• avalanche
    An avalanche activates in Antarctica.

    Photograph by Jessica Lucas, MyShot

    Nature's Weapon
    During World War I, more than 60,000 Italian and Austrian troops died in avalanches while fighting in snowy mountain passes in the Alps. More than 10,000 died in a single day on December 13, 1916. Avalanches killed more soldiers in World War I than poison gas did.

    Train Wreck
    At the end of February 1910, a terrible blizzard struck the town of Wellington, Washington. The nine-day snowfall trapped two trains in the town's depot. On March 1, a thunderstorm unleashed an avalanche, sending a 10-foot wall of snow toward the town. The avalanche hit the train depot and sent the trains sailing 45 meters (150 feet) downhill, killing 96 people.

    During an avalanche, a mass of snow, rock, ice, soil, and other material slides swiftly down a mountainside. Avalanches of rocks or soil are often called landslides. Snowslides, the most common kind of avalanche, can sweep downhill faster than the fastest skier.

    A snow avalanche begins when an unstable mass of snow breaks away from a slope. The snow picks up speed as it moves downhill, producing a river of snow and a cloud of icy particles that rises high into the air. The moving mass picks up even more snow as it rushes downhill. A large, fully developed avalanche can weigh as much as a million tons. It can travel faster than 320 kilometers per hour (200 miles per hour).

    Avalanches occur as layers in a snowpack slide off. A snowpack is simply layers of snow that build up in an area, such as the side of a mountain. In winter, repeated snowfalls build a snowpack dozens of meters thick. The layers vary in thickness and texture.

    The bonds between the layers of a snowpack may be weak. Melted snow that refreezes may cause a slick coating of ice to form on the surface of a layer. A new snowfall may not stick to this slippery layer, and it may slide off. During spring thaw, melted snow can seep through a snowpack, making the surface of a lower layer slippery. Added weight or vibration can easily send the top layers of a snowpack hurtling downhill.

    Sluffs and Slabs

    There are two main types of snow avalanches—sluffs and slabs. Sluff avalanches occur when the weak layer of a snowpack is on the top. A sluff is a small slide of dry, powdery snow that moves as a formless mass. Sluffs are much less dangerous than slab avalanches.

    A slab avalanche occurs when the weak layer lies lower down in a snowpack. This layer is covered with other layers of compressed snow. When the avalanche is triggered, the weak layer breaks off, pulling all the layers on top of it down the slope. These layers tumble and fall in a giant block, or slab.

    Once a slab avalanche starts, the slab shatters into many separate blocks. These snow blocks break up into ever-smaller pieces. Some of the pieces rise into the air as a moving cloud of icy particles. The cloud races downhill at very high speeds.

    The thickness and speed of slab avalanches make them a threat to skiers, snowboarders, mountaineers, and hikers. In the mountains of the western United States, there are about 100,000 avalanches each year. Avalanches kill more than 150 people worldwide each year. Most are snowmobilers, skiers, and snowboarders.

    Avalanche Control

    Storminess, temperature, wind, the steepness of the slope, terrain, vegetation, and general snowpack conditions are all factors that influence whether an avalanche happens and what type occurs.

    Snow avalanches are most likely to occur after a fresh snowfall adds a new layer to a snowpack. If new snow piles up during a storm, the snowpack may become overloaded, setting off a slide.

    Earthquakes can set off avalanches, but much smaller vibrations can trigger them as well. A single skier can cause enough vibrations to set off a slide. In fact, 90 percent of avalanche incidents involving people are triggered by the victim or someone in the victim’s party.

    Currently, scientists are not able to predict with certainty when and where avalanches will happen. However, they can estimate hazard levels by checking on the snowpack, temperature, and wind conditions.

    Many ski areas employ avalanche control teams to lessen the danger by starting slides before skiers head for the slopes. At some ski areas, patrols use explosives to set off avalanches. Or they may blast hazardous slopes with a cannon to shake loose any large, new accumulations of snow.

    In the high mountains of Canada and Switzerland, special military troops are in charge of avalanche control. Many Swiss mountain villages protect homes from snowslides by building large, sturdy structures to anchor snowpacks.

