f > 0 . The non-dimensional parameter α in Eq. 1 characterizes the degree of volumetric change during deformation. For the limit case α = 0 , the model reduces to the standard Von Mises plasticity thory. We consider deformation at low to intermediate strains and use an additive decomposition of the strain tensor ϵ = ϵ pl + ϵ el into elastic (reversible) and plastic (irreversible) contributions. The elastic strain ϵ el is related to the stress via Hooke’s law where we assume isotropic elastic behavior with Young’s modulus E and Poisson number ν . The elastic modulus E in general depends on density and may increase during densification. To describe this dependency, we assume a power-law density dependence as typically observed for solid foams [12], where ρ is the stress-free density of the snow, ρ$_{0}$ is a reference density which we take to be the density prior to compaction, and E$_{0}$ is the corresponding Young’s modulus. To complete our constitutive framework we need to relate the flow stress S to the plastic strain tensor. We assume S to depend both on the density of the snow and on its structure. Phenomenologically, also the yield stress of many solid foams has a power-law density dependence [12]: where the pre-factor Σ depends on the snow microstructure and may be affected by structural softening under shear deformations. The evolution of the stress-free density ρ as a function of plastic strain follows from the continuity equation dρ/dt + ρ Tr( ˙ ϵ ) pl = 0 which can be integrated to ρ = ρ$_{0}$ exp[ − ε pl $_{V}$] where the volumetric plastic strain is given by ε pl V = Tr ϵ pl . Combining with Eq. 3 gives which for negative volumetric strain describes densification-induced strengthening. Shear deformation of snow is associated with failure of bonds connecting ice granules and thus reduces the load bearing capacity of the snow microstructure. We note in passing the proposal of Heierli and Zaiser [13] and Heierli et. al. [14] that volumetric compaction may also be associated with (transient) structural softening. However, the simple viewpoint that densification implies strengthening while shear implies softening, which we adopt in the following, turns out to be fully adequate for understanding the experiments. We characterize the magnitude of shear deformation in terms of the equivalent shear strain ε pl S which relates to the second invariant of the deviatoric part of the plastic strain tensor by ( ε pl S ) 2 = E = E$_{0}$ ( ρ/ρ$_{0}$ ) $^{c}$, (2) S = Σ( ρ/ρ$_{0}$ ) $^{a}$, (3) S = Σ exp[ − aε pl $_{V}$] , (4) 4