Conductivity of the surface affects the propagation of ground waves, with more conductive surfaces such as sea water providing better propagation.[8] Increasing the conductivity in a surface results in less dissipation.[9] The refractive indices are subject to spatial and temporal changes. Since the ground is not a perfect electrical conductor, ground waves are attenuated as they follow the earth’s surface. The wavefronts initially are vertical, but the ground, acting as a lossy dielectric, causes the wave to tilt forward as it travels.