Snow science

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. 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 updraft 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 300 mm (12 in), the snow is melted and the water amount recorded.

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