This article is sponsored by our friends at Arc'teryx, who are dedicated to backcountry education and inclusivity in the outdoors.
There is a certain appeal about recreating in the backcountry during the winter as opposed to ski resorts. The snow, the solitude, and the exercise are just a few examples.
At the same time, avalanches are a very real and potentially deadly threat outside of the ski area boundaries that backcountry skiers and riders must take seriously.
Avalanche safety is comprised of four key elements:
- Human Factor
This article will explore the weather element of avalanche danger, which also has a carry-over effect on the snowpack (weather influences the snowpack) and the human factor (new snow is appealing to ski).
The first thing most people think of when it comes to weather and avalanches, of course, is snow. And rightfully so. Because without snow, there are no avalanches!
But there are other factors that influence how the snow behaves as it’s falling and once it’s on the ground. These factors include wind, temperature, cloud cover, snow density, and other precipitation types.
Snow and Precipitation
All other things equal, moderate to heavy snowfall tends to increase avalanche danger since the new snow adds weight to the snowpack. The heavier the rate of snowfall, the more the avalanche danger will increase.
A mere six inches of new snowfall in a 24-hour period can be enough to increase the avalanche hazard, while a foot or more of new snow in a 24-hour period typically results in a significant spike in avalanche danger.
OpenSnow provides 12-hour and 24-hour snow forecasts out to 10 days for pre-existing backcountry zones as well as custom points for any location and elevation in the world.
The density of the new snowfall is also an important factor to consider. High-density snowfall has more moisture content and adds more weight to the existing snowpack.
Low-density snowfall has lower moisture content for a given amount of snow and adds less weight to the existing snowpack. However, the dry and powdery nature of low-density snow also makes it more susceptible to wind loading.
The most dangerous setup during a storm cycle is when a storm starts out with low-density snow before transitioning to high-density snow. This leads to an upside-down snowpack in which heavy/dense snow is overlying light/dry snow.
In maritime climates such as the Sierra Nevada, Cascade, and Coast Ranges (and occasionally more interior ranges), precipitation can fall in the form of rain over ski terrain during the mid-winter months when warm subtropical air is involved. In addition to creating unpleasant skiing conditions, rain also saturates and destabilizes the snowpack, increasing the potential for wet avalanches.
Temperatures and temperature trends are important factors to consider before, during, and after a storm.
Before a storm, extended periods of dry weather with cold temperatures (<15ºF) weaken the snowpack, especially early in the season when the snowpack is shallow. These cold/dry periods set the stage for greater avalanche danger down the road once the snow eventually returns and accumulates on top of pre-existing weak layers.
During a storm, temperatures influence snow density. Colder temperatures typically result in low-density snow, while milder temperatures approaching the freezing mark result in high-density snow.
Monitoring temperature trends during a storm can help you determine whether or not the snowfall in a storm will be right-side up (low-density snow accumulating on top of high-density snow) or upside down (high-density snow accumulating on top of low-density snow).
OpenSnow’s hourly forecasts are useful in this regard, as you can pay attention to temperature trends alongside precipitation chances to estimate how snow density will change during a storm.
After a storm, temperature trends can also influence snow stability. If temperatures warm up rapidly after a storm (i.e. the day of or the day after), it can cause the new snow to quickly become denser and heavier, and thus more prone to avalanching. This is especially true if the temperature peaks near or above freezing.
Gradual temperature changes over the course of several days do not have as much of an impact on the stability of newly fallen snow. The same is true for temperature drops behind a storm.
Above-freezing temperatures also impact the snowpack, causing it to become saturated and eventually prone to wet avalanches – especially during the spring months when temperatures are warmer, the sun is stronger, and the warmth often persists for longer periods of time.
Wind is a significant factor during and immediately after storms as strong winds can load a slope up to 10 times faster compared to snow falling without any wind. Wind speeds of between 20-60 mph are optimal for transporting new snow.
Wind direction is just as important as wind speed, as the direction determines which slopes will be loaded. For example, a strong southwest wind will load new snow onto northeast-facing slopes at a faster rate, resulting in higher wind slab avalanche potential.
Wind directions and speeds often change during and immediately following a storm, so be sure to follow these trends as well. The hourly forecasts on OpenSnow can also be used to monitor wind behavior during storms.
Cloud cover, or the lack thereof, is a more subtle weather element that influences avalanche danger.
During the early winter season when the snowpack is still thin, a lack of cloud cover at night promotes greater radiational cooling in which relatively warm air near the surface “radiates” back into the atmosphere. As a result, clear nights typically result in cold temperatures, which can lead to weakening/faceting of the snowpack.
On the other hand, nighttime cloud cover will inhibit radiational cooling by acting as a barrier to warm air radiating into the atmosphere, instead deflecting some of this warmth back toward the earth’s surface. This is why cloudy nights tend to be warmer than clear nights, and thus less conducive to faceting of the snowpack.
During the daytime hours, solar radiation from the sun can significantly impact snow conditions and stability, especially later in the season (usually from late February on) when the days are longer and temperatures are warmer compared to mid-winter.
Strong solar radiation can form melt/freeze “sun crusts” that can act as a sliding surface during future storm cycles. Also, solar radiation in combination with warm temperatures during the springtime can result in wet avalanches when the snowpack becomes saturated.
Solar radiation is season-dependent and aspect-dependent. South-facing slopes receive the greatest amount of solar radiation, and north-facing aspects the least amount. Also, the sun angle (and thus, solar radiation) is weakest from November 5 to February 5, before strengthening thereafter during the late winter and spring.
OpenSnow’s hourly forecasts also display cloud cover percentage, which is useful for determining how much cloud cover could inhibit solar radiation.
If you’re looking for an essential weather tool to help plan your days in the backcountry, be sure to check out OpenSnow’s All-Access subscription, which includes a free 2-week trial (no credit card required).
Also, our friends at Arc'teryx are hosting their annual Backcountry Academy in Whistler, B.C. from February 9-12, 2023. The Academy offers an excellent opportunity for backcountry skiers and riders to learn more about safe travel practices in the backcountry while connecting with a community of like-minded snow athletes and outdoor enthusiasts.
This year’s Backcountry Academy is anchored by the Arc'teryx athlete team, riveting and thought-provoking films, and some of the most talented folks in the industry.
Arc’teryx recognizes the growing popularity of backcountry pursuits and will be offering something for every level including overnight hut trips to clinics that support new ski and boarders getting outside in a safe and respectful way with entry-level clinics.
This article was sponsored by our friends at Arc'teryx.