Snow stratigraphy studies at the fracture lines of dry slab avalanches often indicate a situation that consists of a harder thick layer overlying a thin weak layer. Prefacture stress conditions and progressive fracture of snow slabs are discussed here from the point of view that slow shear failure in the weak layer results in Griffith-like critical lengths there, resulting in rapid shear fracture and subsequent avalanche release. The important aspects of the timing of dry slab avalanche release and the relationship between the applied shear stress and the shear strength at the weak layer are discussed in this context. The results show that the main experimental facts on snow and field observations of snow slab avalanches are explainable from this approach, although direct, concrete proof is not presently available.