 Abstract The radar backscattering cross sections of spongy hailstones are considerably influenced by the topological structure of the water-ice mixture. Observational evidence suggests that such an inhomogeneous composite medium can consist of regions of different topologies. We investigate three different kinds of topologies and derive a general mixed topology mixing rule (MTMR). The mixed topology rule is used to calculate the effective dielectric constant of spongy ice as a function of its liquid-water content. This leads to predictions of backscatter cross sections that are in good agreement with measurement. Journal: Journal of Applied Meteorology and Climatology Volume/Issue: Volume 00: Issue 0 DOI: https://doi.org/00.0000/0000-0000