Avalanche ahead.

Chris Lavell and Howard Wormuth recently went skiing on the off-trail slopes of Aspen, Colorado. As the two made their way down the mountainside, a huge white cloud suddenly rumbled overhead. In seconds, this avalanche, a large mass of tumbling snow, hurled toward them. The slide missed Lavell, but buried Wormuth--suffocating him beneath tons of snow. Wormuth didn't have a chance to escape, laments Lavell.

More than 90,000 avalanches occur every year in the U.S., mainly between November and March. Since 1950, when avalanche deaths were first recorded, these snow slides have killed 462 people. More than 80 percent of those deaths occurred in the backcountry--unpopulated, remote areas outside ski-resort boundaries.

The high rate of off-trail fatalities is no coincidence, says avalanche forecaster Bruce Tremper of the Utah Avalanche Forecast Center in Salt Lake City. Some avalanches occur naturally when heavy winds, rain, or the weight of new snow cause old snow to become unstable.

But 95 percent of avalanche disasters are brought about by people skiing, hiking, snow-boarding, or climbing in the backcountry. That's because their added weight on pristine snow can trigger an avalanche. The additional weight of Wormuth and Lavell skiing down the slopes, experts say, probably caused the snow to slide.

CRYSTAL CLEAR

It's hard to believe that fragile snowflakes can cause so much destruction. But as snow accumulates throughout the winter, Tremper explains, each layer of snow changes in texture and strength.

Once the snow hits the ground, its crystals begin to change shape. Some snow crystals bond together to form a uniform layer of snow, Tremper explains. These well-bonded flakes are cohesive--they stick together well.

But as the snow piles up, temperature differences develop within or between the layers of snow. For example, the snow closest to the ground will be warmer than the flakes exposed to freezing air.

The difference in temperature can cause some of the layers within a snowpack to weaken, says Tremper. Disturbing these weak layers can cause an avalanche.

SLIDE CITY

A hard-slab avalanche is one of the most dangerous types. This snow slide develops when wind, rain, or a sun crust--melted snow that refreezes--forms a rock-hard layer of snow (see diagram, below). But in order for this slab to slide, says Tremper, a weak layer of snow must lie beneath it.

One kind of weak layer forms when the water molecules in the snow nearest the warm ground move upward through the snowpack, says Tremper. The molecules rearrange themselves to form sugarlike granides, called depth hoar crystals, which are not cohesive--they don't stick together.

Another type of weak layer may form when sur ace hoar crystals, or frost, build up. These crystals grow on the snow's surface during cold, clear weather. New layers of snow piled on top of the fragile surface hoar can become unstable.

The weight of a single skier can cause any one of the weak layers in the snowpack to fall apart, says Tremper. Once the skier breaks up the snow, the slab and the layers beneath it start cascading down the mountain. These killer slabs can break up into cementlike blocks as big as a bathtub or a car. Slabs can speed down a mountain at 37 to 161kph (60 to 100mph). "Ninety percent of people caught in an avalanche are killed in slab avalanch," says Tremper.

Another kind of avalanche, called a loose-snow avalanche, occurs during or shortly after a snowstorm. In this snow slide, a new layer of snow cascades down the mountain like sand tumbling down a hill. The mass and speed of the powdery avalanche increases as it picks up more snow.

"Avalanches may look beautiful from a distance," says Tremper, "but they pack a lot of power." Like an earthquake or a hurricane, an avalanche can destroy everything in its path-crushing trees, buildings, and people.

SLOPE SAFETY

To forewarn skiers about avalanche risks, Tremper and his coworkers spend a lot of time testing snow on the slopes. He skis around the backcountry about four days a week collecting data on snow texture and strength. Collapsing or cracking are two sure-fire clues that the snow is unstable, he says. Another important sign that an avalanche might occur is the steepness of a mountainside. Slopes ranging from 30[degrees] to 45[degrees] angles are steep enough to produce an avalanche.

"I also dig pits in the snow to look at cross sections of the different snowpack layers," Tremper says. Rigid surfaces indicate strong or stable layers of cohesive snow, while indentations indicate weak layers of snow that are not cohesive.

Tremper also conducts a series of tests to see how much force is needed to make the snowpack shear and break away. For example, he isolates a column of snow and taps it progressively harder until the snow breaks away. "What I'm doing is creating a mini avalanche," he explains. By repeating these tests, Tremper can determine what might happen on a larger scale in a particular area.

Once he's finished making his observations, Tremper writes up a forecast and shares his information with off-trail travelers, like hikers, snowboarders, and skiers. "I tell them which areas are safe and what regions may be dangerous," he says. The Utah Avalanche Forecast Center posts advisories at ski resorts, on the Internet and over the radio.

Even though most athletes are aware of downhill dangers, some feel the challenge of backcountry sports outweighs the risks. Fortunately, Tremper wil continue to use his snow sense to track down powerful slides.