We present new experimental data and review previous experimental results on molecular hydrogen formation from atomic recombination on porous amorphous solid water (ASW) surfaces at temperatures from 10 K to 30 K, i.e. under conditions of relevance to cold dense interstellar clouds. We show that the desorption of molecular hydrogen formed on porous ASW surfaces is well described by a model, in which the molecules are assumed to be completely thermalized to the surface temperature and to be evenly distributed throughout the porous network of the ASW films. These results emphasize that, not only the chemical properties, but also the physical morphology of dust grain surfaces must be considered to obtain a full understanding of molecular hydrogen formation on interstellar dust grain surfaces.