Jan 18th 2023
Spotting Avalanche Danger
Learning to perform your own snowpack analysis and spot the signs of snow instability and avalanche danger is a critical skill to have in the backcountry. While weather reports and avalanche advisories can provide clear warnings or general descriptions of conditions, they cannot tell you if a specific hill is likely to slide. Individual testing and careful observation will give a more precise metric for avalanche danger in a particular location. Your observations can also increase your confidence in local advisories or reveal a danger previously unknown. We highly recommend that all backcountry enthusiasts know how to make basic observations and recognize the signs of avalanche danger. This guide will teach you how to spot avalanche danger and perform a rudimentary snowpack analysis.
Avalanche Ingredients:
Avalanches are the creation of three key elements: (1) avalanche terrain, (2) unstable snowpack, and (3) a trigger. If you remove any of these components, an avalanche will not occur. Careful evaluation of these three elements will determine the risk of riding in a particular area and test the veracity of local advisories and word-of-mouth.
Avalanche Terrain:
Simply put, Avalanche Terrain is anywhere an avalanche can start, run, or stop. So, where do avalanches start? Slides typically occur on Slopes between 30 and 45 degrees. Slopes less than 30º normally won't slide, while slopes steeper than 45º often can't support enough snow to build slabs. This doesn't mean that every slope between 30-45 degrees is dangerous. Always check your local avalanche advisories for details on what elevations and faces slides are expected, and learn to identify the red flags and visual clues of avalanche danger.
Pay close attention to Vegetation. Forest patterns and trees can help you spot slide areas and avalanche danger quickly. Missing or broken trees often indicate that large slides have ripped through the area previously. On the flip side, a grove of solid trees without signs of damage is a good sign that the place is safe to travel.
Other areas to avoid are Terrain Traps. Called "traps" for a reason, Terrain Traps are areas where slides can get hung up or stop – like boulders, trees, and gullies. Be mindful of where a slide will travel when it's triggered. Gullies, ditches, and ravines can fill with avalanche debris and leave a victim trapped under a tremendous amount of snow (remember, most avalanche victims buried under 6 feet or more don't survive). Rocks and trees puncture through a fast-moving slide and can cause traumatic injury. Nearly a quarter (24%) of avalanche fatalities result from trauma and injury. Also note lakes, rivers, and streams. When avalanches stop on ice sheets, their sheer force and weight is easily enough to break through the ice. It is entirely possible to be pulled underwater and drowned as a result of an avalanche. Typical Terrain Traps include ditches, gullies, and ravines; cliffs, boulders, and trees; and lakes, rivers, and creeks.
Unstable Snowpack:
Typically, snowpack can be described as layers of snow (deposited after storms or wind) stacked one upon another. The composition and fusion of snow layers largely determine the overall stability of a snowpack. Although there are many classifications for snow surfaces and layers, the critical identifications are weak and strong layers. Understanding the definitions of storm snow, surface crust, and slabs is also beneficial.
Relatively Weak Layers are made up of snow that fell or formed into loose crystals similar to sugar or beads – try making a snowball out of sugar. The flakes are icy, dry, and refuse to bond or stick. Have you ever tried to make a snowball with dry snow? Weak snow layers behave similarly. Since the snow refuses to compress and fuse, the layer becomes less dense and unable to support the weight of stronger and heavier snow deposits above. The more heavy snow found piled on top of a weak layer, the bigger the avalanche. Luckily, deep weak layers tend to heal faster as the weight and pressure can force the sugary deposits to compress and bond. In contrast, weak layers beneath a shallow snowpack or on shaded slopes can persist for months.
.jpg)
Relatively Strong Layers are the opposite of weak layers. They're composed of that nice, wet, fluffy snow we all love. It's clay-like in nature and can be compressed, stuck, and formed (like into a snowman). Because the snow is so dense and hard, it acts as an ideal foundation for additional layers. Ideally, a stable snowpack is composed of many strong base layers. However, similar to how weak layers can evolve into strong ones, strong ones can also transform into weak ones. The process occurs when dense snow layers dry out, crystalize, and rot over time. Frequent snowstorms diminish this problem by supplying the necessary weight, pressure, and moisture to heal a snowpack.
