A closer look at how avalanches occur | VailDaily.com
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A closer look at how avalanches occur

Richard Chittick
Special to the Daily/Brad Odekirk A slide on Big Mike, an infamous avalanche path, shows the size and magnitude of avalanches in Summit County.
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Snow can’t slide until it has fallen on a mountain, so the weather that creates snow is just as important to causing avalanches as any other factor. Simply put, an avalanche begins as snowflakes in the sky.

In Colorado, snow reaches the ground after traveling hundreds of miles from the Pacific Ocean. By the time it gets here, it falls with very little moisture content. It then sets up in what is known as a continental snowpack that is far different than the wet, heavy snows of the maritime snowpack found in coastal ranges such as the Cascades of the Pacific Northwest or the Sierra Nevada in California.

While flatland areas receive precipitation as a basic function of the way weather works, mountains such as the Colorado Rockies get a boost from a process known as orographic lift. According to Scott Toepfer, a weather forecaster for the Colorado Avalanche Information Center, as a passing weather system rises over a high mountain ridge, the system cools down and unloads its moisture.



This helps explain how a single storm can dump 10 inches in one part of a county while laying a small dusting across another. “It all has to do with orographic uplift, which is how we get the majority of our precipitation,” Toepfer said. “You have to get air to lift and then cool to produce rain or snow.”

Conditions are particularly favorable when high pressure systems set up north and south of Colorado and funnel storm systems straight into the state.



“It’s called the Pineapple Express when they come in straight off the Pacific,” Toepfer said. “It doesn’t happen very often, but when it does, that’s when you lick your chops and know that it’s going to snow hard and snow well.”

‘Silly Putty’



Once the snow is on the ground, it begins to change in many ways. Immediately, the weight of the snow on itself causes flakes to bond to each other, creating the fundamentals of snowpack.

Then many forces go to work.

“Snow is visco-elastic,” said Brad Sawtell, a regional forecaster for the avalanche center who studies slides in Summit County. “It’s like Silly Putty.”

The snowpack can bend and become deformed when the pressure of the top layer of snow fights with gravity. Then pressures can build up or relax within the snow, which can, in turn, increase or decrease the avalanche potential.

The snowpack also tends to form in layers. Some layers are created simply by a single storm’s snowfall while others can be created by wind deposits or even the sun forming a crust on the snow.

“Each layer is a unique one on top of or below another,” Sawtell said. “The problem, however, is that snow is white. Imagine if we could color code snow.”

These layers can be complicated by metamorphic processes which change the consistency of the snow by changing the shape of the crystals that make up the snowpack.

A variety of forces can cause crystals to lose their arms and intricate detail. The crystals turn into microscopic blocks of ice which are known as faceted snow crystals. These layers can fail easily and then act as ball bearings under a slab.

Yet even more layers can form when the ice melts directly into vapor either from solar radiation above or from heat radiation from the ground below. This vapor pressure then can travel through the snow, getting trapped under other layers and once again rotting the snow into faceted layers.

Another issue is the temperature gradient of snow. While it may seem intuitive to think of all snow as frozen, the temperature can vary dramatically under the surface while never going above freezing.

According to Sawtell, a temperature change in the snow of more than one degree every 10 centimeters can easily produce a layer of faceted snow.

Toepfer added: “You can get a big change in temperature from the surface of the snowpack to the ground and it produces that sugary stuff.”

Steep slopes

The next ingredient in avalanche potential is the terrain on which the snow lies. Terrain between 30 and 45 degrees is the most dangerous. Angles below 30 degrees aren’t very conducive to avalanches, while those over 45 degrees rarely hold enough snow to slide.

Sawtell points out that this rule is useful in route finding, as terrain that is flatter than 30 degrees isn’t as likely to slide. Sawtell recommends carrying an inclinometer on every backcountry expedition for determining slope angles.

“It’s sort of like vacationing – you don’t leave home without an American Express card. Don’t go into the backcountry without an inclinometer,” he said.

Toepfer agrees, saying, “Terrain dictates if you’re getting yourself into trouble. An inclinometer is a really important tool, and you also need to take your brain in there.”

Types of triggers

The final ingredient of a slide is a trigger. Some triggers are natural, while others are man-made.

A trigger can be as simple as one snowflake that sends another snowflake tumbling into another and then another. It can be as complicated as a snowmobile breaking free a slide that a skier couldn’t.

Destructive avalanches typically fall into one of two types. The first is a loose snow avalanche that starts at a single point and can gain more mass and depth as it gathers more snow in its path. It is typically recognized by a V-shaped avalanche path that spreads out from the starting point into a large path.

The other type of slide is a slab avalanche. This is the familiar form of slide in which a slab of the snowpack breaks off and slides down the mountain. Such a slide is generally marked at the upper limit by a fracture line and a large snow wall where the slab broke free, which is known as the crown.

Slab avalanches can be triggered by something as simple as the slab’s own weight or backcountry travelers. Even a collapsing cornice, the large overhanging wind formations found on steep ridges, can start a slab avalanche, as can a loose snow avalanche that gains enough mass.

Slabs can be small, on the scale of a yard or two, or extraordinarily large, reaching a mile or more in width, encompassing entire mountain sides.

Spring warmth causes even more danger

Brad Sawtell of the Colorado Avalanche Information Center foresees this spring as being especially vicious. As surface snow melts and percolates through the snow, the density of the snow increases dramatically, as does its weight.

“I have a feeling we’re going to have a very accident-prone spring,” Sawtell said, especially on south and southeast facing aspects. “Maybe it’s time to steer clear of the backcountry until the transition from winter to spring is done.”

A slide that occurs in these conditions is called a wet avalanche and can cause catastrophic damage as the heavier snow packs a more powerful punch.

Spring temperature variances can also cause instability in the snowpack when the layers of water trickling through the snow freeze into a solid sheet of ice deep. Deep inside the snowpack, it gives a potential slide a surface with little restriction.

“It’s like wrapping a mountain in Saran Wrap,” Sawtell said.

Eventually, the cycle of warm spring days followed by cold nights can strengthen the snowpack, when the layers of ice are connected by a lattice n the snow. Until then, conditions can continue to deteriorate.


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