A propagation path of electromagnetic radiation that deviates from the path expected from refractive conditions in a standard atmosphere. In standard propagation conditions, radiation transmitted horizontally at the earth's surface is bent downward along a path with a radius of curvature equal to 4/3 times the radius of the earth. Subrefractive propagation causes less bending of the ray and superrefractive propagation causes greater downward bending than in the standard conditions. AP clutter is an extended region of ground echoes caused by superrefraction. See effective earth radius.