Snow and Course Preparation

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[edit] Cryosphere

The Cryosphere is the big cold, water and snow based system that we work with on this planet. The Snow Cover is what we deal with. The Snowflakes are things that we need to understand.

[edit] Snow

  • Snow Flakes an ice crystal, or more commonly an aggregation of many crystals that falls from a cloud. Simple snowflakes (single crystals) exhibit beautiful variety of form, but the symmetrical shapes reproduced so often in photomicrographs are not found frequently in snowfalls. Broken single crystals, fragments, or clusters of such elements are much more typical of actual snow.
  • Properties of Snow
Snow, from its fall until its full melting, undergoes a structural metamorphism governed by local temperature and humidity fields. Among them, the isothermal metamorphism seems the most accessible to modeling. In this work, we tried to simulate the microstructural transformations of a dry snow sample, placed in isothermal conditions, near 0 degree C. In these conditions, the vapor pressure of water is high: the metamorphism can be considered, in first approximation, as fully curvature-driven. A simple numerical model was implemented on this basis and applied to simulated data and 3-D images from X-ray microtomography. From  Snow Microstructure / MANTO Team Centre d'Etudes de la Neige - French Snow Research Center [1]
Snow, from its fall until its full melting, undergoes a structural metamorphism governed by local temperature and humidity fields. Among them, the isothermal metamorphism seems the most accessible to modeling. In this work, we tried to simulate the microstructural transformations of a dry snow sample, placed in isothermal conditions, near 0 degree C. In these conditions, the vapor pressure of water is high: the metamorphism can be considered, in first approximation, as fully curvature-driven. A simple numerical model was implemented on this basis and applied to simulated data and 3-D images from X-ray microtomography. From Snow Microstructure / MANTO Team Centre d'Etudes de la Neige - French Snow Research Center [1]

Key Words

Transformation of Snow flakes

New Snow see [10]

  • When snow falls from the sky, it is in the classic snowflake form: six arms and three lines of symmetry. This form is responsible for the extremely low bulk density of freshly fallen snow – it can be up to 97% air.
  • As external forces are applied to the snowflake (such as wind), some of these arms are broken off and the snowflakes become simpler, more rounded forms. It is after this point that the more complex metamorphoses begin to take place.
  • In constructive metamorphosis, where there are large temperature gradients (typically in excess of 1°C/10cm), water vapour is produced by sublimation at warm grain surfaces, and is deposited at colder surfaces. Vapour transport occurs through the pore spaces and the direction of the vapour flow is down the vapour pressure gradient (obeying laws of molecular diffusion).
  • Because the snowpack is usually close to 0°C at the ground (bottom of snowpack) and colder at the surface, the ice sublimates rapidly from the lower grains and is deposited on the bottom of colder grains higher up the snowpack in regular, stepped facets. These faceted grains bond poorly to one another and create a snowpack layer which is increasingly weak. If the faceting process continues, the grains form larger cup-shaped grains called depth hoar, which are weaker still.
  • Deconstructive metamorphosis takes place at lower temperature gradients, and results in rounded grains which bond well to each other and create a strong snowpack. Initially, the sharp pointed ends of crystals sublimate, and the resulting water vapour is deposited into the concave areas, changing the crystals into increasingly rounded forms. Like constructive metamorphism, the net effect is that water vapour moves from warm areas to cold, but at a much lower rate which results in a more homogeneous (and therefore smoother) deposition. This type of metamorphism will also take place at the contact points between grains; water vapour is deposited and forms necks between them, creating strong bonds and increasing snow strength. This last process is know as sintering

[edit] Concept Snow Water Equivalent

The most obvious and simplest snowpack property to measure is snow depth (zs). More often we are interested in the snow water equivalent (SWE, zm), or the depth of water that would result if the snow cover were completely melted. SWE can be computed from the depth of snow and the snow density. The density of water = 1000 kg m-3. To determine the depth of snow using snow water equivalent and density, use the following formula: [SWE] ÷ [Density] = Snow Depth (Density must be in decimal form. For example: 25% = 0.25)

[edit] Concept Snow Density

Snow density is relatively constant over an area, but depth may vary considerably. An efficient and relatively accurate estimate of SWE at a given area can be accomplished by making lots of depth measurements relative to the number of density measurements.

Snow density is relatively constant over an area, but depth may vary considerably. An efficient and relatively accurate estimate of SWE at a given area can be accomplished by making lots of depth measurements relative to the number of density measurements.
Snow density is relatively constant over an area, but depth may vary considerably. An efficient and relatively accurate estimate of SWE at a given area can be accomplished by making lots of depth measurements relative to the number of density measurements.

