Know the Snow: Sometimes, less is more

Back in my early avalanche days, I used to collect every piece of information I could find before I made a decision to ski or not to ski a slope. Over the years, as I gained more experience, I have reduced the amount of information I seek to compile a reliable forecast. In hindsight trying to juggle so many pieces of information in my head probably confused me more than helped me to come up with good slope stability assessments.


Today, when obvious signs of instability do not present themselves, I more often than not look to see if the slope is steep enough to slide, if there is a weak layer in the snowpack and if this weak layer was recently loaded; if so, how much and how fast.

This type of evolution in the decision making process is not unusual. The ability to sort the important pieces of information out of a whole collection of the available data is called “situation awareness.”

Developing situation awareness is a normal process that comes with experience in many fields. Still the question remains, am I missing something?

In the last few months, avalanche folks have started to talk about the contribution of surface warming and cooling to snowpack instability. One of the reasons for the renewed interest in surface temperature change was an unusual avalanche cycle in Utah’s Wasatch Range. At the end of February and the beginning of March last year, a heavy snowfall was adding stress to an already weak and loaded snowpack layer. As result, several natural avalanches occurred during or shortly after the substantial snowfall event. Natural avalanche activity slowed down dramatically the next day. However, on the third day, temperatures rose and a new wave of natural avalanches started. This second wave of natural avalanches was about four times larger than the one, which occurred two days earlier.

Drew Hardesty, from the Utah Avalanche Center, took a hard look at this event and could not find any weather event that could contribute to the avalanche cycle three days after the snowfall, other than the rapid warming.

To my knowledge, cooling related avalanche occurrences are not well documented and, to be honest, I have a hard time wrapping my head around why cooling can decrease instability or even trigger avalanches.

On the other hand, the effect of warming on avalanche occurrences is well documented. In fact, in Switzerland, where the best record keepers in the business reside, about one out of every five human triggered avalanches between 1970 and 1999 were during warm and sunny days with no wind. A few years ago, Karl Birkeland and I showed that fractures in weak layers tend to develop more easily when the snow surface warms and starts to become wet. Recently, Benjamin Reuter and Jürg Schweizer showed that slab warming can reduce the slab toughness and make it easier for weak layer fractures to develop.

So, am I missing something by not including surface warming and cooling into my stability assessment? Should we put more weight into snow surface warming or cooling when we assess the slope we are about to ski?

I am always on the lookout for what I may be missing. After all, the details I neglect are the ones that may come around to haunt me. If I chose to dive into the fine details, I use them to reinforce a “no-go” decision and not to justify skiing a slope.

The same goes for warming.

Warm sunny days can, and will, trigger wet slab avalanches in the spring. But not when the main avalanche problems are dry slab avalanches. For temperature changes to make a difference, the snowpack needs to be on the verge of instability as is. In other words: If you are worried that a slope will become unstable (with dry slab avalanches) in the afternoon due to surface warming, you probably shouldn’t ski or ride it in the morning! Furthermore, if you skied a slope in the morning and it avalanched (dry slab) in the afternoon due to surface warming you probably had a lucky morning and not a set of fine-tuned forecasting skills. We don’t need to track temperature change to figure out that we shouldn’t ski under marginal conditions. In most cases, the appearance of a weak layer and the load on it is enough to figure that out.


• Ron Simenhois is an avalanche forecaster who lives in North Douglas; contact him at


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