The International Commission on Snow and Ice devised a classification system in 1954, updated in 1985, that allows better understanding of the structure and stability of snow cover. Various groups, from scientists to hikers to skiers to road maintenance engineers and farmers use the system to describe and determine the characteristics of snow cover in a given area.
This is an important function in understanding avalanche safety, seasonal snow cover, snow mechanics, snow hydrology, and the physics of snow. Even the shape of snowflakes can influence the stability of a snow pack and help determine avalanche safety and the ease of road maintenance.
Snow cover usually occurs in layers deposited during successive storms. Each layer will be more or less homogeneous. However, this can change due to wind patterns and events such as snow falling from trees. Ice layers also occur in a snow pack. These are either vertical channels, horizontal layers, or basal ice.
Snow is very porous, being composed of ice and air and, sometimes, liquid water. The characteristics of a mass of snow are determined by its temperature, its texture, and the proportion of ice, water and air present.
Density, or the weight of a particular volume of snow, is an important consideration, along with the wetness of the snow mass. Other factors are strength, strain or stress on the snow mass, hardness, temperature, and layer thickness. Load bearing capacity is an important consideration for hikers and skiers.
A coarse, grainy type of snow called depth hoar is often responsible for avalanche. This type of snow tends to occur early in the season. Because of the slippery nature of the grain, the layer of snowfall does not bond well to subsequent layers. With the weight of later snowfalls, the bond may fail and cause an avalanche.
To accurately predict the danger of avalanche, it is necessary to carefully record the depth and condition of each layer that makes up a mass of snow. The temperature, wind conditions, steepness of snow, wind direction, and weight of new layers of snow are other factors involved in predicting an avalanche.
Cracks shooting across the surface of a snow pack or small slabs shearing off are signs that a snow mass is weakening. Also significant would be the “whump” or hollow sound that a weak layer makes when someone walks across the surface.
In addition to the other qualities of snow, the international commission has produced a classification system for crystals of solid precipitation. It identifies seven types of snowflake and 3 other types of solid precipitation.
Irregular forms of snowflakes are the most common type. These occur when snowflakes adhere to one another so that the original geometric shape is difficult and often impossible to determine.
Stellar plates are the next most common form. These are produced between 5-28 degrees Fahrenheit (-2 to -15 Celsius). The form is flat and has 6 sides. Markings on stellar plates may be very intricate. They usually show branches that extend from the sides. These further subdivide into sectored plates, stellar dendrites (having longer branches with many smaller branches on them) and fernlike stellar dendrites. The fluffiest snowfall is composed of stellar dendrites.
Hollow columns are very small, six-sided columns of ice, hollow in the center. They may be open or closed on the ends.
A double plate is a hollow column that has formed a plate on each end. One plate is always much larger than the other.
Split plates and stars are a form of double plate in which part of one plate grows larger, and part of the other plate also grows.
Needles look like tiny bits of white hair to the naked eye. They form when the temperature is at 23 degrees Fahrenheit (-5 Celsius).
Triangular crystals form at 28 degrees Fahrenheit (-2 Celsius), proving that for every rule there is an exception. No one knows why these triangular plates form at this temperature. Sometimes the three corners sprout branches.
Twelve sided snowflakes form when a double plate snowflake twists so that one plate is lined up at a 30 degree angle from the other. These are quite rare.
The other three types of solid precipitation are sleet, hail, and graupel.
Graupel is formed when a snowflake encounters tiny droplets of super-cooled water vapor that adhere to its surface until it is covered with them and looks like a small snowball. Graupel is also referred to as “soft hail.”
Sleet forms when rain passes through a layer of cold air and freezes into small pellets.
Hailstones are created in large cumulous clouds when snowflakes are trapped in an updraft. They encounter water vapor that adheres to them and freezes. As the “stone” is bounced around by the air currents present in the cloud, it gets larger until its weight pulls it to the earth’s surface.
Using the classification system, various groups can more accurately predict how much water will be available during summer months due to the quality and size of a snow pack, the level of avalanche danger present, and other important facts related to winter weather.
References and further information:
http://nsidc.org/data/arcss045.html
http://www.hydrology.nl/ihppublications/178-the-international-classification-for-seasonal-snow-on-the-ground.html
http://www.crrel.usace.army.mil/techpub/CRREL_Reports/reports/Seasonal_Snow.pdf
