Errant Ski-Slope
~ Renowned atmospheric optics photographer Pekka Parviainen (Polar Image) imaged this distant Finnish ski-slope as serpent-like it writhed and twisted. ©Pekka Parviainen, shown with permission.

Parts of the ski-slope are being raised or lowered plus being vertically stretched and compressed. Some miraging (multiple images) is also present.

The distortions are the result of a complex and changing vertical atmospheric temperature profile. The different temperatures give layers of different air density. Rays passing across these layers are refracted (deviated). Rays are deviated towards the denser layer. The outcome is that a distant object appears vertically displaced or even distorted – here the effect is extreme.

Old time sailors had words for the different distortions that they saw in ships near the horizon.

Looming – The image of the distant object appears raised up but is not vertically stretched or compressed. A constant, but steeper than normal, decrease in density with height produces looming. Put another way, it needs a less steep fall in temperature with height.

Sinking – The image is lowered but not vertically distorted. The fall in density with height is less steep than normal.

Towering – The image is vertically stretched, it “towers”. In effect the image has more looming at its top. This is the result of a non-linear temperature/density profile with a stronger temperature inversion in the upper air layers.

Stooping – The image is squashed. Stooping can be generated by lower air layers increasing in temperature with height with higher layers having decreasing temperatures.

Pekka’s ski-slope shows all these effects with parts of the image being compressed, parts stretched, raised and lowered. The air temperature profile was complex and changing.


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