A reactive layer of buried surface hoar was observed 45cm down on sheltered north-facing aspects below ridgeline. The surface hoar layer demonstrated a propensity for propagation w/ ECTP results. Buried surface hoar is notorious for persisting in the snowpack for long periods and propagating fractures over long distances. All protected N-E slopes >30 degrees below ridgeline should be considered suspect. Surface hoar layers are really hard to see in snow profiles.
The best way to find out if surface hoar exists in the snowpack is to use upper snowpack tests like CT's and ECT's. If you get a clean planar fracture a foot to two feet down in the snowpack, then you've likely found that persistent weak layer (PWL) and you should keep to slopes < 30 degrees.