While snow and its effects on highways and city streets has been foremost on the minds of residents up and down Colorado's urban Front Range corridor since December, experts say snowpack levels in some mountain regions are below average and avalanche danger generally is no higher than it has been in previous years.
"People have snow on their minds because it's clogged up their driveways for two months," said University of Colorado at Boulder glaciologist and avalanche expert Richard Armstrong of the National Snow and Ice Data Center. "So I understand the trigger."
Armstrong, who traveled recently to southwestern Colorado, said winter has been typical so far in the San Juan Mountains and other Western Slope regions. "You can have some of the heaviest winters, but that does not necessarily translate into greater avalanche danger," he said. "You can have above-average snow with moderate-to-low avalanche hazards."
Armstrong and other experts say avalanche danger ebbs and flows throughout winter based on snowpack layering, temperature ranges, frequency of mountain storms, wind and melt-freeze cycles.
In fact, a coarse, grainy form of snow crystal known as "depth hoar" is often the culprit underlying avalanches. Because of its sand-like structure, depth hoar bonds poorly to other snow layers and creates a very weak layer in the snowpack. Conditions that produce depth hoar occur most often early in the season and the weaker layers are then buried under subsequent snowfall. All too often, depth hoar layers are discovered only after an avalanche has swept off the overlying layers.
Precisely because of the variability of snow conditions, Armstrong and others encourage skiers, snowboarders and other Colorado outdoor enthusiasts to practice good safety habits and to learn to recognize the signs of impending avalanche danger when visiting the high country. Colorado remains the most dangerous state in the nation for avalanche fatalities, with many of them set off by backcountry skiers and others who adventure out across snow-covered terrain.
Avalanches claim more than 150 lives worldwide each year. Over the past 15 years an average of about five to seven people have been killed by avalanches each winter in Colorado, with as many as 13 deaths in a single year, according to statistics from the Colorado Avalanche Information Center in Boulder.
Colorado has tracked avalanches since the 1950s. The state's avalanche season runs from November through May, depending on the season.
Ann Mellick, a CAIC avalanche forecaster, said snowpack tension -- which can trigger a snow slide -- generally is at its greatest during and immediately following a storm. On unseasonably warm winter days and in spring, snow on south-facing slopes can heat up, turning snowpack "into a big Slurpee" and increasing the odds of snow sloughing, Mellick said.
In addition, high winds can redistribute snow along certain slopes and contribute to increased instability and therefore higher avalanche danger, even with moderate amounts of snow, she added.
With many homes and structures along the Front Range still bearing signs of abundant snowfall from the past two months, roof avalanches could be a greater concern for some city dwellers, Mellick noted.
"The first big warm-up -- that's when you should have your hackles up," she said.
For more avalanche awareness information, visit www-nsidc.colorado.edu/NSIDC/EDUCATION/AVALANCHE/ and .