Sea ice plays a key role within the global climate system. It covers some 7% of earths surface and processes a strong seasonal cycle. Snow on sea ice even amplifies the importance of sea ice in the coupled atmosphere-ice-ocean system, because it dominates surface properties and energy balance (incl. albedo).Several quantitative observations of summer sea ice and its snow cover show the formation of superimposed ice and a gap layer underneath, which was found to be associated to high standing stocks of algae. Superimposed ice forms from the refreezing of snow melt / fresh water (Fig. 1+2).Here we present properties of melting snow (Fig. 4-6), processes of superimposed ice formation based on field measurements and ice-laboratory analysis (Fig. 7-10), as well as first results from a numerical model (Fig. 11+12).