Literally, refraction that is not normal or typical.
Anomalous or abnormal is frequently used to characterize the conditions that give rise to mirages, such as in this example: "Abnormal refraction responsible for mirages is invariably associated with abnormal temperature distributions that yield abnormal spatial variations in the refractive index." Yet mirages and the conditions that give rise to them are very common. It would appear that, when these terms are used, normality is something defined by the free atmosphere, far from surfaces. Certainly, by that measure, the refractive index gradients near a surface are abnormal. It is misleading to imply that either the refractive structures or the mirages are abnormal or anomalous when they are so common near the earth's surface, merely because they are not as common farther away from that surface.