|Norwegian Fjiord Mirage by Lars Brubæk looking 6km across Sunndal fjord near Kristiansund.. The miraged opposite shore is that of the island of Bergsøy. ©Lars Brubæk, shown with permission.|
This fjord has to my knowledge never been known to freeze, and there has been reports that recent sea temperatures are warmer than before. I therefore assume that the sea surface was considerably warmer than the air temperature and that a somewhat steeper temperature gradient existed close to the water surface than higher up.
Picture taken with a 300 mm lens on DX-format sensor, from ca. 4 m above sea level, across 6 km of sea."
Houses and shore features close to the water are mirrored upside down. The mirroring occurs not at the water level but higher up at what is named the vanishing line. The distant shoreline below the vanishing line is not visible. The uneven upper edge of the the fjiord water is not real - it is the lower edge of the mirage.
The mirage is an inferior mirage because the image is below the real object. Light passing at low angles across the different temperature air layers is refracted so that rays coming from the opposite shore appear to be coming upwards.
The ray diagram attempts to show what is happening but like all mirage ray diagrams the vertical scale has to be grossly exaggerated. All possible rays between the eye and the distant shore are shown. Two rays from (A) reach the eye. The upper one is slightly curved downwards towards the warmer air. The lower ray from (A) is sharply curved and appears to the eye to come from shore details reflected by a mirror. Rays from point (B), lower down, do the same. Position (C) is different. Only one ray reaches the eye. Rays from lower down the shore cannot reach the eye at all and so it is invisible.
The level (C) is the level of the mirage vanishing-line. The 'real' view and inverted mirage view join at the vanishing line and the eye can see no details of the scene that are below it.