Just a few degrees difference in slope steepness turned an otherwise safe backcountry family ski trip last week near Aspen fatal, the Colorado Avalanche Information Center reported.

The death of Arin Trook, an environmental educator with the Aspen Center for Environmental Studies, is the second avalanche fatality in the state this ski season.

Trook, 48, was staying in the Markley Hut the weekend of Jan. 21 just outside Ashcroft with his spouse and young child, another couple and their young child, Brian Lazar of CAIC wrote in the incident report said. Saturday and Sunday, the group skied on Green Mountain in pairs while the other two stayed at the hut with the kids.

The first two days of skiing “were enjoyable and uneventful,” Lazar wrote. Other than one “collapse” — the falling of an upper layer of snow that can precede an avalanche — the group saw no signs of instability.

“Their mindset was risk averse [sic],” the report said.

The morning of Jan. 21, Trook and another skier left the hut at 6:30 a.m. and skied a dense tree line about 35 degrees in steepness to the east of where they had safely descended two days prior. Trook’s partner skied part of their route first, stopping where he could still see Trook.

Trook had descended about 150 to 200 feet when he triggered an avalanche that was 24 to 28 inches deep and 400 feet wide, and ran 400 vertical feet, the report said. He was fully buried in a thicket of trees, which made the attempted rescue by his ski partner “difficult.”

The first skier was able to dig out Trook within 10 minutes of the slide, but Trook did not have a pulse nor was he breathing. Multiple rounds of CPR were unsuccessful.

Avalanches can occur in areas with slope angles generally between 25 and 45 degrees. Most slides, however, occur at between 36 and 38 degrees, according to data from the University of Colorado.

The slope that slid was about 5 degrees steeper than what Trook and his partner had skied without incident the two days prior. Thirty degrees or lower is accepted as “low-angle,” which CAIC advises skiers adhere to during elevated avalanche conditions.

“It may not seem much steeper, but in this case it made a very important difference,” CAIC wrote. “On some days and with some avalanche problems, the difference between 30 degrees and 35 degrees might not be significant. However, it can be significant when dealing with a Persistent Slab avalanche problem, especially when remote triggers and triggering from well down the slope is possible. It is not uncommon for humans to move into more risky terrain when there is no feedback to suggest things are dangerous. It is hard to recognize this slow drift when you are making a series of decisions throughout a day in the field.”

On the day of the accident, avalanche danger was considerable near and above treeline, and moderate below treeline. Persistent slabs were the only avalanche problem identified for the Aspen zone by CAIC.

Lazar also noted that Trook was using non-release telemark bindings. Only one of his skis was ripped off in the avalanche, which could have contributed to a deeper burial.

Trook was killed 16 days after an avalanche near Red Mountain Pass in southern Colorado took the life of Peter Marshall of Longmont. Marshall, 40, was buried in a remotely triggered avalanche during an advanced avalanche course with the Silverton Avalanche School.

The CAIC incident report said the group made a series of minor mistakes that led to the accident.

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