Avalanche equations that determine the relative risk at any moment of traveling in the wintertime backcountry are complex formulas with many variables that constantly change. Science-based formalized evaluations used by experts to calculate slope avalanche potential or in-season forecasts or hazard have 10 or more criteria that are assessed by region.
Ski areas try to take virtually all the avalanche risk out of snow ventures in that they do all the evaluation and abatement work on a daily, sometimes hourly basis before letting customers on the slopes. They mark unstable or unmanaged terrain as out-of-bounds, direct skiing to slopes where snow generally stays put and regularly release cornices or slab faces in steep, exposed zones. Most importantly, they constantly watch for changes in weather conditions and conduct regular avalanche evaluations.
In the backcountry, however, each individual is his or her own first line of evaluation and avalanche avoidance. The American Institute for Avalanche Research and Education (AIARE) advises all winter backcountry visitors to learn to use these red-flag indicators to identify and avoid avalanche risk:
1. AVALANCHES IN THE MOMENT
Avalanche hazard does not turn on or turn off abruptly, it builds and subsides over time. The brightest red flag or indication of hazard says AIARE specialists is the obvious: recent or observed snow movement. When you are approaching a jumping-off place, always look for evidence of new avalanching or small snow slides that have released naturally or have been triggered by human activity. Check with people coming out to get information about their experiences, what they saw and concluded. AIARE experts say snow movement before your eyes is cause to reconsider and change your plans for that day.
2. WEATHER CHANGES
Storms with snow accumulation rates of one inch or more per hour and/or winds strong enough to blow loose snow around are a second red flag. Rain or warming air temperature also are contributors to building avalanche hazard. These conditions could be happening as you arrive or could have occurred just prior to your outing. Snowpacks on every slope must have time to adjust to the increased weight, internal melting or transformation of snow or ice crystals within or at layer boundaries. Sometimes the snow pack must release or slide to stabilize. Avalanche risk is most frequently highest just after changes in weather.
3. STATE OF SNOWPACK
Cut into any snowpack and you will readily see the distinct layering of snow accumulations through the winter. The relative strength or weakness of bonds between layers is a key indication of snowpack stability or instability in that locale. There are two observable red flag indicators that snow slopes are ready to slide that visitors can hear or see for themselves. One is 'woomphing', that unique sound of snow layers or boundaries collapsing under weight (as you walk on it). So are shooting cracks that create mini slabs as the top snowpack layer is broken. Snowmobilers are advised to periodically "stop and walk" in each locale they motor through.
Case history studies of avalanche fatalities often find that these red flag indicators had or were occurring or were conspicuous and that they went unacknowledged or unheeded by victims.
In the mountain backcountry, you are responsible for your decisions. Risky behavior, influenced by 'summit fever' or 'shredding' bravado and abetted by inattention or ignorance will lead to tragedy.
Source: AIARE: the American Institute for Avalanche Research and Education