You’ll have this very popular area all to yourself in the winter. Mt. Ritter from San Joaquin Ridge.
Winter is a wonderful time to visit the backcountry! In the winter you’ll have the most crowded locations to yourself. Overused areas are refreshed with a tinsel of snow and places you’ve visited a dozen times are exciting and new! However the snow of winter adds challenges and requires additional skills. You’ve got to know how to stay warm and dry both while moving quickly on sunny and stormy days….and once your activity level drops in camp. Navigation must be performed quickly in stormy conditions with limited visibility…when the trails and even trail signs are buried. The snow that adds so much freshness to the backcountry also adds the deadly danger of avalanches. Before you head into your winter wonderland you’ve got to know the season’s snowpack, the weather you’ll be facing and how to assess the risk of avalanches in the terrain you plan to cover and make the best route decisions to minimize those risks.
This article presumes that the reader has a basic knowledge of how snow behaves. It assumes that you will travel in a group and that each group member will carry an avalanche probe, beacon, and metal-bladed shovel and can deploy them within 10-15 seconds. Avalanche probes, beacons, and shovels can save lives but they are no guarantee of safety. While it is critical to know how to use them successfully (and their use is beyond the scope of this article), you cannot count on your beacon and probe to save you! You’ve got to know where, when and how to travel and rigidly follow the rules you’ve set up. You must approach snow with the mindset that coming back alive is more important than completing a route or skiing a fun but dangerous line. That mindset is crucial.
Let’s dispel a few myths. First, most avalanches are not huge avalanches that rip out trees and send house-sized boulders cascading into valleys. They are small and release after storms and changes in weather. Second, most avalanches do not kill by burying their victims. They are more likely to kill by sending an unsuspecting skier over a cliff or by smashing them into rocks or trees. Third, most avalanches are not random events from a side ridge. They are usually triggered by their victim. Finally, having a probe and beacon does not guarantee your safety. They may save your life and certainly make it easier to find your body but alone their use means a life threatening event has already occurred. Let’s keep that from happening!
A small avalanche below a steep glacial slab. Going over the cliff above would not have been healthy.
Knowledge is Light! (Scientiae perfusorius est)
As with so much in lightweight backpacking it’s not what you carry but what you know that keeps you safe. Understanding that a beacon and probe won’t protect you means that you have to carry a cautious attitude and a working knowledge of snow safety into the backcountry when you visit. Fortunately these don’t add anything to your baseweight!
Sastrugi give evidence to the transformative power of wind.
Snowpack Evaluation and Transformations
Evaluating the snowpack begins with the first snows of the season. This is true even if your trip doesn’t happen until March. I follow the base layers of snow, the temperatures at which they fell and subsequently settled, and any rain especially early in the snow season. Snow falls as a series of layers. Once it falls it is changed by a combination of wind, sun, rain, a high temperature differential between the ground and the air, and warm temperatures. Each of these factors can increase or decrease the avalanche risk.
Thin snowpacks and high differential temperatures between the ground (32 F) and air promote the formation of dreaded “depth hoar.” Depth hoar is a type of reformatted snow that develops from the constant melting/refreezing cycles driven by large temperature gradients. Depth hoar has few sharp edges and behaves more like crushed ice. It has little cohesiveness. This is a loose crystal type of snow that is very unstable. Put a load of heavy snow on top and it is prone to slide. Early depth hoar development can leave unstable layers in the snowpack that can last the entire season. And you’ve got to know about them. Rain on top of snow often forms an icy layer that provides another unstable surface prime for avalanches. Really warm days before a cold storm can also lead to icy layers. Icy layers don’t hold snow well. New snow will often release from the top of an icy layer.
Sudden and new increases in the ambient temperature (often increased by the “reflector oven” effect of bowls) result in less bonding force within the snowpack, and this is a major time of increased avalanche risk. If you are traveling on the first unseasonably warm day of the season or any day above recent average temperatures, watch out! In the Sierra we often travel before sunrise and only until 12-1pm on warm days to avoid avalanches. (But if we’re making miles we may start again at dusk and travel well into dark.)
Fresh snow on top of this icy surface would not hold very well. It is a prime avalanche location.
When snow falls it usually does not stay where it fell. Storms are almost always associated with wind and inches of snow can be redistributed to the leeward side of ridges and gullies. These “wind loaded” slopes get way more snow deposition that the windward side during a storm. The snow is often packed together into “slabs” that behave as a unit and are much more prone to avalanches until that snow has settled. This is especially true if there is an unstable layer of powder, hoar, or ice beneath it. One of the important wintry backcountry skills is determining where the winds have deposited the last storm’s snow. Just by looking at the scene you need to be able to see how the snow has been influenced by the weather and where possible avalanche sites have formed. Also the wind direction needs to be determined from the current wind direction as well as the snow ridges behind rocks and trees; snow will build up behind trees and rocks on the side away from the wind. Cornices are also a good indicator of where new snow may be deposited and where possible avalanche areas may be found since they form on the leeward side of the ridge. In short, the leeward side of ridges and ribs are much more unstable for several days after a storm. Don’t travel there!
The illustration below (from Allen & Mike’s Avalanche Book by Allen O’Bannon and Mike Clelland) and the video shows how snow loads on slopes.
Loose windloaded snow pillows–could send you to a permanent sleep. Top of the “Golden Staircase” John Muir Trail.
