This cluster of papers focuses on radar wave propagation, refractivity estimation, and atmospheric ducting. It includes research on parabolic wave equations, evaporation ducts, tropospheric refractivity, and the impact of meteorological parameters on radio wave propagation. The studies cover a wide range of techniques and models for estimating refractivity from radar sea clutter and ground-based GPS, as well as the development of tools for propagation prediction over variable terrain.