A paper from CSAW.....interesting.

At this years CSAW, Ian McCammon presented a paper on slope angles and slope meters. I found it very interesting so I posted it here. This is the kind of SnowMagining I want this site to be about.

Everyone who has ever taken a level 1 avalanche class knows that most avalanches happen on slopes between 30 and 45 degrees. But 38 degrees is often cited as the “magic number” where most slides happen. I believe that 38 degrees is also the "angle of repose" but i know this depends on many factors. Is this a wives tale? Actually, this magic number can vary between 38 and 51 degrees, according to Ian McCammon, who spoke at the Colorado Snow and Avalanche Workshop (CSAW). The event is organized by the Colorado Avalanche Information Center (CAIC).

The magic number can depend on snow climate, avalanche type, and grain type, said McCammon, who studied several hundred avalanches since 1972 in which the slide was skier-triggered and the starting zone’s slope angle was actually measured. While continental and intermountain snowpacks slide most frequently at 38 degrees, maritime snowpacks slide most frequently at 40 degrees. Hard and soft slab avalanches are most likely to start at 38 degrees, wet slabs at 40 degrees, wet loose slides at 42.5 degrees, and loose snow avalanches at 51 degrees, he said.
Regarding grain type in the weak/failure layer, surface hoar creates the most headaches, with a median slope angle of 38 degrees but a lopsided tendency toward slopes even gentler than 36 degrees. Depth hoar fails most at 38 degrees, but with less frequency on those gentle slopes. Wet grains fail most frequently at 45 degrees.

The moral of the story, according to McCammon: Beware of surface hoar, even on slopes well below the “magic number” of 38 degrees, which isn’t so magic after all. His other caveat: most slopemeters are not accurate within +/- 2 degrees anyway, at least at the time he did his research. Bruce Edgerly, of Backcountry Access , told me just yesterday, that BCA's slope meter is spot on accurate and passes all test.