The chance that an avalanche will not occur, relative to a given trigger (usually the weight of a human).
The snowpack struggles through its life in a constant state of conflict between opposing forces—the strength of the snowpack and the stress on the snowpack. We can use a simplified analogy of a furniture mover trying to carry different kinds of household appliances. The furniture mover represents all the forces that hold a slab in place (crystalline bonds, friction and anchors). The household appliances represent all the forces that try to make the weak layer fracture (the weight on top of the weak layer and the shear stress on the weak layer). In very stable conditions, a big, burly furniture mover carries an empty cardboard box. In less stable conditions, the burly furniture mover carries a microwave oven. When things are very unstable, the Mr. burly is trying to carry a refrigerator. In other words, we have the same strength, but different stress.
Also notice that we can tweak the equation from the other side. The burly guy can carry the refrigerator or a wimpy little guy could try to carry the refrigerator (same stress, different strength). One way works but the other way doesn’t. When we do stability analysis, our job is to figure out where we sit in the conflict between stress and strength.
The bad news is that we're not only dealing with hundreds of different combinations between slabs and weak layers, but we’re dealing with something that's invisible. The good news is that most of the time the invisible becomes visible through observations and tests, but only as long as you know how to ask the questions and listen for the answers. In this chapter, we'll explore a number of simple observations and tests that can reveal snow stability.
Judging snow stability is a lot more like playing the Wheel of Fortune than using any rules of thumb or equations. In other words, you never, ever get all the pieces of the puzzle in front of you at one time--there's always missing pieces that you have to fill in with your imagination. The more knowledge and skill you have, the easier the game becomes. In other words, stability evaluation means INTEGRATING lots of different pieces of information--putting the pieces of the puzzle together in an organized way. It's a very serious mistake to latch onto just one test or observation and base your whole opinion on that one test--kind of like deciding to get married on the first date. Bad mistake. No, you need to shop around, do some homework, put them through a rigorous series of stress tests, travel for a month in a third world country, meet the in-laws, remodel the kitchen together. Then you'll have a lot better idea whether to invest your life in this person--or, in this case, invest your life in the slope you are about to cross.