The immediate effects of a storm on a snowpack, both during and for hours or days afterward, can significantly influence the instability of the snowpack. This is due to the added stress of Storm Snow, especially if it has yet to bond to existing surface snow adequately. As the snowpack adjusts and compresses from fresh powder, avalanches and collapses are expected.
Surface Crust is formed when water or snowmelt freezes, creating a thin hard shell on the top of the snowpack. This is caused by melt/freeze cycles, rain, wind, sunlight, and pressure. Snow Crust is dangerous because it creates a hard bed surface for future avalanches or, when buried, can encourage the formation of a weak layer directly above it.
A Slab is a relatively strong layer of dense, cohesive snow directly above a weak layer or bed surface (like snow crust or the ground). Slab avalanches are among the most deadly types, as slabs increase the load on underlying weak layers. The larger, more dense the slab, the bigger the avalanche.
Utilizing their shovel and a snow saw, backcountry travelers can carefully excavate a small trench in a safe area of a hillside to analyze and determine the snowpack's structure and reveal any hidden dangers. Nearby hills facing the same direction with a similar slope will likely have the same snowpack composition. Avoid avalanche terrain with extensive weak layers and hard bed surfaces with large slabs deposited above them. Be extra careful during and after a storm.
Triggers:
Avalanches can be triggered naturally, through human activity, or remotely. You don't necessarily have to be on an avalanche slab to trigger the avalanche. Avalanches can be started from directly above a slope, next to a slope, or dangerously from a flat or gentle area beneath a slope. Be mindful of how avalanches are triggered in your area—you can find this information in your local avalanche advisories. For example, naturally triggered avalanches are more common during or immediately after a storm. Identifying the common trigger can help you determine the sensitivity of the snowpack.
Red Flags & Visual Clues:
Although local avalanche advisories provide vital information about current avalanche conditions, the snow is constantly changing. Backcountry travelers must know how to spot red flags. We've already discussed a few, like recognizing avalanche terrain, understanding snowpack stability, and identifying triggers. Now let's shift focus to some specifics.
Fresh Snow – we all love that pow pow. Unfortunately, 90% of human-triggered avalanches occur in fresh snow (during or after a storm), not to mention an increase in natural avalanches. Fresh snow causes a major spike in avalanche danger and necessitates extra caution.
Signs of Recent Avalanche Activity – are prominent warnings that avy danger shouldn't be taken lightly. If you see signs of recent activity (crown lines & any debris), take extra precautions. Note the elevation, direction/aspect, and slope of any slide, as similar hillsides have a high potential for avalanches.
Collapsing or Cracking – now that you understand the structure of a snowpack, you know that avalanches often fail when weak base layers collapse. If you see or hear collapsing or cracking, you are in immediate danger and should carefully vacate the area. It's common to feel the snow collapse beneath you, see surface cracking, or hear a "whumping" sound.
<!--
-->
.jpg)
Wind – the wind is elemental to avalanche formation. Wind erodes snow in such a way that once deposited, it packs into a heavy and dense layer of snow that can easily overload existing weak layers. This process creates what is known as a wind slab – a deposit of dense snow located above a weak layer or bed surface. Here's what to look out for: (1) eroded snow – rough looking, often hard, and challenging to navigate. Since eroded snow has lost weight and load, it is actually more stable. (2) deposited snow – smooth and round, pillow-shaped, hard on top, and flaky below. Deposited snow can sound like a hollow drum—the more drum-like, the more dangerous. It's common to see cracks shooting away from you or to frequently fall through the slab. Wind slabs are a volatile form of snowpack and a common cause of avalanches. This is because wind-deposited snow can accumulate faster than storm-deposited snow. Adding that much weight, that fast, to a snowpack creates tremendous stress and can cause massive failure in the weak layers below.
Spotting these warning signs and performing your own evaluation can save your life. Our guide covered the basics you need to know to navigate the backcountry safely. We recommend exploring this topic further by taking an avalanche course and utilizing the resources available at avalanche.org.