The word avalanche comes from the Old French “avalanste” meaning “to let down” or “to go downstream.” Come along with Tidbits as we out-run avalanches!
• There are about 9,000 known snow avalanches per year in the western U.S., although unrecorded avalanches may top 100,000. About 100 avalanches per year cause serious problems in the U.S., and around 150 people per year are caught in avalanches worldwide, of which an average of 14 are killed.
• There are two types of avalanches. Loose snow avalanches occur when large loads of freshly fallen snow, which has not had a chance to settle, descend in an inverted V-shape. Slab avalanches happen when many successive layers of snow, which have formed various levels of cohesion over time, break loose over a broad area. Slab avalanches are responsible for about 95% of avalanche deaths.
• One scenario that often precedes avalanche activity is a long over-night snowstorm that deposits a great deal of snow in the mountains. In the morning, the skies clear and the sun shines upon the slopes, rapidly warming the snow, which settles and creeps downhill before finally breaking away.
• Snow layers are highly compressible, because 85 to 95 percent of a typical layer of new snow is composed of empty space.
• Temperatures of a snowfield will be lowest at the surface of the snow and highest near the ground. Because warm air can hold more moisture than cold air, this means that air at ground level in a snowfield may contain 100% humidity while air at surface level contains much less. As pressure forces air from the ground to the surface, moisture precipitates out of the air and condenses around snow crystals, making conditions ripe for avalanche.
• In between snows, rising temperatures may partially melt exposed surface layers which often re-freeze and create a slicker, less stable surface for the next snowfall.
• When an avalanche encounters a building in its path, the structure often explodes like a bomb hit it. This is because snow entering through broken windows and doors acts like a piston, generating enough pressure inside the building to blow it apart at the seams.
• Snow that is water-logged will avalanche on even the slightest slope, as was the case at a Japanese ski resort where an avalanche killed seven skiers who were practicing on the beginner’s “bunny slope” that had an angle of only ten degrees. But 90% of avalanche activity occurs at slopes of 30 to 45 degrees. Mountainsides that are steeper than 45 degrees produce no avalanches because they are so steep that snow will not build up.
• Snowfalls of 6 inches or less seldom produce avalanches. Snowfalls of six to 12 inches produce a few small slides. Snowfalls of one to two feet produce avalanches of considerable size. Snowfalls of two to four feet are very dangerous, and snowfalls greater than four feet produce major avalanches
• The rate at which the snow accumulates is critical, with a snowfall of three feet (90cm) that falls in a single day being more dangerous than if it falls over three days. A snowfall rate of one inch per hour or greater, which is sustained for ten hours or more, is a great hazard, especially when accompanied by wind.
• Avalanches are most common on slopes facing north, east and northeast. That’s because slopes that are shaded throughout the day undergo less melting and bonding that can make the snow layers stronger.
• About one-third of avalanche deaths are due to trauma, usually to the head and neck, from being thrown against rocks, trees, and other debris during the fall. The other two-thirds of the deaths are due to suffocation, with a mere 1% dying of hypothermia.
• If rescued within the first 15 minutes, 86% of victims survive, but if the rescue time stretches to 30 minutes, survival rates drop to 50% and lower. Only one out of every three victims will survive if buried for an hour, and only one out of ten after three hours. 75% of those who end up with a hand or foot sticking out of the snow survive.
• Twice as many people survive if buried face up rather than face down, probably due to the fact that the body and head will sink a little bit after coming to a stop, and this creates an air space allowing easier breathing. When face down, the mouth and nose easily become solidly packed with snow, while the warm breath creates water that turns to ice, further blocking passage of air.
• About 66% of those who have survived were rescued by members of their own party. Only 20% were rescued by an organized rescue team. A trained rescue dog can cover in a single 25-minute period what takes a team of 20 men four hours to search.
• One of the deadliest avalanches was when Hannibal crossed the Alps in 218 B.C. while leading the Carthaginian army from Spain in order to conquer Rome. His troops were walking on fresh snow that had fallen on old snow, perfect conditions for avalanches. In the end, 18,000 out of 38,000 soldiers were buried under snow, along with 2,000 horses and several elephants.
• Movie photographer John Hermann worked for Disney and was sent to Berthoud Pass, Colorado, in 1957 to film avalanches. On April 8, he set up his cameras in an area where a week-long storm had dropped 79 inches of snow. The skies had cleared; the sun came out; and the Colorado Dept. of Highways was going to use a howitzer to shoot explosives to release an avalanche so he could film it. Although this took place in an area that had not been hit by an avalanche in the previous 24 years, the slide was much bigger than anticipated, moving at 100 mph, snapping off 80-foot trees, and covering the entire highway. Hermann ran for his life but was buried under 15 feet of snow, killing him. Also killed was a highway worker whose job was to stop traffic during the avalanche. Hermann’s cameras were recovered and contained spectacular footage of the avalanche that killed him.
• Red Mountain Pass near Ouray, Colorado, has seen its share of avalanches, especially in the area known as East Riverside. In 1992, snowplow operators Dan Jaramillo and Eddy Imil were clearing snow from the highway from a previous avalanche when another avalanche hit. Their rig was buried under 15 feet of snow, killing Imil. Jaramillo found a snow shovel in the cab of his truck and was able to begin tunneling through the snow. Fifteen hours later, he broke through to the surface.