Why is the radio horizon for VHF and UHF signals more distant than the visual horizon?

As VHF and UHF radio waves travel through the Earth's lower atmosphere, they pass through layers of air with varying densities and temperature profiles. These atmospheric conditions act somewhat like a lens, slightly bending or refracting the radio waves downward toward the Earth's surface. Because of this slight refraction, the radio waves can follow the curvature of the Earth just a bit farther than visible light can. As a result, the practical "radio horizon" is generally about 15 percent farther away than the true visual horizon.