Image:Wt of 1 m3 of snow.jpg

  • Pure water density is around 1000 kg (or one tonne) per cubic metre or 62.4 lb per cubic foot.
  • Pure ice is just slightly less dense than pure water at about 917 kg per cubic metre (that is why ice floats on a pond or in our drink).
  • Snow on the other hand has less density because it contains more air in a given volume than the ice.
  • Heavily compacted snow, called firn, can be nearly as dense as ice (around 910 kg per cubic metre)

  • Newly fallen snow more typically weights in at 70 to 150 kg per cubic metre (4.4 to 9.4 lb/cubic foot), but this increases rapidly once snow is on the ground and it begins to compact due to wind, the addition of liquid water and its own weight.

Newly fallen snow more typically weights in at 70 to 150 kg per cubic metre (4.4 to 9.4 lb/cubic foot), but this increases rapidly once snow is on the ground and it begins to compact due to wind, the addition of liquid water and its own weight.

  • Typically winter snowpacks have a density of 200-300 kg per cubic metre (12.5-18.7 lb per cubic foot),
  • Vary with the moisture content (SWE) of the snow and that can vary across climatic regions.
  • Environment Canada reports that one cubic metre of old snowpack in;
    • Winnipeg, Manitoba typically weighs around 190 kg (419 lb)
    • Quebec City the same volume weights 220 kg (485 lb)
    • Whistler, BC (a wetter climate of British Columbia it is generally higher), 1 cubic metre may weigh 430 kg (948 lb).

Reference [11]

The figure here from the National Atlas of Canada shows the mean snowpack density across Canada for March. The spatial pattern is characterized by higher snowpack densities in warmer coastal regions and lowest densities over the boreal forest zone.
The figure here from the National Atlas of Canada shows the mean snowpack density across Canada for March. The spatial pattern is characterized by higher snowpack densities in warmer coastal regions and lowest densities over the boreal forest zone.

Environment Canada, 40 centimetres of wet snow on an area of 40 square metres -- like the size of a driveway -- will weigh 8,000 kilograms, or eight metric tonnes. Consider that these tonnes are not just pushed aside, but lifted a considerable height, this is great weight to move.

[edit] Concept Snow density

Boot Packing The mechanical reworking of snow to harden it and to prevent depth hoar formation, usually by a large number of people walking up and down the slopes. This affects deeper layers than ski-cutting. This compresses any depth hoar which has formed and helps prevent its formation by decreasing pore space. Boot packing increases snow density and strength. This method is generally limited to small areas due to the manpower required.
Boot Packing The mechanical reworking of snow to harden it and to prevent depth hoar formation, usually by a large number of people walking up and down the slopes. This affects deeper layers than ski-cutting. This compresses any depth hoar which has formed and helps prevent its formation by decreasing pore space. Boot packing increases snow density and strength. This method is generally limited to small areas due to the manpower required.

Snow density is commonly given as the density relative to water ( ρs/ρw), or specific gravity, expressed as either a dimensionless proportion, or as a percentage. For example, if we have a snowpack with;

  • an average depth of 1.0 m,
  • with a density of 35%,
  • SWE would be 1.0m x 0.35, or 0.35 m.
Condition Density
(dimensionless)
Fresh snow (v. calm, cold conditions) <0.05 to 0.05
Fresh snow (typical range) 0.07 to 0.15
Melting snowcover 0.30 to 0.40
Older, compacted snow (will support a person without skis) 0.30 to 0.35
Older, compacted snow(adult’s foot leaves slight impression) 0.35 to 0.40
Old, compacted, crusty snow (adult’s foot leaves no mark on surface) >0.40

[edit] A snow study on a ski slope

Impact of artificial snow and ski-slope grooming on snowpack properties and soil thermal regime in a sub-alpine ski area Reference Annals of Glaciology 38 2004 [12]

