Superior mirages result from cold air beneath warmer - a temperature inversion. Rays from the object are refracted by the vertical temperature/density gradients. Rays tend to curve towards the denser, cooler, layer and are thus generally concave towards the sea or ground.
The precise form of the optical distortion, mirage if there is more than one image formed, depends on the exact form of the temperature profile and gradients. Mirages can range from a classical inverted image above the object through to the towering cliffs and castles of a Fata Morgana.
At left the ray 'a' from the ship's mast passes through only cold air and is hardly deviated - the lower image of the ship appears it would be with no mirage taking place.
Ray 'b' from the same point on the ship grazes the boundary between the cold and warm air layers and is refracted downwards - refracted rays always curve towards the colder and denser air. To the eye, ray 'b' appears to be coming from a point above the real position of the ship.
Now look at one of the rays from the ship's hull - ray 'c'. Its increased angle to the sea causes it to penetrate deeper into the cold/warm boundary and it is refracted more strongly. To the eye it appears to come from a point higher than that of ray 'b'. The miraged image of the ship is inverted and above the ship itself.
Ray 'd' from the ship's mast also penetrates deep across the cold to warm air temperature gradient and is strongly refracted downwards again. Ray 'e' is even more strongly refracted. It crosses ray 'd' and forms yet another inverted image. Crossed rays give inverted images.
The Redcar mirage is more complicated than the above schematic. There are complexities in the temperature gradients that yield weak and possibly multiple overlapping and tilted first inverted image(s). The upper erect image, if indeed it is erect, is vertically compressed and all the images are truncated to the left.