long enough. All are formed in cloud, except for rime, which forms on objects exposed to supercooled moisture, and some plate, dendrites and stellars, which can form in a temperature inversion under clear sky.
Physical properties Each such layer of a snowpack differs from the adjacent layers by one or more characteristics that describe its microstructure or density, which together define the snow type, and other physical properties. Thus, at any one time, the type and state of the snow forming a layer have to be defined because its physical and mechanical properties depend on them. The International Classification for Seasonal Snow on the Ground lays out the following measurements of snow properties (together with their codes): Microstructure of snow is complex and hard to measure, yet has a critical influence on the thermal, mechanical, and electromagnetic properties of snow. Although there are multiple means for characterizing microstructure, there is no standard method. Grain shape (F ) includes both natural and artificial depositions, which may have decomposed or include newly formed crystals freeze-thaw or from hoar frost. Grain size (E ) represents the average size of grains, each measured at its greatest extension, measured in millimetres. Snow density (ρs ) is the mass per unit volume of snow of a known volume, calculated as kg/m3. Classification runs from very fine at below 0.2 mm to very coarse (2.0–5.0 mm) and beyond. Snow hardness (R ) is the resistance to penetration of an object into snow. Most snow studies use a fist or fingers for softer snows (very soft through medium) and a pencil (hard) or knife (very hard) below the hardness boundary of ice.