Simulated snow-depth and snow-density profiles for the ski slope (a) and the off-piste site (b) for winter 1999/2000. For the off-piste site, measured snow depths (black dots) are shown for comparison. With regard to snow density, a considerable increase was noticed throughout the winter season. The groomed ski slope had a density of 500 kgm^3 in mid-December and gradually compacted to become a mixture of hard snow and pure ice lenses with a maximum density of 700 kgm^3. At the off-piste site, we measured a density of 400 kgm^3 at the end of winter. The snow-density pattern with layers of low density in early winter and after new snowfalls, which later settled to layers of higher density, was reflected adequately by the model. Reference Annals of Glaciology 38 2004 [2]
Simulated snow-depth and snow-density profiles for the ski slope (a) and the off-piste site (b) for winter 1999/2000. For the off-piste site, measured snow depths (black dots) are shown for comparison. With regard to snow density, a considerable increase was noticed throughout the winter season. The groomed ski slope had a density of 500 kgm^3 in mid-December and gradually compacted to become a mixture of hard snow and pure ice lenses with a maximum density of 700 kgm^3. At the off-piste site, we measured a density of 400 kgm^3 at the end of winter. The snow-density pattern with layers of low density in early winter and after new snowfalls, which later settled to layers of higher density, was reflected adequately by the model. Reference Annals of Glaciology 38 2004 [2]
Relationship between snow density and thermal conductivity (a), and between snow density and snow hardness (b) measured on 12 January 2000 on the ski slope (black dots) and in the natural snowpack (grey dots). In (a) the model function for ksnow (Equation (3)) is indicated (solid line), representing the fitted parameters p1 and p2, and (dotted line) with the default parameter values (Jordan, 1991).The function suggested by Sturm and others (1997) is also shown (dashed line). Reference Annals of Glaciology 38 2004[3]
Relationship between snow density and thermal conductivity (a), and between snow density and snow hardness (b) measured on 12 January 2000 on the ski slope (black dots) and in the natural snowpack (grey dots). In (a) the model function for ksnow (Equation (3)) is indicated (solid line), representing the fitted parameters p1 and p2, and (dotted line) with the default parameter values (Jordan, 1991).The function suggested by Sturm and others (1997) is also shown (dashed line). Reference Annals of Glaciology 38 2004[3]
Simulated and measured soil temperatures at 0.03 and 1.0 m depth for the off-piste site and the ski slope. Reference Annals of Glaciology 38 2004[4]
Simulated and measured soil temperatures at 0.03 and 1.0 m depth for the off-piste site and the ski slope. Reference Annals of Glaciology 38 2004[4]


[edit] Concept Snow Strength and Snow Density

Digital Snow [13]

Detection of grain boundaries in snow from 3D images Grain boundaries play an essential role in snow mechanics. As the weakest link in the structure they have a huge impact on snow properties. We present a method to identify grain boundaries in three dimensions from volumetric micro-CT images based on their characteristic neck-like shape. The method will be useful to calculate the grain boundary surface and the effective bond thickness which can be used to simplify snow geometry as input for numerical models. Reference WSL Institute for Snow and Avalanche Research SLF see [14]

See WSL Institute for Snow and Avalanche Research SLF Picture gallery CT-Images [15]

Fresh Snow WSL Institute for Snow and Avalanche Research SLF Picture gallery CT-Images [5]
Fresh Snow WSL Institute for Snow and Avalanche Research SLF Picture gallery CT-Images [5]
Deep Hore Snow WSL Institute for Snow and Avalanche Research SLF Picture gallery CT-Images [6]
Deep Hore Snow WSL Institute for Snow and Avalanche Research SLF Picture gallery CT-Images [6]
Ice Crystals onto of Snow, from WSL Institute for Snow and Avalanche Research SLF Picture gallery CT-Images [7]
Ice Crystals onto of Snow, from WSL Institute for Snow and Avalanche Research SLF Picture gallery CT-Images [7]
Wet Snow, from SLF WSL Institute for Snow and Avalanche Research SLF Picture gallery CT-Images [8]
Wet Snow, from SLF WSL Institute for Snow and Avalanche Research SLF Picture gallery CT-Images [8]
Different Density of the snow Grains are represented by the different colors, from WSL Institute for Snow and Avalanche Research SLF Picture gallery CT-Images [9]
Different Density of the snow Grains are represented by the different colors, from WSL Institute for Snow and Avalanche Research SLF Picture gallery CT-Images [9]

[edit] Snow and Weather Changes


[edit] Moving Snow Around

[edit] Preparing Freestyle Competition Slopes, Tracks and Features

Preparation and maintenance of pistes Fauve, M.; Rhyner, H.; Schneebeli, M., 2002: Preparation and maintenance of pistes. Handbook for practitioners. Davos, Swiss Federal Institute for Snow and Avalanche Research. 134 S. see [16]

Preparation and maintenance of pistes
Preparation and maintenance of pistes

[edit] Also See


Return to Working with Snow, Snow and Weather Glossary or Freestyle Skiing



also see The FIS Freestyle Technical Officials - Introduction

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