The following discussion of basic snow properties is presented to provide the uninitiated reader with the background to understand the discussion of snow physics and modeling in Chapters 2-4.
Snow originates in clouds at temperatures below the freezing point. As moist air rises, expands and cools, water vapor condenses on minute nuclei to form cloud droplets on the order of 10 microns in radius. When cooled below 0 °C such small droplets do not necessarily freeze and may "super cool" down to -20 °C and occasionally down to -40 °C. Once a droplet has frozen it grows quickly at the expense of the remaining water droplets because of the difference in saturation vapor pressure between ice and water. The form of the initial ice crystal, columnar, platelike, dendritic, etc. (see Fig. 1.3) depends on the temperature at formation, but subsequent
Snow and Climate: Physical Processes, Surface Energy Exchange and Modeling, ed. Richard L. Armstrong and Eric Brun. Published by Cambridge University Press. © Cambridge University Press 2008.
growth and structural detail also depend on the degree of supersaturation (Hobbs, 1974 and Chapter 2 of this book). During its fall to earth a snow crystal may undergo considerable change due to variations in temperature and humidity with altitude. The character of a surface layer after a snowfall depends on the original form of the crystals and on the weather conditions during deposition. For example, when a snowfall is accompanied by strong winds, crystals are broken into smaller fragments favorable for close packing. After deposition snow may dissipate rapidly by melting or sublimation or it may persist for long periods. If it persists it will undergo metamorphism, changing its grain texture, size, and shape, primarily as a result of the effects of temperature and overburden pressure as it becomes buried by subsequent snowfalls. Snow metamorphism can occur rapidly because the crystals are thermodynamically active due to their large surface area to volume ratio (complex shape) and because their temperature is at, or proportionally close to, the melting temperature. Over the winter the typical snow cover accumulates and develops as a complex layered structure made up of a variety of snow grains, reflecting both the weather and climate conditions prevailing at the time of deposition as well as the persisting influence of metamorphism within the snow cover over time (Armstrong, 1977; Colbeck, 1986; Colbeck et al., 1990 and Chapter 2, Fig. 2.7).
The three basic properties used to describe snow cover are the related parameters of depth, density, and snow water equivalent (SWE). "Snow depth" refers to the thickness or height of snow, typically expressed in centimeters. Maximum snow depths range from a few centimeters in regions with ephemeral snow cover, to several meters in moist cold mountain regions. "Snow density," as with any material, is simply the ratio of mass to volume for a given sample. The standard unit of measurement is kilograms per cubic meter (kg m-3) with typical values for newly fallen snow of 30-150 kg m-3 increasing to a maximum seasonal snowpack density of approximately 400-500 kg m-3. However, wind-deposited snow may rapidly achieve densities of300-400 kg m-3 and crusts that form following the refreezing of melting snow may have densities of about 700-800 kg m-3. For reference, the density of pure ice (bubble free) is 917 kg m-3 and the density of water is 1000 kg m-3. Thus the bulk of the seasonal snow cover is typically composed of 50 percent or more of air by volume throughout the winter. This simple fact has great significance with respect to the processes of metamorphism that depend directly on the amount of water vapor contained in the air spaces surrounding the snow grains (Armstrong et al., 1993). SWE is the thickness of the layer of water resulting from the melting of the initial volume or thickness of snow and is typically expressed in kg m-2 or mm. Further details on these basic snow properties and the instrumentation used to measure them are provided in Chapter 5.
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