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Gather observations while moving through the terrain.
Gather observations while moving through the terrain.
Monitoring conditions helps us track relevant changes and incorporate them into our travel decisions. Specifically, we look for clues as to how weather and snowpack conditions are affecting the formation of avalanches in the terrain. Weather conditions that promote unstable snow conditions include heavy snowfall, recent drifting snow, and rapid warming. Signs that the snowpack is unstable include recent avalanche activity, cracking or collapsing (“whumphs”), overhanging cornices, or poor snowpack structure (e.g., a slab overlying a weak layer). These observations are vital for recognizing when we might need to simplify our terrain selection. The AIARE Fieldbook contains references to help draw important connections between the forecasted avalanche problems and the relevant conditions to monitor in the field.
Certain weather and conditions warrant stopping your group to discuss. These are highlighted in a quick reference in the Fieldbook.
Snowpack
Snowpack
Signs of Avalanche Activity
Signs of Avalanche Activity
Recent avalanches offer direct evidence of unstable snow. To better incorporate this observation into decision making, it is important to consider when the avalanche occurred, whether the release was associated with recent snowfall, wind, or warming, where it released in the terrain (i.e. elevation, aspect), and how the observed avalanche activity compares with the forecasted conditions. These considerations help you put the observed avalanche activity in the context of the anticipated conditions and corresponding trip PLAN.
Whumphing
Whumphing
Cracks that initiate at the tip of a snowshoe, ski, or snowmobile and that shoot across the snow surface indicate unstable conditions. Travelers may also hear or feel whumphing, a term that describes the sound of a weak layer collapsing under a traveler’s weight. Groups should avoid steep terrain if they encounter these signs of unstable snow.
Bruce Jamieson
Cracks in the snow surface due to the collapse of a weak layer.
Cornices
Cornices
Cornices form when wind transports snow over a ridgeline. These dense, wave-like formations of snow overhang the ridge, creating a hazard. Traveling in terrain with cornices is a challenge because it is difficult to predict where and when cornices will break. Additionally, a cornice collapse has the potential to trigger a slab avalanche on the slope below, increasing the destructive potential of the event. It is important for groups to limit their overhead exposure, give cornices a wide berth when traveling on ridge-tops, and recognize conditions that have the potential to bring cornices to their breaking point (i.e., intense snowfall, strong wind, or rapid warming).
Cornices can break off significantly further back from the ridge than expected.
A Slab Overlying a Weak Layer
A Slab Overlying a Weak Layer
Another sign of unstable conditions is an observation of the snowpack structure. Where a stronger layer (i.e., a slab) overlies a weaker layer, there is increased avalanche hazard. This condition is often reported in the avalanche forecast or by field observers conducting snow profiles or snowpack tests. There are a variety of ways to identify the presence of a slab over a weak layer, from quick tests with probes or hand shears to digging a snow profile and conducting snowpack tests. However, interpreting snowpack observations is challenging, requiring expertise collecting and analyzing data from multiple locations. Backcountry travelers with basic knowledge should follow the advice given in the avalanche forecast, even when they don’t see the signs of instability themselves. The best way to incorporate observations is to put them into the context of the avalanche forecast and to revisit the PLAN.
Weather
Weather
Heavy Snowfall
Heavy Snowfall
Significant accumulation of snow in a short timespan (12” in 48 hours) increases avalanche hazard. Snowfall accumulation can be tracked by measuring the new snow with a pole or probe. Accumulation rates of greater than 1” (2 cm) an hour is cause for concern. If there is wind, rapid accumulation can occur in drifted areas despite lesser snowfall amounts.
Colin Zacharias
Snowfall accumulation can be measured or estimated from visual clues or foot penetration.
Recent Drifting Snow
Recent Drifting Snow
Recent drifting snow indicates that wind slabs may be forming downwind of ridgelines or other terrain features. Wind moves snow from the windward slope and deposits it on the leeward side, creating cohesive slabs that can avalanche. Wind blowing across mid-slope features can have the same effect. Look for plumes of snow at ridge-tops, cornice growth, snow blowing off trees, and rippled or variable snow surfaces.
Wind moving snow and loading cross slope features
Wind moving and depositing snow over a ridge
Recent Warming or Rain
Recent Warming or Rain
Rapid warming from solar radiation, temperature, or rain increases avalanche hazard. Signs that snow is becoming unstable due to warming include pinwheels or roller balls on steep slopes. Exercise extra caution with warming right after a storm.
Alison Mehravari
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