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ence between this and the condition necessary for looming is that for 

 superior mirage there nuist be a more sudden change from cooler to 

 warmer air at a certain height. 



The observer on a ship near land usually sees mirage as an unnatural 

 image of the coastline, single, double, or triple, or as an appearance of 

 the coast much nearer to him or farther from him than it actually is. 



At sea, ships and icebergs are the mirage subjects more generally seen. 

 Ocean fog is also associated with mirage since the temperature and 

 humidity variations which favor condensation of moisture as fog in the 

 air are often factors in causing mirage. An attendant mirage is, of 

 course, not observable while dense fog actually obstructs the vision, but 

 mock fog or the typical refraction band is often seen under such condi- 

 tions and may lead to the recording of damp, or true, fog which does 

 not exist. 



The not uncommon phenomenon of mirage has been responsible for 

 many false estimates of remoteness of newly discovered land features 

 which have been seen by explorers within the polar regions, combined 

 as it has been with the underestimates of distance due to the unusual 

 clarity of the atmosphere. In many cases of snow-covered lands, there 

 is not enough individual character in the coastal features to permit 

 identification from different ship positions, and in such cases coasts 

 have frequently been placed upon charts on the basis of the direction 

 and estimated distance from positions offshore. These estimated dis- 

 tances are often as much as 40 to 50 miles too low because of atmospheric 

 clarity alone, and can be as much as 300 miles too low as a result of the 

 existence of a superior mirage. 



As already indicated, abnormal refraction can be recognized only by 

 its effect on the appearance of land, or such objects as ships, or icebergs. 

 Temperature inversions may also give rise to abnormal dip of the 

 horizon, which may seriously affect the accuracy of sextant observa- 



Figure 26. — Diagram illustrating the conditions under which superior mirages may be formed 

 off large ice masses. The inversion layer has been warmed adiabatically in descending the 

 glacier surface. The dust-free nature of the air leads to great underestimation of the distance 



of the coast. 



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