Snow is a material composed of air, ice and sometimes liquid water and impurities. Snowflakes can take numerous different forms controlled by the atmospheric conditions. Once deposited on the ground, snow continues to evolve. Indeed, its high porosity and its temperature close to the ice melting point promote rapid changes in the microstructure through water vapor transport or melting/refreezing processes. This time evolution is called snow metamorphism. As a consequence, the snow material is characterized by a wide range of densities and microstructural patterns classified into different snow types (see above).

The different snow microstructural patterns are traditionally classified into snow types, discrete classes based on the observation with 10 x magnification lens of the shape of individual snow grains. For instance, one distinguishes precipitation particles (PP), rounded grains (RG) or depth hoar (DH) (see above). The state-of-the-art snow models such as Crocus or SNOWPACK describe the morphology of the snow grains by somehow converting these discrete classes into continuous variables. For instance, Crocus uses dendricity, sphericity, grain size and a historical variables, from which the snow type can be determined. None of these variables can be unambiguously measured nor defined. Efforts have been conducted to get closer to a physical description of the snow microstructure (e.g. dendricity and grain size replaced by specific surface area) but this rude representation still limits the performance of the model.