The Science Of Avalanches

Avalanches on average kill on the order of 30 people every year in the United States, and one season is often particularly dangerous: Springtime brings the nasty combination of prime avalanche weather and lots of climbers, skiers, snowmobilers, snowshoers and other outdoor enthusiasts taking advantage of warming temperatures and lengthening daylight. These often-massive, swift-cascading snow slides – devastating and potentially fatal for anyone caught in their path – aren't always easy to predict, but in many cases warning signs abound. Most fatal avalanches are triggered by their victims (or others in their party), so it definitely pays to know your Avalanche 101 before heading into those ravishing, thrilling, and – yes – hazardous heights.

Two general categories of avalanche are (1) loose-snow avalanches, also called point-release avalanches or, especially when small, sluffs; and (2) slab avalanches. Sluffs are typically surface-level slides that often result from fresh-fallen snow overcome by gravity and fanning downslope. Because sluffs usually release below a person who triggers them, and because they tend to be on the small side, they're often considered less dangerous than slabs, but loose-snow avalanches can still be plenty deadly: sweeping victims over drop-offs or into glacial crevasses, or burying equipment, tents, and trails. Very large loose-snow avalanches are called powder avalanches.

Slab avalanches – statistically the most dangerous by far – are generally larger and deeper than sluffs. They form when a slab of upper snow detaches from an underlying bed surface, typically because of a weak intervening layer or precarious contact between the slab and the bed. Notorious weak layers include buried hoarfrost, graupel (ice-glazed snow pellets) and depth hoar (loose granular ice crystals formed within the snowpack).

Another, broader classification is between wet and dry avalanches. Wet avalanches result when warm temperatures or rain-on-snow events permeate the snowpack with water. In most cases they're slower than dry avalanches (which may churn downhill at 80 miles per hour), and tend to follow terrain contours more faithfully. There are wet and dry varieties of both point-release and slab avalanches.

There are other species of avalanche beyond sluffs and slabs, meanwhile. When the wind-sculpted furls of snow overhanging cliffs or ridgelines (a.k.a. cornices) collapse and their frozen ruins spray downslope, cornice-fall avalanches result. Ice avalanches occur when icefalls – where glaciers spill over cliffs or especially steep slopes – shed significant debris. Both cornice-fall and ice avalanches can also trigger slab avalanches, whether through the force of their plunge on unstable snowpack or more indirectly and farther off by reverberations or spreading fractures.

The wet avalanches called glide avalanches, rarely triggered by people and tough to forecast, happen when the entire snowpack, lubricated by meltwater beneath, slides downhill. This "glide" often happens as a slow creep, but can also occur in a catastrophic release, avalanche-style.

Avalanches require a certain steepness of slope for gravity and weight to overcome friction – typically at least 25 degrees, although shallower slopes can generate avalanches if the snowpack has an exceptionally weak or slippery layer. Very steep mountainsides, meantime, tend to shed snow too regularly to build up snowpacks prone to big slab avalanches. Most avalanches occur on slopes between 35 and 45 degrees.

The bottom of an avalanche is its runout zone, where the collapsed snow slows and comes to rest. The runout zone often encompasses a gentler slope below a steeper one, or a basin or valley flat under mountain walls. The take-home message is if you're traveling or camping in the runout zone, you're still at risk of avalanche, even though you're not on a slope sharp enough to release one. You can also trigger an avalanche above if the snowpack is hard enough for fractures to propagate within it over long distances; the same goes for sparking slides below you or on adjacent slopes.

Gullies and chutes may channel avalanches loosed from surrounding mountainsides. And leeward slopes may be especially prone to avalanches, because prevailing winds drift snow over ridge spines and peaks and deposit wind slabs in their lee – plus the cornices that may develop above such slopes pose their own avalanche risk.

Terrain sets the stage for avalanches, but weather supplies the essential ingredients and conditions. Snowfall loads slopes with the white stuff; if they're overloaded, they'll avalanche. Cold, clear weather can form the surface hoar (hoarfrost) atop the snowpack that, buried by subsequent storms, becomes a weak layer that could prompt a slab avalanche sometime down the line. Rapidly warming temperatures or rain may destabilize the snowpack and trigger slides.

The rate and type of precipitation and the progression of temperature during a single mountain storm help establish the relative avalanche danger. If temperature falls during a storm, the snowpack (all else being equal) is likely to be more stable, as warmer, wetter, heavier snow falls first and overlying snow will be colder, drier and lighter. But if temperatures rise during a storm – as may happen with passage of a warm front, for example – the denser, wetter snow will pile atop lighter, looser layers, creating instability.

If snow falls faster than the snowpack can stabilize, avalanches are more likely. Snowfall of an inch or more per hour for eight hours or more increases avalanche risk considerably.

Straight-falling snow is one thing, but wind can pile up snow 10 times as fast. Falling snow and windy conditions together make a bad combination, but wind scours and drifts snow even without precipitation. If winds reach 10 or 15 miles per hour or so, avalanche danger edges upwards.

Let's round out this discussion of avalanche science with some sobering numbers, courtesy of the Colorado Avalanche Information Center. Last year, 12 people died by avalanche in the U.S.; 29 were killed in 2016, 11 in 2015 and 35 in 2014.

Between 1951 and 2016, the following activities resulted in the most avalanche deaths in the country: backcountry touring (skiing, snowshoeing, etc.) at 263, snowmobiling at 251 and climbing at 182. In recent years, snowmobilers have suffered the most avalanche-related fatalities of any recreation group.