    Dangers of an Avalanche

    An avalanche is one of the most powerful events in nature. A fractured mass of snow may flow down a slope or become airborne. As a large avalanche speeds down a mountainside, it may compress the air below it, producing a powerful wind that can blow a house apart, breaking windows, splintering doors, and tearing off the roof.

    Avalanches strike suddenly and can be deadly. In 1970, a massive avalanche of rocks and ice destroyed the town of Yungay, Peru, killing 18,000 people.

    If you are caught in an avalanche, the first thing to do is try to get off the slab. Skiers and snowboarders can head straight downhill to gather speed, and then veer sideways out of the slide path. Snowmobilers can punch the throttle to power out of harm’s way. If this is not possible, reach for a tree. As a last resort, try to “swim” up out of the snow. The human body is three times denser than avalanche debris and will sink quickly. This makes finding and rescuing avalanche victims much more difficult.

    If buried in an avalanche, try to clear some space in front of you to breathe, then punch a hand skyward. Once the avalanche stops, it settles like concrete. Bodily movement is nearly impossible. Most avalanche victims are rescued, but those who aren’t die of suffocation as the snow hardens and buries them.

    Avalanche beacons are the most common tools to help rescuers find avalanche victims. Avalanche beacons are “beepers” that emit consistent noise when activated. Beacons can help rescuers locate a buried victim more than 80 meters (262 feet) away.

  • Term Part of Speech Definition Encyclopedic Entry
    accumulation Noun

    a buildup of something.

    airborne Adjective

    transported by air currents.

    avalanche Noun

    large mass of snow and other material suddenly and quickly tumbling down a mountain.

    Encyclopedic Entry: avalanche
    avalanche beacon Noun

    device attached to people on mountains that emits a noise signal able to be detected by other beacons, even when buried in layers of snow. Also called avalanche beeper.

    avalanche control Noun

    intentional triggering of avalanches to produce frequent, smaller snowslides, when people can be cleared away to a safe distance.

    cannon Noun

    very large gun used for firing heavy projectiles.

    compress Verb

    to press together in a small space.

    debris Noun

    remains of something broken or destroyed; waste, or garbage.

    dense Adjective

    having parts or molecules that are packed closely together.

    dozen Noun

    a group of 12.

    earthquake Noun

    the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

    emit Verb

    to give off or send out.

    estimate Verb

    to guess based on knowledge of the situation or object.

    explosive Noun

    material that can quickly and violently expand due to a chemical change.

    hurtle Verb

    to move very quickly, often with violence.

    ice Noun

    water in its solid form.

    Encyclopedic Entry: ice
    incident Noun

    event or happening.

    landslide Noun

    the fall of rocks, soil, and other materials from a mountain, hill, or slope.

    Encyclopedic Entry: landslide
    military Noun

    armed forces.

    mountain Noun

    landmass that forms as tectonic plates interact with each other.

    particle Noun

    small piece of material.

    rock Noun

    natural substance composed of solid mineral matter.

    slab avalanche Noun

    dangerous slide of snow that falls as a large, flat whole (slab).

    sluff avalanche Noun

    small slide of loose snow.

    snow Noun

    precipitation made of ice crystals.

    snowpack Noun

    layers of snow that naturally build up during snowfalls.

    snowslide Noun

    large mass of snow tumbling down a mountain.

    soil Noun

    top layer of the Earth's surface where plants can grow.

    temperature Noun

    degree of hotness or coldness measured by a thermometer with a numerical scale.

    Encyclopedic Entry: temperature
    terrain Noun

    topographic features of an area.

    thaw Verb

    to melt, or turn from ice to liquid.

    throttle Noun

    lever or button controlling the amount of fluid, such as gas, that goes into an engine.

    troop Noun

    a soldier.

    unstable Adjective

    unsteady or likely to fall apart.

    veer Verb

    to lean or change direction.

    vegetation Noun

    all the plant life of a specific place.

    vibration Noun

    rapid movement back and forth, usually of small particles.

Tell us what you think