localization is bound to occur, otherwise, deformation is expected to proceed in a homogeneous manner associated with monotonic hardening. The difference is manifest not only in laboratory experiments, but in everyday experience: Walking on snow is, in the second case, associated with a foot resistance that increases with increasing penetration depth – as a consequence, we can walk on the snow. In the strain softening case, on the other hand, once the critical load needed to nucleate a compaction band is exceeded the foot travels downwards together with the compaction band at constant or even decreasing load – we have the feeling of breaking through the snow. This behavior, commonly interpreted in terms of snow heterogeneity ("there is a crust on the snow"), in fact reflects the intrinsic interplay of strain softening and strain localization characteristic even of homogeneous snow. The formation of compaction bands has close analogies in deformation of metallic foams, where the initial stage of compressive deformation is often characterised by localized deformation bands which can be explained in terms of localized structural softening (buckling or failure of cell walls) [17, 9]. However, repeated nucleation and propagation of deformation bands, which is a conspicuous feature of the present experiments, has never been observed in solid foams. The difference is readily understood if we consider the extremely high homologous temperatures in the present experiments where rapid sintering may lead to strength recovery on short time scales [18, 19]. As a consequence, if the characteristic time required for propagation of a deformation band is on the same order of magnitude as the characteristic time of strength recovery in a snow sample, repeated band nucleation may be observed. In the context of snow, the interplay between softening and ageing has been modelled by a number of authors (e.g. Louchet [20], Reiweger et. al. [21]. However, models published to date cannot describe spatio- temporal strain localization, either because they implicitly assume homogeneous deformation [20] or because they consider spatial couplings within a mean-field framework which does not account for spatial structure of the deformation field [21]. We have demonstrated in the present paper that generalized continuum models (here: gradient plasticity), combined with a simple phenomenological description of strain softening and age hardening processes, offer a framework capable of quantitatively describing the observed spatio-temporal phenomena. Beyond the scope of the present study, such models may offer a generic framework to describe snow deformation also under heterogeneous and time-dependent loading conditions. T.B., T.S. and J.B. designed the apparatus and DIC imaging system, T.B. carried out the experiments, S.S. and S.L. analyzed the data, G.W. wrote the code and performed the simulations, M.Z. formulated and Author contributions 10