Old snow

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.