How does the VHF/UHF radio horizon compare to the geographic horizon?

While VHF and UHF radio waves primarily travel via line-of-sight, they do not travel in perfectly straight lines. The Earth's atmosphere is denser near the surface and thinner at higher altitudes. This gradient in atmospheric density causes the radio waves to gradually refract, or bend, downward toward the Earth's surface. Because the radio waves are bending slightly along the curvature of the Earth, they can reach beyond the true visual or geographic horizon. This atmospheric refraction makes the effective radio horizon approximately 15 percent farther than the geographic horizon.