Deviation but we used to observe anaprop all the time when I was at school. On the beach at St Bees we could see the Isle of Man up in the sky with the sea out to the horizon below it. All ducting. It is just refraction of light in the atmosphere and it was sort of possible to predict based on the weather conditions.
A common cause of this phenomenon is an atmospheric event known as a temperature inversion, which occurs when a layer of warmer air forms above a layer of cooler air, reversing the normal altitudinal trend and interfering with the radar beam’s path. This often happens on calm, clear nights within areas of high-pressure; after sunset, the ground begins to lose heat quickly, cooling the near-surface air, while subsiding air aloft (associated with the high) warms as it sinks, eventually forming a relatively stable blanket atop the cooler air below.
The effect is not dissimilar to sub-surface convergence zone propagation.