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

The earth's atmosphere has varying layers of temperature, pressure, and moisture. This creates a density gradient that acts like a weak lens, causing radio waves in the VHF and UHF spectrum to bend or refract slightly downward toward the earth as they travel. Because of this downward bending, the radio waves can follow the curvature of the earth a bit further than the human eye can see in a straight line. As a result, the radio horizon is generally about 15 percent farther away than the true visual horizon. While dust or moisture can affect certain extremely high frequencies, they do not cause the horizon extension. Additionally, radio waves are electromagnetic waves and travel at exactly the speed of light, so they do not move faster than light.