By Jonny Lupsha, Wondrium Staff Writer
On September 26, a skier went missing in the mountains of Nepal. An avalanche swept her off-course and her body was discovered two days later. Avalanches have as many causes as consequences.
Pioneering ski mountaineer Hilaree Nelson was killed in Nepal on September 26. She and her partner, Jim Morrison, were skiing down from the 26,775-foot summit of Mount Manaslu. Morrison said she inadvertently caused a small avalanche while skiing and was swept away by it, with poor weather conditions delaying a search for her.
Approximately one million avalanches occur per year. In his video series The Science of Extreme Weather, Professor Eric Snodgrass, Director of Undergraduate Studies for the Department of Atmospheric Sciences at the University of Illinois at Urbana–Champaign, dissects the reasons avalanches form in the first place.
Pineapple Express
“If you live in the Western U.S. or ever plan to visit, the most important weather feature you should watch for is the position of the jet stream,” Professor Snodgrass said. “The West has a distinct dry season, and April through December is normally very dry. By contrast, during the wet season of January through March, meteorologists in the Western states watch for the development of the Pineapple Express.”
The Pineapple Express, he said, is the name given to a specific branch of the subtropical jet stream. This specific branch starts in Hawaii and flows east and eventually north toward California. It’s an “atmospheric river” that connects Hawaii to the mainland. He also said that research has shown that during the rainy season, as much as half of the West’s total precipitation comes from atmospheric rivers.
“They are called rivers because they carry lots of moisture, but also because their clouds appear on satellite as a narrow band that flows across the world’s oceans,” Professor Snodgrass said. “When an atmospheric river sets its sights on the West Coast of North America, incredible amounts of rain and snow can fall.”
Where Do Avalanches Come From?
According to Professor Snodgrass, avalanches, which occur when large amounts of snow slide down a mountain, have several, possible causes. Some avalanches are triggered by winds. Others begin due to heavy snowfall caused by atmospheric rivers. Still, others begin due to shifting snow, or a thaw/freeze cycle.
“The type of avalanche depends on the snow conditions,” he said. “At high elevations, the temperatures are often cold enough to support fine, powdery snowfall. As feet of snow pile up, the additional snow becomes unstable along the mountain slopes and begins to slide, pulled toward the valley by gravity. Powder snow avalanches race downhill at speeds that can exceed 100 miles an hour.
“The air is filled with white dust.”
Fatalities due to avalanches have increased in the time that records have been kept. Nearly 30 avalanche-related deaths per year occur in the United States alone, constituting about 20% of global avalanche fatalities. More than 2,000 avalanches occur each year in Colorado alone. According to Professor Snodgrass, the Colorado Avalanche Information Center estimates that 70% of avalanche fatalities happen within four days of a previous accident.
The Science of Extreme Weather is now available to stream on Wondrium.