The propagation of radio waves under conditions of superstandard refraction (superrefraction) 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 greater than 40 N-units per kilometer. Superstandard propagation produces greater than normal downward bending of radio waves as they travel through the atmosphere, giving extended radio horizons and increased radar coverage. It is caused primarily by propagation through layers near the earth's surface in which the dewpoint temperature is rapidly decreasing or the temperature is increasing with height. Such conditions are commonly observed near coastlines when a layer of warm dry air overlies a cool moist layer adjacent to the ocean surface. A layer in which the downward bending is greater than the curvature of the earth is a radio duct. Frequently, the general term, anomalous propagation, is used for superstandard propagation. See standard propagation, substandard propagation.