Miraged Icebergs over Disko Bay, Greenland pictured from Ilulissat by Poul Oestergaard July 3, '05. ©Poul Richard Oestergaard, shown with permission.

The distant bergs have mirrored companions floating above them. Above those are even hints of more bergs, this time the right way up.

This is a 'superior' mirage - superior because the images are formed above the miraged object. An extreme type of superior mirage is the fabled Fata Morgana (1,2,3). Everyday 'inferior' or hot road mirages have the image below the object creating them.

A temperature inversion is responsible. The icy cold sea cools the air it is in contact with forming an unusually cold layer beneath warmer air. The cold air is visible against the 50-90 km distant mountains as a dark band.

Light rays passing between the dense colder air and the upper warm air are refracted. The ray paths become curved and concave to the horizon level. I.e. they tend to curve downward. The curvature is always such that the rays tend to curve towards the cooler - denser - air. The ray curvature is strongest where the vertical temperature gradient is greatest.

In the diagram at right the ray 'b' from the real berg top is slightly curved making it appear to originate from a point above. Ray 'c' from the berg's base encounters a higher region of even greater temperature gradient and is more strongly refracted. A rule of thumb is that where ray crossing occurs the resulting image is inverted. An upside down berg floats over the lower one.

When the inversion layer is thick enough, rays from the berg top have a third route 'd' available through upper warmer air before curving down towards the eye or camera. They form a third image - this time the right way up. Traces of it show in the picture where the berg base starts to bulge upwards.

Erik the Red is said to have visited the area in c.980. It was also the final call for many early polar expeditions before they quickly got themselves icebound while searching for the North West Passage.


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