Juneau Area Avalanche Advisory
2005-11-13
Mt. Troy
by Bill Glude, SAAC Observer
Home Advisory Home Send Us An Observation Next Observation Previous Observation
Text
We found much better snow today for traveling, with the thaw crust from November 8 to 10 now frozen and the 15 cm of snow on it now sugary faceted grains topped with surface hoar. Wet spots, smaller creeks, and puddles have solidly refrozen. The snowcover is still a bit thin everywhere and the snow is very lumpy and rain-runneled below 600m.

We went to the same test slope in the upper West Bowl of Mt. Troy as the last two field days, where both the Cutback AK and AK Blocks yielded moderately weak #4 (first gentle jump) shears at Q2 (average shears) on 40° on a layer of sugary faceted grains below the now refrozen thaw crust 22 cm down. The Rutschblock sheared at a strong #6 (multiple hard jumps) and Q3 (irregular shear) on the same layer. The structure rated 5 lemons (very weak), but the stress was low, the energy low, and there were no signs of instability.

This snowpack is a setup for persistent weakness and an avalanche cycle with the next significant snowfall or wind. The melt freeze crust is weak and faceted to 2cm below its top, the snow on top is very loose and weak faceted grains, and the surface hoar layer is likely to be very sensitive when buried.

Field Notes
Photos
Surface hoar has grown rapidly in the last few cool, clear, and calm days. These large feathers and facets of it are in Hilda Meadows, along the upper cross-country loop. Surface hoar froms when the snow surface radiation cools by radiating heat off into space, which snow does very efficiently, and moisture moves from the warmer air to the cooler snow surface. As the water changes from vapor to solid phase, it makes hoarfrost crystals and transfers heat from the air to the snow and then off into space, working like a heat pump to cool the air. At our test site late in the afternoon today, the air temperature was -10°C and the snow surface had radiation cooled to -18°C (0°F), a difference of 8°C.
On twigs, grass, and branches, and along waterways, surface hoar often grows in these needle forms rather than the feathers and facets commonly seen on the snow.
Hilda Meadows is carpeted in surface hoar. While delightful to look at or ski in when it is on the surface, it is a very sensitive and persistent weak layer when buried by subsequent snowfall.
Bethan Gilmartin shears the Cutback AK Block on 40° at CAK4 (first gentle jump) Q2 (average clean shear) on a layer of sugary faceted grains 22cm down, just under the thaw crust from November 8 to 10. This crust was still soft three days ago, and this weak layer was relatively strong rounded grains then, so our fractures were on the ground. The AK and CAK Blocks today had identical results, while the Rutschblock sheared at RB6 (multiple hard jumps) and Q3 (irregular break).
Midwinter alpenglow light on the upper slopes of Mt Troy.
Home Advisory Home Send Us An Observation Previous Observation Next Observation