Ground wave

Ground waves are radio waves propagating parallel to and adjacent to the surface of the Earth, following the curvature of the Earth beyond the visible horizon. This radiation is known as Norton surface wave, or more properly Norton ground wave, because ground waves in radio propagation are not confined to the surface.

The normal line-of-sight distance to the horizon is about 80 to 100 kilometers (50 to 60 miles) at best, for an antenna on a tall tower. But signals at medium or low frequency are observed to travel several hundred miles beyond the horizon, up to several thousand kilometers in some cases. This "hugging the ground" effect is called ground wave, as opposed to direct wave or sky wave. This effect does not occur for signals in the VHF or UHF frequencies, which typically propagate only about 100 km (60 miles) to the horizon.

AM radio stations rely on ground wave propagation to cover their listening areas, which may be a several hundred miles radius.