Atmospheric ducts capable of propagating microwave signals often form over what geographic feature?
Why Tropospheric ducts form when a temperature inversion puts warm, dry air above cool, moist air, creating a sharp drop in refractive index that traps VHF/UHF/microwave signals in a waveguide-like layer. That layering happens most reliably over large bodies of water, where cool water chills the surface air and keeps it humid while warmer air sits above. Marine ducts over the ocean, the Gulf of Mexico, or the Great Lakes routinely support contacts of hundreds of miles on bands from 144 MHz up through the microwave bands.
Watch out Clouds themselves, whether stratocumulus or nimbus, are not the mechanism; a duct is about the refractive index gradient of the air, and stratocumulus decks are simply a common visible sign of an inversion rather than the duct itself. Mountain ranges support knife-edge diffraction and rain scatter, not ducting.
Ducting loves water: cool moist air under warm dry air over the sea makes a natural waveguide.