The Weakest Link--so how's it work?
as mentioned earlier a slab needs 3 things to slide (a bedsurface, a cohesive area of snow--- slab, and a weak layer between them). the strength of the weak layer must be greater than the force exerted on it. so what is a weak layer? simply put it is a layer of snow that is poorly bonded and can't hold up to either shear or compression weight of a slab above it. in a snow pit a weak layer will reveal itself as you run your hand down the wall. it will break up into grains and fall into your hand easily compared to the cohesive layers around it (slabs). buried surface hoar, depth hoar, graupel, light density uncosoilated snow, facets, rain/sun/windscoured crusts can all be weak layers.
when snow comes in light density and then turns heavy you will feel it when you push your pole into the snow. it gets easier to push when you get to the light layer. winds can pile up loose snow (making it heavy and cohesive) ontop of the poorly bonded new snow and create a slab. you will likely be the trigger. even w/o a slab you can still easily get point releases to run far in these conditions (density inversions).
buried surface hoar can be hard to feel with a pole probe. the layer is thin and easy to miss. the crystal are not sintered (rounded and formed good bonds to other crystals) and look like feathers before buried. it is formed on cold humid nights just like frost at your house. it tends to be formed where cold air pools on clear nights like the bottom of valleys or higher up when there is cloud cover or fog it will form just above the fog. it is a persistantly weak layer that doesn't go away qiuckly. so how do you get rid of surface hoar? before buried it can be destroyed by wind, sun or rain (mechanical morph). after burial one way is to have it avalanche. though sometimes surface some buried surface hoar will linger and not go down with the avalanche like most weak layers. rain on the snowpack will melt the weak layer and another freeze will solid it up. sun can increase the temperature and do the same.
facets or plates are made by temperature gradients in the snowpack. if a warm layer of porous snow falls, gets buried and then you have rapid drop in temps it can cause that snow to change. it needs room to grow, hense the porous snow being better. shallow snowpacks will exaggerate the temerature gradient and are notorious for these types of weak layers. depth hoar or sugar snow is considered an advanced faceted grain that is formed much the same. rock outcroppings may gather heat from the sun and allow this layer to flourish especially since it often has room to grow next to the rock. these facets persist on the shady slopes(north slopes around here) longer since the sun can't melt/freeze that aspect as easily. an avalanche will get rid of the layer and a rain event may percolate down and refreeze the layer. advanced facets are strong in compression but weak in shear. the strength of the snowpack above it is what keeps it from shearing. use compression tests instead of shear to check this one.
grauple is ball bearing like snow. it's small, round and doesn't bond well with friends. it can become a slab or a ball bearing like weak layer that just rolls the slab off it. it is typically found in the compression zone or under cliffs. these layers stabalize within a few days after a storm (provided more weight from wind transport isn't added).
rain, wind and fire can all change things. the wind can take all the loose snow off and leave you with a finely buffed boilerplate that dry new snow dousn't want to stick to. sun crusts, which are typically on se to west aspects are rougher than rain crusts and bond a little better with new snow than rain crusts. if the snow comes in wet it will bond better. likewise if the crust is still wet from a thaw when the snow comes it will bond nicely. facets can form just above and below the crust due to hige temperature gradients created by the crust. rain crusts are more slick than a sun crust and just like the sun crust need a good bond to keep things in place. new snow and feceting behaves very similar to that of sun crust but the rain crust can be found on all aspects.
it's important to remember that these weak layers are only troublesome when the force exerted upon them is greater than that of their strength. these layers can be completely fine until a storm or wind finally pust enough weight on it to go naturally or it may be fine until your weight comes along and rapidly overloads the layer. it seems like 180 lbs is incosequential to a snowpack with hundreds of tons upon it but remember that the snow fell slowly and that allowed the layers to adjust to the added weight. you don't move that slowly. also your traverse tracks may be removing the tensile or compression strength that the snowpack is relying upon to stay in place.
http://www.uoregon.edu/~opp/snow/ava...itytension.gif http://www.uoregon.edu/~opp/snow/ava...cturesites.gifhttp://www.uoregon.edu/~opp/snow/ava...onrelease2.gif
in the above right avalanche, the slide was triggered just below a convexity. it also appears to be the steapest point of the slope.
