Ground Waves are radio waves that follow the curvature of the earth. Ground waves are always vertically polarized, because a horizontally polarized ground wave would be shorted out by the conductivity of the ground. Because ground waves are actually in contact with the ground, they are greatly affected by the groundís properties. Because ground is not a perfect electrical conductor, ground waves are attenuated as they follow the earthís surface. This effect is more pronounced at higher frequencies, limiting the usefulness of ground wave propagation to frequencies below 2 MHz. Ground waves will propagate long distances over sea water, due to its high conductivity.
Ground waves are used primarily for local AM broadcasting and communications with submarines. Submarine communications takes place at frequencies well below 10 KHz, which can penetrate sea water (remember the skin effect?) and which are propagated globally by ground waves.