There are few measurements of Poisson’s numbers for snow reported in the literature, with reported values being typically very small [9]. In the present context we can obtain an upper estimate of Poisson’s number from our own lateral force measurements. Identifying the side plate normal direction with the z direcation and assuming that the lateral force is exclusively due to elastic cross-expansion, we find from σ$_{zz}$/σ$_{xx}$ = 0 . 025 = ν/ (1 − ν ) that ν < 0 . 025. This allows us to approximate ν ≈ 0 which yields the simple elastic constitutive relation σ$_{ij}$ = Eϵ el $_{ij}$. Fundamental to our constitutive formulation is the parameter α which governs the vol- umetric response in the plastic regime. To determine this parameter, we observe that the experimental deformation behavior is characterized by the emergence of localized deformation in the form of compaction bands that are oriented perpendicular to the compression direction. As discussed by [10], such behavior occurs for α = 1 / 2. We note that α = 1 / 2 is also quite commonly used in deformation of metallic foams, see the pa- rameterizations of [10], [11] and [12]). Again, we can use our lateral force data to derive an upper estimate for the deviation δ = 1 / 2 − α from this value. For δ = 0, the lateral plastic expansion is, to lowest order in δ , given by ε pl zz = (2 / 3) δε pl $_{xx}$. Assuming that the lateral force at the end of the deformation experiment is exclusively due to the elastic accomodation of this lateral plastic expansion, and using the elastic modulus value given above to evaluate ε pl zz = σ$_{zz}$/E , we find an upper estimate δ ≤ 10 − 3 which shows α ≈ 1 / 2 to be a very good approximation. The parameters characterizing the strain and time dependence of the yield stress in our model – the exponent a relating strength to density, the initial strength Σ = Σ 0 $_{S}$+Σ$_{R}$ , the residual strength Σ$_{R}$, the softening strain ε$_{S}$ , and the relaxation time τ – were obtained by matching simulated and experimental stress-strain curves. This yielded the values a = 7, Σ = 5 × 10 4 Pa, Σ$_{R}$ = 0 . 25 × 10 $^{4}$Pa, ε$_{S}$ = 0 . 05, and τ = 10 s. The fitted strength values are well within the range reported in the literature [5, 13, 14], and the same is true for the relaxation time τ [15, 16]. For α = 1 / 2, instability of homogeneous uniaxial deformation with respect to formation of localized bands is associated with perturbations of strain in the axial direction, i.e., bands are oriented occur at 90 degrees to the stress axis [10]. Instability occurs if the derivative of the yield function with respect to the plastic strain becomes positive. Neglecting for the time being the gradient term, the stability requirement is thus: Strongly localized perturbations are of negligible influence on the stress given by Eq. 4, hence we obtain the simple stability criterion This is tantamount to requiring that the rate of hardening due to densification, with structural strength fixed, exceeds the rate of softening due to bond breakage, calculated with density fixed: Densifcation-induced hardening must outweigh shear softening. 4 Stability analysis df de ∣ t = ∂s ∂e − a (Σ$_{S}$ + Σ$_{R}$) exp[ − ae ] + 1 ε$_{S}$ Σ$_{S}$ exp( − ae ) ≤ 0 . (7) aε$_{S}$ ≥ Σ$_{S}$ Σ$_{S}$ + Σ$_{R}$ (8) 3.2 Parameters of the plasticity model 4