Terrain Tip No. 2: Be Wary of Wind’s Ways

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Terrain Tip No. 2: Be Wary of Wind’s Ways

Wind flow over mountainous terrain is arguably the most important driver of both snowpack variation and avalanche formation. When we discuss avalanche terrain near treeline and in the alpine, exposure to wind and sun are the primary factors.

Each weather system moves with a unique pattern, resulting in varied wind direction and speed, as well as varied precipitation intensity and duration. This wind flow combines with local terrain features and snowfall to create a dizzying complexity of scoured or wind-crusted slopes, corniced ridges and lee-slope deposits, and a layered snowpack of varying density and distribution.

To survive all this uncertainty, we need to watch and learn.

Photo No. 1: Given an adequate incline, slopes that collect drifting snow are most likely to produce avalanches.

The Avalanche Handbook succinctly states, “Given an adequate incline, slopes that collect drifting snow are most likely to produce avalanches.” Most of us think of the leeward slopes (below cornices, just below ridges, and the entrances of couloirs) as the most likely locale for windslabs. And we’re not wrong. But windslabs can also form lower on the slope, below convexities, or near slope-parallel ridges and other features that protrude from the surrounding terrain. 

The more time we spend observing and remembering what these wind-exposed slopes look like before, during, and after a snowfall and wind event, the better we will understand what terrain we will want to avoid. When it comes to wind’s effect on snow-covered terrain, expertise is synonymous with long-term local experience.

Photo No. 2: Wind-drifted snow can accumulate three to five times faster than falling snow.

The backcountry avalanche forecast provides important information on recent and forecasted winds. In the absence of this data, look closely at exposed ridge tops for signs of blowing or drifting snow. (The windward side of the mountain is called the “fetch,” and the lee side of the mountain is the “deposition zone.”)

Recent and forecasted wind speed and direction at ridgetop is critical information that helps you estimate the windslab problem. A speed of 20 kilometers per hour (12 miles per hour) is all that is required for drifting snow to occur. Light, new snow favors drifting. If the wind is blowing from the southwest, then snow is likely to be transported towards and deposited onto the north- through east-facing slopes.

Wind-drifted snow can accumulate in the start zones at a rate of three to five times that of fallen snow. Consider that snowfall amounts are observed and recorded in sheltered areas. A measurement of 15 to 20 centimeters (6 to 8 inches) of new snow sounds like good skiing and not much of a hazard. However, on the lee side of an exposed ridge, the newly formed windslab could be 60 centimeters deep (24 inches) from moderate winds overnight. Less snow transport occurs when the snow surface is moist or already wind-packed.

Drifting snow fractures snowflakes into uniform particles that, when deposited, quickly bond and form a firm wind drift or wind slab.

Photo No. 3: Cornices point towards wind loaded slopes.

In this photo, large, fresh cornices sit at ridgetop and overhang steep, shaded northwest and northeast slopes that have been recently loaded by southwest winds. Cornices themselves are a hazard worth avoiding—let alone the loaded slope below. 

Photo No. 4: Cornices, however small, still point towards a wind loaded slope.

This ridge in Colorado’s San Juan Mountains is exposed to frequent winds that both strip snow and load snow. Bare ground illustrates the windward, scoured, and shallow side of the ridge. The skiers in the photo study the leeward slope. They expect that the strong winds resulted in lee-slope loading and crosswind loading and scouring. The snowpack will be unpredictably layered with a varied depth and strength. The windslabs may be both hard to assess and also found farther down the slope. After discussion, they move to descend plan B, the lower angled basin farther east and out of this picture. 

Photo No. 5: The wind's story is told by the changing shape of the cornice.

This cornice reveals that the prevailing southwest winds shifted and came out of the northeast for some time. The scalloped shape of the cornice shows that recent winds eroded the cornice from both directions.

Photo No. 6

This slope is the shadowed one just below the ridge shown in photo No. 5. This slope was stripped early season by strong southwest winds, leaving a shallow, weaker snowpack. It was later gently loaded by northeast winds from the opposite direction. The eroded cornice seen in photo No. 5 provides a terrain clue that something had changed. The northeast winds are less common and may have caught the skiers by surprise.

Photo No. 7: Even experienced backcountry users can overlook crosswind loading.

