On slopes, danger comes in many shapes, angles, and sizes.
Slope angles between 30 and 45 degrees are the main originators of avalanches, although slides can start on inclines from 25 to 55 degrees (fig. 12-45). Angles above 55 degrees are generally too steep to collect a lot of snow, which tends to sluff off immediately after falling. Angles lower than 25 degrees are usually safe except for danger from
very wet, and usually slow, avalanches. Climbers commonly overestimate the angle of a slope. Get a clinometer, built into a compass or as a separate instrument, if you want to end the guesswork.
Slope shapes also affect the hazard (fig. 12-46). Snow on a slope that is straight, open, and moderately steep presents the most obvious danger. Snow on a convex slope, under tension as it stretches tightly over the curve of the hill, is more prone to avalanche than snow on a concave slope. Coming down a convex slope, you may not know how steep it will get until you're past the curve obstructing your view and are down on the face. Regardless of the shape of the slope, look at them all, large and small, for avalanche hazard. They all can slide.
Fig. 12—46. Convex and concave slopes
Bowls and cirques have a shape that invites wind-deposited snow. Once an avalanche starts, it often spreads to an entire face and dumps a great depth of snow into the constricted area below.
Couloirs are enticing, because they offer a direct route up a mountainside, and intimidating, because they are natural avalanche chutes. Always weigh the merits of these gullies against the dangers, and trust them only at certain times under certain conditions. Take into acccount the snow conditions and the amount of snow both in the couloir and adjoining slopes. A slide on a slope within the drainage system of a couloir can sweep the main channel. This slide, by undercutting every tributary on its descent, may pick up bigger loads or leave the tributaries poised and ready to dump their loads at the slightest provocation. Beware of lower-angled couloirs and canyons that are obvious collection areas for avalanche debris.
Forested slopes offer some protection but don't put too much trust in trees. Slides aren't likely to originate in a dense forest, but they can smash through from above. Look around as you climb. Do you spot shattered trees in avalanche fans, and wide swaths cut through old timber? This is evidence of large avalanches penetrating even thick forest. Does a slope grow only brush and small trees, all downslanting? This is probably a slope that avalanches so often that timber has no chance to grow. Are tree limbs missing from the uphill side in open timber? It might be the result of avalanches. There's little or no avalanche protection in open timber, such as you can easily ski through. Another type of vegetation—grass—can increase avalanche danger by providing a slick surface for a slide.
In a valley, the question to ask is whether an avalanche on the slopes above could reach you. The danger is obvious in a narrow-floored canyon, but large avalanches have swept for more than a mile across a valley, even climbing the opposite wall. As you trek through a valley, keep an eye out for damaged trees and other signs of avalanehing. Remember to pitch tents outside the potential reach of an avalanche.
A route beneath a cliff is a gamble unless you're convinced that conditions are in your favor. Cliffs above flat terrain may harbor cornices, icicles, or ledges piled high with snow—temporarily. All this stuff is ready to drop, and if it doesn't get your party, it could start an avalanche that will.
Safe at last! Well, perhaps not quite. A ridge, especially a forested ridge, is usually the safest route when the avalanche hazard is high. However, a ridge presents its own problems of cornices and of crests that can be too jagged to serve as a practical route.
Climbers pay attention to how sun and wind hit a slope for invaluable clues to avalanche danger.
The direction a slope faces determines how much sun it gets—and this tells a great deal about its avalanche potential. Here's how it works in the Northern Hemisphere, and of course it's just the opposite on mountains south of the equator.
South-facing slopes receive more sun, and therefore snow settles and stabilizes faster than on northern slopes. In general (with plenty of local exceptions), this makes south-facing slopes safer in winter. They also release avalanches sooner after a storm, so if they are avalanehing it's an indication that slopes facing in other directions may soon follow their lead. As warmer spring and summer days arrive, south slopes become prone to wet-snow avalanches and north-facing slopes may be safer.
North-facing slopes receive little or no sun in the winter, so consolidation of the snowpack takes longer. Colder temperatures cause depth hoar within the snowpack, creating weak layers. Therefore, in general (again with local exceptions), north slopes are more likely to slide in midwinter. In spring and summer, as south slopes become dangerously soft, look to the north side for firmer, safer snow.
Windward slopes—those that face into the wind—tend to be safer. They retain less snow because the wind blows some of it away, and what snow remains is compacted by the blast of the wind.
Lee slopes—those that face the same way the wind is blowing—collect snow rapidly during storms and on windy days as snow blows over from the windward slopes. The result is cornices on the lee side of ridges, snow that is deeper and less consolidated, and formation of wind slabs ready to avalanche.
The snow is full of clues to the prevailing wind direction. Cornices face the same way the wind is blowing. Deposits of rime, on the other hand, face into the wind: the larger the deposits, the stronger the wind. The steep faces of sastrugi and the rounded ends of snow drifts around trees and rocks also head into the wind (fig. 12-47).
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