Written by: Brandon Levy, CAIC Central Mountains Highway Forecaster
Avalanches can happen at any elevation, depending on weather and snowpack conditions.
Colorado Avalanche Information Center (CAIC) forecasters use the terms "Below Treeline," "Near Treeline," and "Above Treeline" to describe where we expect avalanches or where an avalanche occurred. These terms refer to broad elevation bands and aren't always obvious or intuitive.
For example, skiing through trees could be "Below Treeline" or "Near Treeline." An open slope might be an "Above Treeline" face or a "Below Treeline" meadow. An avalanche can start "Above Treeline" and run all the way down to the valley floor.
It's important to understand the concept of these elevation bands so you can interpret the avalanche forecast, even though the "lines" aren't sharply defined.
Here's a breakdown of what each elevation band generally means in Colorado:
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Below Treeline (below ~11,000 feet) is the lowest of the three elevation bands. It extends from valley floors or the snowline up to where the forest starts to thin. Below Treeline describes low-elevation areas that get less snow and areas with less wind—either because they are at a lower elevation or sheltered by trees. However, open areas and slopes with sparse trees are possible Below Treeline.
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Near Treeline (~11,000 to ~12,000 feet) is the middle of the three elevation bands. It is a transition zone between dense forests and treeless alpine areas. It is the narrowest of the three elevation bands, extending only a few hundred vertical feet above and below the treeline. At times, slopes in this elevation band can be the most dangerous as they often have the deepest and most continuous snow cover, but can also harbor buried weak layers under deep wind drifts.
- Above Treeline (above ~12,000 feet) is the highest of the three elevation bands. This elevation band is the alpine area where trees don’t grow, where the most snow falls, and where the winds are the strongest. It begins where sparse trees transition into open slopes and extends through exposed ridges to the tops of the highest peaks.
For each of these elevation bands, it’s important to note:
- The elevation bands do not describe an exact elevation. Rather, they are a characteristic of the terrain.
- The exact elevations that characterize each elevation band vary by geographic location. (For instance, treeline is lower in elevation as you go north in terms of latitude).
- The elevation of each band varies locally and is not a constant elevation or width.
A general overview of a couple of slopes in the Tenmile Range. The “blurry” lines define the elevation bands.
Understanding these bands can help you plan safer travel through the backcountry, depending on the conditions.
Let's look at a hypothetical avalanche forecast:
CAIC forecasts CONSIDERABLE avalanche danger, meaning human-triggered avalanches are likely. The avalanche problem is "Persistent Slab," caused by wind-blown snow building up over a weak layer deep in the snowpack (like depth hoar).
The forecast's Aspect/Elevation distribution rose highlights that this problem is most likely in the "Near Treeline" and "Above Treeline" areas.
Aspect/Elevation distribution rose highlighting avalanches are most likely Near and Above Treeline on slopes that face West, Northwest, North, Northeast, East, and Southeast.
In this scenario, you could generally find safer travel by avoiding terrain Near and Above Treeline.
However, avalanche safety is never that simple — unless you avoid traveling on or under slopes around 30 degrees or steeper — and this scenario presents other considerations:
- While the avalanche problem is most likely at higher elevations, it's important to be aware that some Below Treeline slopes can still have wind-hardened snow over weak layers, posing a concern.
- Additionally, selecting safer Below Treeline terrain is complicated because dangerous slopes may be above and connected to your Below Treeline choices.
The final caution is to consistently stay at lower elevations and avoid unintentionally moving up to more sparsely covered slopes Near Treeline or Above Treeline. It can be particularly difficult to distinguish between Below Treeline and Near Treeline.
Use cues such as harder surface snow, pillowy drifts, wind-eroded surfaces, or sparser vegetation to recognize you are gaining too much elevation and moving into more dangerous areas.
Below Treeline terrain sheltered from wind impact offers safer options and probably better snow with our given hypothetical avalanche forecast.
The key takeaway is that avalanche conditions vary significantly by elevation. Understanding these elevation bands is critical for developing safe travel plans.
By increasing your awareness of elevation bands and knowing where you are and where you intend to go, you can significantly improve your safety while still enjoying the snow.