What portion of a radio signal is directly affected by the surface of the Earth?

The ground wave is the portion of a radio signal that travels directly along, and is in contact with, the surface of the Earth. It essentially 'hugs' the terrain as it moves outward from the transmitting antenna. Because it travels along the ground, its propagation is strongly influenced by the physical characteristics of the Earth's surface, such as soil conductivity, bodies of water, and terrain roughness. High conductivity, like salt water, allows ground waves to travel much further. In contrast, sky waves (ionospheric waves) interact with the upper atmosphere, and space waves travel line-of-sight above the immediate surface, unaffected by the ground's conductivity.