...The Avalanche Triangle: Terrain...

Some snowy spots have frequent avalanches while just a stone's throw away, they never happen. To figure out which is which, the first question to ask yourself is...

It's common to broad-brush avalanche danger and assume that if it's High in one place, it must be High everywhere else, too. This just isn't true. Even on very unstable days, you can still go to the mountains but you must be selective about your route and learn to recognize the steep terrain that is capable of producing an avalanche.

Snow is a granular substance, like sand. If you've ever watched sand fall through an hourglass, you know that the pile at the bottom never gets any steeper, just bigger. That's because it can't. The angle of repose--or the steepest angle a granular substance can maintain without collapsing under the pull of gravity is 38°. This is the bulls-eye angle for avalanches, too.

Unlike sand, snow grains can stick together or interlock, so avalanches can start on slopes in the range of about 25-55°. Most avalanches start on slopes of about about 35-45°, which is quite steep. On lower angled slopes, the snow can only start moving if the slope is extremely heavily loaded. On steeper slopes, snow sluffs off continuously, rather than forming dangerous slabs.

      Some tips:

• People tend to over-estimate slope angles. 30° is about the steepness of a blue square run at a ski resort. 35° is a black diamond and 38° is a double black diamond slope. 50° is uncommon inside ski area boundaries. It feels like you are clinging to a wall and will unbuckle your boots as you make turns. Carry an inexpensive slopemeter to help you develop a sense of true slope angles. Many of these have scales for estimating slope angles from topo maps, too.
• If snow conditions are scary, you can stay on tamer slopes while continuing to enjoy winter gravity sports in safety.
• Windward slopes will usually also be safer.
• Since ridges tend to be lower in angle than surrounding slopes, they provide relatively safe peak access if you stay off the leeward cornices.

So, all you need to know before blasting out there is "is this slope steep enough to slide?" and if it isn't, then no problem, right? Hang on there!...

      There are some other things to consider when you are evaluating slope angles!...

If the slope you are playing on isn't steep enough to be an avalanche starting zone that doesn't mean you won't get hit by an avalanche that started above you and is just passing through. In fact, in a shallow snow pack, it is possible to trigger avalanches from apparently safe terrain far away from the starting zone. This was the scenario in the fatal 1992 avalanche accident here in the La Sal Mountains.

Sometimes runouts are obvious swaths through dense timber, but sometimes they are not obvious at all. Here are some ways to identify runouts:

• The expected runout distance of an avalanche starting zone is predicted by the alpha angle. The maximum avalanche for the path can be expected to run downhill until the angle from the debris toe back up to the starting zone reaches 15° to 25°, depending on the snow load and your geographic location.
• Vegetation clues can help you identify avalanche paths and runouts.

In the spring, the snow pack carries free water around the snow grains. Therefore, in a wet snow slide, the avalanche will behave more like water and can be expected to release and run over much lower angle slopes. There is an infamous case in Snowy Torrents where a Japanese climbing team was killed in their tents by a freak wet slide that initiated on a 10° slope.

Don't lie awake at night worrying about this kind of avalanche. Generally speaking, if the overnight temperatures drop below freezing for several hours and you get off the slopes before you sink in deeper than your boot tops, then you will never have a problem with this type of avalanche. Skiers will naturally do this, because these conditions are not fun. A snowboarder or snowmachiner will be more inclined to stay too long, however, because boards and sleds easily push through the mashed potatos. Ice climbers down in shady gullies need to consider the warming effect of the sun on feeder slopes high above.

But all steep slopes are not equal avalanche producers! Pay attention to the following factors to determine which slopes are more dangerous than others on a given day...

If the slope is steep enough to slide, then it eventually will when the snow conditions are right. But many slopes only avalanche every fifty years, so it's silly to avoid them all the time! The type and density of slope vegetation can tell you a lot about the frequency of avalanche activity on a given slope.

You should remember that once surface features like rocks and bushes are buried, they will not anchor snow layers above. It pays to know the summer terrain!

