A rather shallow, almost horizontal layer in the atmosphere through which vertical temperature and moisture gradients produce a refractive index lapse rate greater than 157 N- units per kilometer. Conditions necessary for the formation of ducts are strongly increasing temperature and decreasing relative humidity with height. The resulting superstandard propagation causes the curvature of radio rays to be greater than that of the earth. Radio energy that originates within a duct and leaves the antenna at angles near the horizontal may thus be trapped within the layer. The effect is similar to that of a mirage (sometimes called “radio mirage”), and radar targets may be detected at phenomenally long ranges if both target and radar are in the duct. The greater the elevation angle between radar and target, the less the possibility of serious distortion due to transmission through ducts. Ducts may be surface-based or elevated, with typical thickness ranging from about 10 to 300 m. When elevated ducts occur, they are generally associated with subsidence or frontal inversions. Elevated ducts are rarely found above 5 km. See anomalous propagation, skip effect.