When I was a youngster, a couple of us went for a winter hike on a hill behind a ranch where we’d been hunting that fall. A few hundred feet up we walked across a snow-filled coulee that seemed adventurous and innocent, but soon “whumped” (the sound of the snow collapsing, a precursor to sliding). That scared us into turning around.
In January 1993 a boy was killed and two friends buried in an avalanche hiking under a cornice on Mount Jumbo on the outskirts of Missoula. In February 2014 a big slide in the same area crashed into a house in a residential neighborhood at the foot of the same mountain, burying three (who survived).
I have seen the hill a hundred feet above my house in Red Lodge break off a cornice and slide. All this is to say that snow slips (a European term for avalanches) can happen in unlikely places if conditions are right, and can be dangerous to unsuspecting people doing innocent things. More frequently we are reading about people who knowingly venture into avalanche danger to ski, snowboard, snowshoe or snowmobile and get caught when something big slides.
Every one of us who travels into the mountains to recreate or work in winter should have some minimal knowledge of what can be dangerous, and try to avoid that stuff. For example, what does a 30-degree slope look like and feel like; is there a ridge I can walk instead of crossing a couloir or steep snow field; is there steep snow above me such as a cornice; are there people or animals above me?
One general answer: it’s very hard for snow to naturally slide down a hill that’s less than 30 degrees (get a cheap slope meter and practice measuring and looking at and feeling the steepness).
Steeper slopes far above can slide and inundate shallower slopes below (slides can even cross valleys and come up the other side). Years ago an avalanche came all the way down to the Lake Fork Trail in the Beartooth Mountains, snapping off trees and brush, stopping just short of this popular route. Because you cannot see the steep slopes far above that section of trail no one suspects it is a danger zone.
Knowing the history of snowfall in the area since winter started is helpful. Was it big at first, then weeks or months of dry weather (like 2019-20 in the Beartooths)? Was the snowfall cold and light; or warm and heavy? Have there been temperature swings; wind; which direction was the wind?
How do you learn this stuff? Travel with someone you trust who has experience in the winter backcountry. Read the literature. Read it again. Check area avalanche centers on the internet. Take avalanche courses. In Level 1 you will learn some of this information. Then you will have a little knowledge, which can be a dangerous thing.
There’s a phenomenon I call the sophomore slump. In many activities people relax and let their guard down after they’ve learned a lot, but they are still nowhere near being experts. They don’t have enough experience. That’s when you take a Level 2 avalanche course (designed for serious folks like professional guides, ski patrollers, and others who are in the steep and deep a lot). In Level 2 professionals become very attuned to the social dynamics of an outing, one of the biggest avalanche hazards that get people into deep trouble.
A couple of things snow science tells us for sure: slides are unpredictable; people are involved, so their behavior can be the biggest factor.
Surf the web for avalanche courses and check area avalanche reports, like the Gallatin National Forest Avalanche Center at mtavalanche.com, for professional information.
Get out there, enjoy winter and pay attention.