• Grass is a slippery surface that will not anchor the snowpack unless it is mowed, like at some ski resorts.
• Brush--unless it lies flat and smooth like dogwood, alder or willow--will anchor the lower snow layers in place. The same is true of deadfall timber, boulders and other terrain features.
• Dense trees will anchor the snow in place, but they must be so thick that they are annoying to travel through in order to provide adequate protection. They will not protect you from avalanches that start above the forest and run down into it, however.
• Vegetation, like bare rock, is relatively warm, so expect faceted crystal growth and hollow tree-wells.
• Sparse trees can act as stress-risers in the snowpack. They resist the downhill flow of the snow above them, creating tension.
• Avalanches frequently break across open slopes between trees or exposed rocks for these reasons.

      Vegetation can provide evidence of past avalanche activity, too.

The type and age of the vegetation on a steep slope indicates the size and return interval for avalanches in a given path. Swaths through dense trees are an obvious sign of frequent large avalanches. Dogwoods, willow and alder indicate regular seasonal avalanche activity. Young aspens indicate intervals of 5-15 years. Large, dense evergreens indicate a return interval of 50 years or more. Unusual snow conditions produce unusual avalanches.	Damaged or "flagged" trees indicate previous avalanche activity. They are yelling out that you are in a slide path!	Look at the snow around the base of trees for other clues. "Settlement cones" indicate that the new snow is settling and stabilizing; wind scouring should alert you to the possibility of wind-slabs.	Most avalanches happen during or shortly after storms. If the snow in the trees hasn't settled, the snow on the ground may not have had enough time to stabilize, either.

	Shady, northerly aspects take longer to stabilize after a storm and favor the growth of weak faceted crystals. In the Northern Hemisphere, these slopes are often recently glacier-carved and steeper than similar slopes of other aspects. Deeper snow on these shady, protected aspects tends to cover anchors, but trees will often be denser, as well.
Snow stability may be influenced by regional weather patterns. Here in the La Sals, storms usually come in from the SW, then turn to the NW as they leave. Our western aspects receive more snow due to orographic lifting, but quickly lose it due to strong winds...		...while our eastern aspects tend to collect wind-drifted snow. Morning sun and afternoon shade create crusts and associated weaknesses on these aspects.
	South aspects stabilize quicker under the influence of direct, warm sunlight, but are more prone to spring wet slides. South aspects often have less vegetation, fewer dense trees to anchor the snow and a shallower, crusty snow pack.

Higher slopes tend to get more snow which favors stability, but they are more susceptible to wind and are colder, encouraging faceted crystal growth in continental snow packs.

Lower slopes are warmer and stabilize faster after storms, but the shallower snow favors faceted crystal growth in continental snow packs. Lower elevations are more prone to rain-on-snow events that overload and weaken the snow pack or create icy problem layers.

Certain locations pose a disproportionate avalanche hazard. Terrain traps--like the permanently closed "Snake Pit" at Alta--are areas a person might overlook or be drawn into where even small avalanches can result in deep or unexpected burials. Examples are ravines, gullies, depressions and short, steep slopes.

Some terrain features cause increased stress on the snowpack and can act as the "sweet spot" where avalanches can be triggered. Many avalanche accidents in the past involved slopes that had tracks on them already. Steer clear of potential sweet spots and make sure that the snow is not weak, even if someone else got away with it!

• Watch out for convexities that lure you from basically flat terrain onto an avalanche slope in just a few feet. Here, the snow is stretched over the bulge.
• Exposed rocks and trees interrupt the smooth flow of the snowpack downhill, creating stress points. They also cause faceting that leads to weaker spots in the slab.
• Terrain features like ridges and gullies can collect wind drifted snow on their leeward sides, causing a heavily stressed area.

Always assess the consequences of taking a ride when evaluating avalanche terrain! Will you be carried down an open, flat bowl, or through a rocky cliff band? Will you be deposited in a deep gully where the snow will accumulate deeply, or onto a broad, smooth alluvial fan where the debris will be shallower?

Remember, it's not all or nothing! Avalanche hazard evaluation, like life, is a process of evaluating the risks and deciding if the rewards are worth the cost. You can always find a safer slope and come back to the dangerous one on another day.