This is not a lee slope. There is no obvious cornice at the ridge. The steep slope is sometimes affected by cross-slope winds. The wind-drifting and snow-loading patterns are harder to recognize. The winds sweep across the face in the direction indicated by the arrows (from left to right). Mid-slope features, such as a line of trees or a small ridge parallel to the fall line, can act like a snow fence, causing transported snow to be deposited on the lee side of the trees and load into the shaded gullies. It is from this mid-slope area that avalanches frequently release. 

Photo No. 8: Convoluted mountain ridges result in a high degree of snowpack variability.

This gullied sub-peak in the center of the photo faces north, a slope aspect that is usually sheltered by wind in North America. But on a mountain scale, it sticks out like a thumb near the head of a large valley, so it catches every breath of wind, regardless of what direction it’s coming from. Therefore, despite being in a deep coastal snow climate, it has a highly variable snowpack. Protruding rocks and bushes, slope-parallel ridges and bumps are exposed to the varying effects of the wind. The snowdrifts and cornices and exposed earth identify the localized wind pattern. An experienced guide would take one look and leave this slope alone. This slope frequently avalanches and never feels predictable.

Photo No. 9: Beware of on-slope protrusions like downslope ridges, bulging convexities, exposed rocks, and trees.

This photo was taken of a north aspect near treeline in the Monashees, which has an intermountain snow climate. The cat skiing groups are careful to avoid terrain features (marked with an X) that jut out from the surrounding surface as they may have wind slabs or weaker snow and trigger points. The guides led the skiers down through the trough-like features where the snowpack is deeper, more consistent, less layered, and with fewer trigger points. The ridge-like features and protruding convexities are more exposed to cross-slope wind and are therefore less predictable. (The photogenic Alaskan spines, where loose dry avalanches peel away on either side of your line, are often in a coastal climate with deep, dense snow.)

That’s not to say, however, that troughs, draws, and gullies always offer the safer route—sometimes they form windslabs from funneled and accelerated down-flowing surface wind. (Whenever wind is compressed, as in a gully, it accelerates. It is much like water in a river; the narrow sections always flow faster. The accelerated wind can deposit drifts of snow in the gullies, while the snowfield above remains unaffected. Experienced backcountry skiers read the surface of the snow to determine what’s wind-affected and what’s not.) Always be cautious when you’re looking at features that protrude in a surrounding slope; the safest route down depends on the wind.

Photo No. 10: With an already shallow mid-season snow depth, any feature that slightly protrudes up above the uniform terrain will be shallower and weaker.

This shot is of a north-northeast–facing alpine bowl in Colorado, which has a continental snow climate. Exposure to strong winds has resulted in a highly variable snow cover. 

Photo No. 11

This terrain is near the start of the same run pictured in photo No. 10. The skier sticks to the bowl-like feature with deeper snow and avoids the wind-scoured mid-slope ridge identified by the exposed rock visible in the foreground. The skier leaves a “buffer zone” (see Terrain Tip No. 1) of safer terrain between himself and the weaker, faceted terrain to the skier’s left. 

Photo No. 12

This shot is of a north-facing alpine bowl in an intermountain snow climate with significant recent wind effect. Even with a settled snow base of deeper than 4 meters (12 feet), backcountry skiers expect the local wind to play a role on the exposed alpine slopes. Here the skiers are careful to avoid triggering shallow, weaker snow formed by cross-slope and downslope wind-scouring.

Photo No. 13: This photo shows multiple hazards of cornice formation, lee slope loading, and crosswind effect. The wind has spoken; it's up to you to listen.

Summary

In this article, we discussed how wind flow over terrain is an important driver of snowpack variation and avalanche formation. We noted that a steep lee slope under a cornice formation is the most obvious wind-related terrain hazard. But we also recognized that backcountry travelers often underestimate the rate that wind-drifted snow can accumulate and the short time it takes for slabs to form. Only a few hours of wind drifting can turn a light snowfall with moderate winds into a potential hazard.

We also looked at an example of how the shape of the cornice can illustrate changing wind direction—and point towards recent wind loading. And we provided examples of slope scale features that protrude out of the start zone, get affected by the wind, and create a potential trigger point—especially in continental and intermountain snow climates, where faceting (leading to weak layers) is a common snowpack theme. Features such as lateral moraines, large convex bulges, and small ridges parallel to the fall line seem to be magnets for cross-slope and downslope wind-effect.

Photo No. 14: Avoiding the wind-effect does have its benefits.

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