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| Juneau Area Avalanche Advisory | ||||||||||||
| 2005-12-25 | ||||||||||||
| Mt. Stewart | ||||||||||||
| by Bill Glude, SAAC Observer | ||||||||||||
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| We got out for a quick run today on our day off but did snow tests because the conditions were interesting, so we are posting our notes and photos. We had the best of holiday presents in this global warming non-winter, fresh snow! We did not much care that it was only 5cm deep because it still made a smooth turnable layer over the icy crust from the last thaw and coated the trees so it looked wonderfully wintry.
The overnight snow level was about 550m and there is not enough base to ski even with caution below 650m, but there are some smooth slopes higher up if you know where the rocky spots are and carefully avoid them. As a moist riming mist fell out of the fog today, the night's light 110 kg/m3 powder was being moistened and coated by rime. It was quite sensitive in slope and traveling tests, cracking and sliding between and around our skis. This was the only sign of instability and slope test result though, and the surface layer was so thin it did not present a problem. In our snowpit tests we did an AK Block, a Cutback AK Block, and a Rutschblock. None fractured at all even with multiple hard jumps (AK7, CAK7, RB7). Sensitive but soft surface layers like this often fail to fracture in large block tests because the skis or snowboard sink right through them without applying enough shear force. We rated the strength ~, stress +, energy -, and structure - (4 of 5 lemons, indicators of weak structure), where + designates stronger snowpack, - weaker, and ~ neutral. Given the extremely thin slab layer, the moderate strength, high energy, and weak structure are not a concern until there is further load. Or if the partial clearing we saw this evening creates enough cooling, it could make weak sugary faceted grains in the surface layer that could be very sensitive when loaded by a bigger snowfall. |
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| Field Notes | ||||||||||||
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| Photos | ||||||||||||
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| Most of the day was foggy, with a light moist rime falling out of the mist. | ||||||||||||
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| The 5cm of snow from last night started to fall as fairly dry snow with a density of 110 kg/m3, then acquired a light moist rime coating that made the surface both damper and harder, giving it a density of 160 kg/m3. Heavier snow falling over lighter, combined with a poor bond to the slick icy crust below, made the surface layer crack between our skis.
Despite the weakness, our block tests had no fractures. This is a common result with soft but sensitive surface layers, because the skis sink below the surface layers without shearing them. Small block tests yield more reliable results in these conditions. In any case, today's new snow at 5 cm thickness did not present a danger. |
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| High on the mountain, all the rock outcrops and boulders were being coated with rime, an accumulation of below freezing but unfrozen water droplets that freeze on contact when they hit anything they can freeze onto. The rime feathers grow into the wind, toward their moisture source. Note how the rime orientation indicates the airflow pattern around this boulder. | ||||||||||||
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| The fog broke late in the day, revealing broken skies above, valley fog below, and a clear snowline at about 550m. | ||||||||||||
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