The propagation of radio energy under conditions of substandard refraction in the atmosphere; that is, refraction by an atmosphere or section of the atmosphere in which the refractive index decreases with height at a rate of less than 40 N-units per kilometer. Substandard propagation produces less than the normal downward bending, or even upward bending, of radio waves as they travel through the atmosphere, giving closer radio horizons and decreased radar and radio coverage. It occurs primarily when propagation takes place through a layer in which the specific humidity is constant or increases with height. See standard propagation, superstandard propagation.