Slopes with angles between 30 and 45 degrees of steepness are the most prone to avalanches. Most of our travel occurs on slopes less than 40 degrees so it’s critical to pay attention to slopes in the 25-40 degrees. Even slopes less than 20 degrees can avalanche–typically on very warm days. Steeper slopes usually shed their snow in small slides and shallower slopes don’t provide enough kinetic energy for snow to slide unless conditions are unusual. Many compasses have a clinometer and it’s worth having one to measure slope angles.
An in depth look at the most common slope angles for skope failure. (credit: Allen & Mike’s Avalanche Book by Allen O’Bannon and Mike Clelland)
Small features in the terrain also give clues to and dictate avalanche risk. Aside from the obvious tree cleaned avalanche path look for places where the slope angle steepens–this increases the force on the snow pack at the top of the steeper slope and is a more common area for an avalanche to triggers. If chutes and gullies collect snow and increase avalanche risk the top of a ridges or ribs are unlikely to be snow loaded and less likely to avalanche. Mature trees reduce the risk of avalanches–but traveling in trees is no guarantee of safety as summer travelers can attest from the massive piles of downed trees they sometimes see below avalanche chutes.
They all point one way and that points to how they got there: a massive avalanche! Trees are no guarantee of safety!
If you take an avalanche course you’ll be taught about digging a snow pit to assess for unstable layers. Digging a snow pit can help assess risk–but you need quite a bit of experience digging and interpreting tests for unstable layers. You also have to understand that a snow pit at one elevation and on one aspect of a slope may give very different information from one on another aspect 400 m away or 1000 ft below. Wind loading, sun exposure, and base layers can be dramatically different short distances apart. Don’t be overly reassured by snow pits!
Snowpacks behave differently in different regions. In the Sierra we get a very heavy “marine” snowfall and most storms are relatively warm. While this means we’re often skiing “Sierra cement” it makes for a safer snowpack. Generally the greater snow density means less blowing snow (but this rule is much less reliable at higher elevations) and more rapid consolidation of new snow. The biggest risks usually come within 48 hours of a snowfall and on the leeward side of peaks and ridges. Three to four inches of snow can translate into 1 to 2 ft in leeward gullies. Big avalanche risks often come late in the season with the first warm days. As the snow warms it loses its cohesiveness and it is less able to support the heavy and deep snow that has fallen. Snow on granite slabs is especially dangerous – melting water percolates through the snowpack and lubricates the snow sitting on smooth slabs. This is a recipe for disaster and is why travel on warmer days is limited to the cooler times of day.
In the Rockies the lighter volume of snow and dramatic temperature difference between the ground (32 F) and air often lead to “depth hoar”* and unstable layers. Many seasons will have persistent unstable layers that develop early and can make for very unsafe conditions through the entirety of the season. The very light powder that falls is a skiers dream: but if it blows into a gully and wind loads it this dreamy powder can quickly become a nightmare.
High in Cloud Canyon before a ripping descent down Tamarack Canyon. King’s Canyon.
So what’s the bottom line? What is a snowshoer or skier to do to enjoy the backcountry safely? In addition to the mandatory beacon and probe, here’s my approach to researching the avalanche risk and minimizing my chances of meeting one. (They’re as nasty as trolls!)
- Check the avalanche risk and weather forecast before you go. Avalanche.Org and NOAA.gov can give detailed avalanche and weather forecasts for specific regions. Don’t travel in avalanche prone terrain if the avalanche danger is more than moderate. If the general risk is lower know the details of what slopes and aspects have a higher avalanche risk. If the big avalanche risk is on the northeast aspect of slopes above timberline don’t travel there. Change your plans to minimize risk.
- Pay attention to what you see, feel, and hear where you’re traveling. If you see avalanches even in the distance–especially fresh ones–avoid similar slopes and aspects. If you hear deep “whumps” in the snowpack or trigger small slides go somewhere safer. A whump sound can indicate deeper unstable layers. Pay attention to the signs of wind direction and wind loading–and change your route to avoid wind loaded slopes.
- Travel spaced out across potential avalanche terrain with beacons on and probes ready to deploy.
- Take an avalanche assessment course. Learn even more about how to assess risk and how to (partially) mitigate the risk through specific route choices.
- Always choose safety over fun. The mindset and caution you bring into the backcountry is probably the most important guarantee of your safety.
Looks like a fun ski slope! I wonder why there are no trees. Weird?!
One other consideration
A final additional snow risk to consider is that of collapsing snow bridges – especially over lakes and rivers. (spoiler alert for “The Last Season”) Randy Morgenson, an experienced backcountry ranger, likely died this way in the Window Peak drainage of the Sierra Nevada. Water undercuts snow and especially on a warm day the thin and weakened snow bridge can collapse–and suck you down the river, under the snow. I visited this region a few years ago and identified the location Randy likely fell through the snow: just below an avalanche runout at a geologically determined bend/pond in Window Creek.
Water undercuts snow. Tyndall Creek is 8 ft down. This weakness is obvious, others aren’t.
A “crevasse” at the edge of a lake. Another place an unsuspecting hiker could fall in.
Winter is a great time to visit the backcountry. Don’t be chilled by the thoughts of cold weather and avalanches! Develop the knowledge and skills to stay warm, travel safely and you’ll open up months of time you can travel in the mountains. Use this knowledge to help make cautious decisions so you can enjoy your private paradise for many years to come!
Additional Information can be found online.
There’s a lot of fun to be had in the winter if you’re careful. Horse Creek Canyon, Northern Yosemite.