Weather is the carpenter that constructs the snowpack and makes the snowpack stable or unstable. Knowing what weather has occurred in the past can give us a sense if the snowpack is likely stable or unstable. Likewise, listening to or reading weather forecasts gives us some indication of how future weather events may affect the snowpack and make it more or less stable.
The following are weather conditions that almost always cause the snowpack to become unstable and liable to slide:
1) Significant Precipitation: Snow and/or rain adds weight to the snowpack. The harder it snows or rains, the more difficult it is for the snowpack to adjust to the additional weight or load and the more likely it is for the snowpack to avalanche.
- Rain falling on cold unconsolidated snow will almost always lead to unstable conditions and avalanches.
2) Wind: Wind blown snow often creates dense slabs on the leeward or down wind sides of ridges and other terrain features. When wind transports snow from one location and deposits it at another location (see photo to the right) it adds weight to the existing snowpack which almost always causes the snowpack to become less stable and more apt to avalanche.
3) Rising Temperatures: Warming temperature can make the snowpack less stable in the short term and more stable in the long term. Rapid warming typically causes individual snow layers to creep and can cause the entire snowpack to glide downhill (see the snow "gliding" off the old car to the right). Creep and/or glide add stress to the snowpack and often make it less stable and more apt to avalanche.
- Prolonged temperatures well above freezing melts the surface of the snowpack and which allows often allows free water to saturate the snow pack and weaken the bonds between individual layers and increase the likelihood of wet snow avalanches.
However, when prolonged periods above freezing are followed by prolonged periods well below freezing and when temperatures fluctuate frequently between just above freezing to several degrees below freezing, the snowpack tends to become more stable and weak layers tend to strengthen.
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