Understanding snow and avalanche
- New snow
- Old snow
Most skiers know that snow conditions can vary tremendously from place to place, and throughout the season. Snow can be soft and fluffy, soft and damp, porridgy, in icy grains, or very hard and icy. Occasionally, it's dry and powdery, and sometimes, even perfect! Although these conditions are all snow, they vary so much that it is worth understanding how and why conditions can be so different.
Snow is defined as ‘the aqueous vapour of the atmosphere precipitated in partially frozen crystalline form and falling to the earth in white flakes’. (Macquarie Dictionary 1998).
When the temperature falls to below freezing, drops of water are deposited on dust particles in the air and snowflakes form. Their size and shape depend on the amount of moisture in the air and on the temperature, but each snow crystal is hexagonal with unique features, The hardness and sharpness of these crystals determines snow properties. The lower the temperature, the drier and harder the snow crystals.
While falling, the snowflakes are worn down and rounded by the wind. When they reach the ground, a change process starts. As fallen snow gets older, it settles, further rounding off the crystals. The crystals are broken down into fine grains at a rate that increases with the temperature. At temperatures above freezing, the crystals start to melt. The higher the temperature, the wetter and softer the snow crystals, and the lower the temperature, the harder and drier the crystals. After a few days, the crystals have usually disappeared and been replaced by coarse grains or ‘corn’ snow.
Snow can be categorised according to its age and the effects of weather. Newly fallen snow looks like the traditional six-sided crystals. In Australia, this type of snow frequently falls in large ‘clumps’, and remains on the ground in that form for some time. Freshly fallen snow maintains its crystal structure for a long time, providing the temperature remains below freezing and the snow crystal structure is not broken by the wind. Most newly fallen snow is ideal for skiing on, since there is a good deal of air mixed in with the mass. The snow will compact easily and provide a smooth base for skiing. Newly fallen snow that is mostly air is known as ‘powder snow’ and is not all that common in Australia.
After some time the new snow will begin to break down and increase in density, becoming more humid (or wet). This coarse-grained snow is most common in Australia and is the result of having been through the cycle of partial melting and refreezing many times. The water content of this snow is high and the humidity of the snow structure will be almost 100%. An easy method of determining snow type is to take a handful of snow and squeeze it in your gloved hand. If the snow is loose and powdery or blows away when you open your hand, it’s dry. If it forms a snowball, it’s wet.
Snow lying on the ground consists of various layers, with different types of snow crystals in each layer. The differences between the layers result from differences in air temperature, humidity, and wind strength at the time of each snowfall. These factors determine how cohesive each layer is with the next. The process of these changes is not very important if the layers are on a level surface. On a slope, downward movement affects the snow cover as it settles.
The different layers of snow on a slope will move at different speeds as the snow settles. The snow near the surface will move down the slope more quickly than the snow near the soil. This causes tension throughout the snow cover and tension between the layers. If all the constraining forces in the snow cover balance each other, changes occur slowly and the conditions stabilise. But if a sudden change happens, such as a heavy snowfall, a storm, rapid temperature changes or even the added weight of a skier, the balance can be upset, resulting in an avalanche.
There are two main types of avalanche: loose snow avalanches and slab avalanches. Both kinds can involve either wet or dry snow and may involve parts of the snow cover, or all of it down to the bare ground. A loose snow avalanche is characterised by starting at a particular point and then spreading out progressively in a pear-shaped course. At the release point the gradient must be at least 35°. A slab avalanche has a distinct line of cleavage through the snow at right angles to the surface, and it achieves its full width of front immediately. A slab avalanche can occur on any slope above 25°.
Avalanche prediction is, at best, an inexact science. So many variables affect snow on different slopes under constantly changing weather conditions that it is very difficult to assess or predict avalanches accurately. High avalanche danger results from storms with heavy snowfalls, when a fresh snow fall does not bond with the underlying layer, which may have been coarse or icy. The unstable layer does not have to be the top or most recent layer. A very thin layer a metre below the surface, with little bonding to layers above and below, could be triggered into an avalanche by the weight of a skier. Slopes to the lee and cornices tend to be particularly unstable. Eighty per cent of avalanches occur during or just after storms. Most avalanches which trap people are triggered by those people.
One cubic metre of new snow weighs between 30 kg and 66 kg while a cubic metre of damp, coarse grain snow can weigh 400 kg to 600 kg. An area 20 m wide and 25 m long covered to 20 cm contains 100 m3 of snow, therefore this cover weighs between one half and fifty tonnes!
Although avalanches are not all that common in Australia they have occurred, and there have been at least two deaths in recent years. In the northern hemisphere, only about 20% of those trapped by an avalanche are rescued alive by an organised search. About 20% of people caught in avalanches are killed more or less immediately by shock or mechanical injuries. After an hour, there is only a 40% chance of survival, and it diminishes by half for every half hour after that.
If you find yourself in an area of high avalanche risk, proceed with great caution. Make sure you can release your pack, poles and skis quickly, by undoing straps, safety bindings etc, as these will drag you down or injure you if caught in an avalanche. Try to avoid areas which show any signs of previous avalanche activity. Avoid wide, open snow areas, and any vigorous turning or movements which will increase the load on the snow cover. Leave the danger area by going up or down the fall line, on foot if necessary. Cross any suspected avalanche area as high up on the slope as possible. Groups should cross one at a time, keeping a distance of at least 50 m between skiers. Appoint someone to watch the slopes above while the rest make their way across.
If you are caught in an avalanche, try to move to the side of its downhill path, and head for rocks, trees or any other feature which may provide shelter. Move your arms in a swimming motion to try to stay on the surface of the snow. If buried, your best chance is probably to curl into a ball and protect your chest and respiratory passages with your arms in front of your body.
Skiers in powder snow and avalanche areas tie a 10–15 m length of red cord or cloth around their waist which would help in the event of a search party trying to locate them if buried in an avalanche. Another aid is a portable transceiver radio which sends out a continuous signal. Any other skier not swept away can set their radio to receive signals, and aid in locating the buried person.
As a fifth of those caught in avalanches are killed instantly, survival chances are not good, but a prompt search by companions may bring you out alive. Statistics show that most people surviving avalanche burial are rescued by their companions. An externally organised search takes time to arrange, and crucial time is lost getting help.
If searching for someone buried, start your search at the bottom edge of the avalanche debris, looking for skis, poles or other objects sticking out of the snow. Try to follow the likely path the person may have taken, searching around fixed objects such as trees and rocks in the avalanche path, or where the avalanche may have changed course. Remove the baskets from your poles, and use them as probes, working back up in the direction from which the avalanche came. Always be wary of further avalanches. Any detour, however long, must be worth the extra effort if it means not being caught in an avalanche and potentially losing companions, or killing yourself.
Trundle I. 1991. Australian Nordic Ski Instructor Manual.
Crawford-Currie R. 1982. Cross Country Skiing. Pelham Books, London, UK.
Dostal G. 1988. Cross Country Skiing, 3rd edn.
Main L. 1980. Mountaincraft Mountain Safety Manual 3, 2nd edn. NZ Mountain Safety Council, Wellington, New Zealand.
Hurley O. 1975. The Snowshoe Book, 2nd edn.