What effect is responsible for propagating a VHF signal over 800 km?

When VHF signals travel distances exceeding 800 km, they are typically taking advantage of an enhanced atmospheric condition known as tropospheric ducting. This occurs when a sharp temperature inversion creates a trapped boundary layer in the lower atmosphere, functioning much like a waveguide or fiber-optic cable for radio waves. VHF frequencies are generally too high to be bent back to Earth by the ionosphere under normal conditions, meaning options like D-region or ionospheric refraction are incorrect. Tropospheric ducting is the primary mechanism that allows these high-frequency signals to follow the Earth's curvature for immense distances.