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, forecasts
indicate if anticipated future weather events are apt to make the snowpack 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 either snow or rain adds weight to
the snowpack; if added very quickly the snowpack may not have time to adjust to
the additional weight or load and it may fail and fracture. Rain falling on unconsolidated
snow will almost always lead to unstable conditions.
2) Wind Blown Snow can create 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 it adds weight to the existing
snowpack which almost often causes the snowpack to become more unstable and more
apt to avalanche.
3) Warm Temperatures can make the snowpack less stable in the short
term and more stable in the long term. Rapid warming typically causes snow layers
to creep 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 often saturate the snowpack with free water which increases
the likelihood of wet snow avalanches.
In the absence of these factors, especially when temperatures are around freezing,
the snowpack tends to become more stable and weak layers can strengthen.
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