Remember that the avalanche danger rating levels issued by regional avalanche centers are only general guidelines. Distinctions between geographic areas, elevations, slope aspect and slope angle are approximate and transition zones between dangers exist. No matter what the current avalanche danger, there are avalanche safe areas in the mountains. The following are factors that contribute to avalanche danger:
Avalanches most frequently occur on slopes of 30 to 45 degrees, but they may occasionally release from either gentler or steeper terrain. The diagram below illustrates the slope angles where avalanches most commonly start.
Dangerous slab avalanches are more likely to begin on convex (raised or curving outward) slopes but may also begin on concave (sunken, or curving inward) slopes. Short slopes may be as dangerous as long slopes, especially if an avalanche carries its victims into or over terrain traps like cliffs, trees, rocks, creeks or crevasses. Almost half of all avalanche fatalities result from slides running less than 300 feet (about 90 m) slope distance.
North-facing slopes may be slower to stabilize than slopes facing in other directions. South facing slopes are especially dangerous in the spring when heated by the sun. Leeward slopes, slopes facing away from the wind, are dangerous because this is where the snow collects and may form an unstable slab. Windward slopes that face the wind generally have less snow and are usually more stable.
Large rocks, trees, and heavy brush help anchor the snow. Smooth, open slopes without these natural anchors are more dangerous. However, avalanches can start even among trees. When the snow depth covers natural anchors, additional snow layers will slide more readily. Snow depth also determines the rates of internal snowpack changes, like the development of weak sugar-like snow. Snow depth over an upper-level rain crust can also affect how rapidly snow layers weaken or facet above the crust when air temperatures are low - shallower depths mean more weakening.
Make a habit of testing the layering and bonding of the snow structure by using ski or probe poles. Feel how the strength of the various snow layers changes as you push your probe through the snowpack. Test snow layering often as you move from area to area and use increasing caution if the resistance to a probe or pole decreases with increased depth. Snow structure and its stability can change significantly from slope to slope. Pay particular attention to very weak or very strong layers buried beneath the snow surface. The strong layers may act as a sliding surf ace for avalanches, especially if overlain by a weak layer.
Old Snow Surface
It is important to know the condition of the old snow surface when trying to assess developing snow stability. For example, cold snow falling on a hard, refrozen snow surface, such as a sun or rain crust, may form a weak bond and lead to a rapid increase in the danger.
Rapid changes in weather conditions (wind, temperature, snow/rainfall) can quickly change snow pack stability; therefore you need to stay aware of the weather and the trends in the weather at all times. The s now pack is very dynamic, and weather may change what was a stab le snow pack in the morning, into an unstable snow pack by afternoon.
Sustained winds of 15 mph or more, even during clear weather, may increase danger rapidly since such winds can quickly redistribute large amounts of loose surface snow. Snow plumes from ridges and peaks indicate that snow is being moved onto leeward slopes, which can accumulate ten times as much wind- blown snow as nearby sheltered valleys. This can quickly create dangerous wind conditions on leeward slopes and large changes in slope stability as you traverse fr om windward to leeward slopes.
Cold temperatures (well below freezing) tend to maintain an unstable snow pack, while warmer temperatures (near or above freezing) allow snow to “settle”, bond and strengthen more quickly, thus making the snow pack more uniform and stable. Prolonged cold air temperatures result in internal weakening of the existing snow pack through strong vertical temperature differences in the snow pack and associated development of new weakly bonded crystals (faceting or depth hoar). Meanwhile prolonged warm temperatures result in introduction of liquid water into snow layers that acts to lubricate and weaken snow pack bonds.
A high percentage of all avalanches occur during or shortly after storms. Be especially aware of storms that start cold and then warm during snowfall.
Rate of Snowfall
Snow falling at the rate of 1 inch (2-3 cm) per hour or more increases avalanche danger rapidly, and allows underlying weaker layers less chance to safely accommodate the new load.
Be alert to dangerous conditions with a foot or more of new snow. Remember that new snow depth may vary considerably with slope elevation and aspect.
Rainfall can rapidly weaken surface snow and overload buried, weak layers, sometimes causing avalanches to occur almost instantaneously with the start of rain. Rain may also percolate through the snow until it reaches an ice layer or a layer of smaller grains. It can then pool or lubricate the snow near or within this layer and produce large, wet-slab avalanches. During sustained heavy rainfall, a series of avalanches may occur on the same slope as progressively deeper snow layers are weakened or stressed. Wet- slab avalanches are also produced in the spring by strong sunshine or radiation through clouds that melt and weaken the snowcover. When a warm day is followed by clouds overnight that prevent the snow surface from refreezing, dangerous avalanche conditions may develop the next day when temperatures increase, and larger deep slabs may result.
While clear, calm skies during winter often allows for excellent recreating opportunities and great visibility, it may also produce surface hoar, the ice equivalent of dew. Fragile ice crystals may be deposited directly on the snow surface during clear nights, and may provide very weak attachment to subsequent new or wind transported snow. In such instances, very sensitive slab slides are possible.
Among the basic factors terrain, snow pack, and weather - lies the subjective human factor . All too often people ignore factual information, and make decisions that are neither prudent nor wise and are based on human issues. Some of these human considerations include:
Overconfidence, ego (pride or greed), stress, conflict, impatience, euphoria, hormones, lion syndrome (first tracks or rush to the summit).
Fatigue, cold, wet, schedules, equipment, injury, “gizmo reliance”.
Poor communication, poor planning, time management, tunnel vision, incorrect assumptions, disparate skill or physical levels, “herding instinct” (safety in numbers), “lemming instinct” (always follow), “horse syndrome” (head back to the barn), goals.
Do avalanche skills match travel skills? Other factors - good weather; familiarity smugness; positive reinforcement, it-won’t-happen-to-me beliefs, “flatland vs. avalanche eyeballs”.
- USDA Basic Principles for Avoiding and Surviving Avalanches: http://www.emd.wa.gov/preparedness/documents/Schools_L2L3_Handout_Ava_Avalanches.pdf