1. The process of combining different surface characteristics from neighboring heterogeneous regions into an average value for the area. It is used in boundary layer studies for surface fluxes, drag, and roughness. This process is often necessary to define surface characteristics for numerical models that have coarse horizontal grid mesh and that cannot resolve the individual surface areas. 2. The process of clumping together of snow crystals following collision as they fall to form snowflakes. This process is especially important near the melting layer where snow particles stick to each other more easily because of the liquid water on the surface. It also occurs at lower temperatures especially between dendritic snow crystals and occasionally rosette crystals in cirrus.