Wind slab produces areas of increased snow hardness and cohesiveness that are much more prone to avalanche and are often triggered when confidence is increasing and terrain becomes more aggressive, more open and more exposed. If you see signs of wind effect on the snow surface, expect wind slab conditions.
As beautiful six-sided snowflakes fall to the ground they may get knocked around by wind. Turbulent winds break the arms off the flakes and smash them into smaller pieces which allow them to pack tighter. These pieces are then deposited into sheltered (lee) areas on mountainous terrain to form a harder, more cohesive, and more sensitive deposit of snow that has more weight and stresses weak layers.
Reading the wind is similar to reading current in a river, and noticing wind affect is an important skill to develop. Signs to watch for include changes in snow hardness, hard snow over soft snow, snow drifts or pillows, and scouring as the snow has loaded somewhere else.
Depending on the characteristics of the wind, snow crystals may get jammed together with such force that an isolated hard slab forms even though the majority of terrain has powder and is slab free. Classic wind loading zones are under cornices and ridgelines but, somewhat more mischievous, wind can get to the snow surface in open meadows and cut blocks in the trees.
Wind slabs come in all shapes and sizes, but in cross-section most look like a lens. For example, a 50 metre round wind slab may have a 50 cm thick center which tapers to nothing at the edges and it literally jumps out of its location when triggered.
One afternoon during good snow stability, I witnessed a wind slab release. On a seemingly uniform slope, the left line was skied safely while another party skied the right side that later proved to be wind loaded.
It took the weight of four people skiing spaced 30 m apart to make the slab fail. This slab was 120 cm deep, about 150 m long and close to 60 m across.
The entire unit slid for about 40 or 50 m before it started to break into smaller pieces. One fellow fell over and then barrel-rolled until he fell off the edge, and the other three skiers simple skied off the moving slab to safety.
It was a close call and luckily all was well at the end of the day, but it demonstrates how a seemingly uniform slope can be powder in one area and a wind slab in another. Probing would have shown 40 cm of surface snow on the left increased to 120 cm on the right, plus the increase in hardness.
This was one, large, isolated, hard wind slab that slowly built up from light winds drifting snow through the pass and depositing it on the right side of the slope and not on the left. It was quite a large slab and it had tapered edges and literally jumped out of its place and started tobogganing down the slope as a single unit.
Another example of wind slab was a fine cold, clear morning in the southern Rockies, with 10 cm of new snow overnight at -15 C, light to no wind overnight, and a snow stability forecast in the morning of good to very good.
We got caught again, and in retrospect our only warning was the observation that the 10 cm that had fallen was mostly scoured off wind exposed areas. It had drifted into isolated lee pockets and formed into slabs that were up to 50 cm in depth with little surface area, but they were scattered like landmines across the slopes and were very reactive to skier weight.
It took less than a run to change the stability rating to poor and to move everyone to more sheltered, less steep terrain. The power of observation is critical and with the understanding of how wind moves snow and how it is deposited is an important step in making safe decisions after wind events.
It doesn’t take much imagination to forecast a scenario for a warm snow storm of zero to -5 C with strong winds. Warmer snow will pack tighter and quicker then cold snow and, with the wind’s energy, one can assume that wind slabs would be prevalent in most wind affected terrain and should be avoided.
Watch for blowing snow and take note of wind direction as all this information can be used in making slope use decisions.
There are many tests for checking wind slab and a good example is found on YouTube, “triggering a slide in upper teepee basin.” Also check out “windslab over weak layer 12 Dec 2010.”