RESTRICTED 



polar coasts are among the most favorable of these regions because 

 of the frequently prevalent marked difference between sea and air tem- 

 peratures. In polar regions excessive visibility or some form of 

 mirage is often manifest when comparatively warm and light winds 

 blow over the cold ice surfaces or when cold winds blow over open 

 water. A milder temperature over open water than over the ice-clad 

 adjacent shore also leads to refraction phenomena. 



In the polar regions the most common forms of abnormal refraction 

 tre looming and superior mirage. Looming is the apparent raising of 

 an object above the horizon. It is quite common at sea, especially in 

 high and middle latitudes, and results in the appearance of distant 

 objects which in many instances may actually be below the normal 

 horizon at the time of observation. There are two types of looming. 

 In one case, the object (island, iceberg, ship) is seemingly increased in 

 elevation though not in size ; in the other case, the object appears to be 

 enlarged and brought much nearer to the point of observation. 



The atmospheric condition that produces looming is one in which 

 there is an abnormal decrease in the density of the air from the surface 

 upward and hence a greater than normal downward curvature of the 

 paths of light rays. The more rapidly the density decreases with 

 elevation, the more unnatural and impressive becomes the phenomenon. 

 If the rate of this decrease is variable at low elevation, the shape of the 

 looming object is distorted, and strange bulging, thinning, flattening, or 

 pointing may occur. Thus, a distant rounded peak might loom in its 

 natural shape, appear with perfectly flat summit, or with a misshapen 

 summit drawn much nearer the observer than the base. Likewise, the 

 appearance from the masthead may be different from that at deck level. 



Superior mirage is the apparent reflection from a more or less mirror- 

 like atmospheric condition where there is a pronounced temperature 

 inversion at a distance of several feet above the surface. This inver- 

 sion introduces an abnormal change in density, and extraordinary 

 refraction results. Its most frequent appearance is that of an inverted 

 image above the object, but under suitable conditions a second mirage is 

 seen erect, close above the inverted one. Sometimes the object is not 

 observed directly and the inverted image or the upper erect image of an 

 object below the horizon may be seen. 



The formation of superior mirage is illustrated in figure 26. It is 

 best and most frequent in Arctic and Antarctic regions but it may be 

 observed down to middle latitudes. As with looming, the condition 

 requisite for its formation is a warm layer of air existing over the sea at 

 a suitable height ; that is, an inversion of temperature. The only differ- 



110 RESTRICTED 



