NATURAL PHENOMENA 



G. E. Buckwalter 



Maritime Safety Division 



U. S. Naval Oceanograpkic Office 



Mariners, by the oery nature of their calling, have excellent 

 opportunities for observing natural phenomena which may be of great 

 value to the astronomer, oceanographer, and meteorologitit. The 

 scientist that delves into the secrets of nature depends upon accurate 

 data obtained from all sources, but obaervations of phenomena from 

 remote areas of the world add significantly to the sum of man's 

 knowledge particularly when geographic distribution studies of the 

 occurrences of such phenomena are made. Also, such observations 

 may give the clues to unlocking the secrets of nature. 



Observations of phenomena in the marine environs reported by 

 mariners to the U. S. Naval Oceanographic Office are not only useful 

 in studies made by this office but are forwarded to the various 

 govern ment agencies and institutions having related interests. 

 Mariners are therefore encouraged to forward observations con- 

 cerning natural phenomena, as well as other hydrographie 

 observations. 



The following are selected examples of natural phenomena to 

 which mariners have contributed significant knowledge as well as 

 being of general interest. Inquiries concerning them have been 

 received from time to time. 



ABNORMAL REFRACTION 



Deceptions of vision at sea are produced by abnormal refraction 

 of light which in the more extreme cases give rise to false images 

 of land, ships, or other objects. Generally, abnormal refraction at 

 sea is due to an inversion of temperature in a layer of air, the varia- 

 tions in density thus produced causing the light rays to be bent 

 considerably in excess of normal conditions. 



The most frequent and most favorable conditions for excess 

 refraction, under which most of the more fantastic forms of mirage 

 and distortion take place, occur when a layer of warm air is in 

 contact with cooler water. The air next to the surface of the sea is 

 cooled, and consequently the upper layers are warmer than the lower 

 so that instead of the usual decrease there is an increase of tem- 

 perature with height. Most refraction phenomena are formed at the 

 boundary between this cold dense layer of air at the surface of the 

 sea and the less dense warm air above. This condition is identical 

 to that which is responsible for the formation of most sea fog; 

 therefore, the presence of fog is an indication that excessive refrac- 

 tion is likely to be encountered. 



Similar inversions may be caused by the presence of cold air over 

 warm water. A marked difference between air and sea tempera- 

 tures is therefore another guide to the presence of excessive 

 refraction. 



Although abnormal refraction is not restricted to particular 

 geographical areas, certain regions of the globe are so situated with 

 respect to general meteorological conditions as to be more favorable 

 than others for the occurrence of abnormal refraction phenomena. 

 The polar coasts are among the most favorable of these regions be- 

 cause of the frequently prevalent marked difference between sea 

 and air temperatures. 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. 



Fin lire 1 



Mirage 



OIAGDAM 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 IHE COAST 



In the polar regions the most common forms of abnormal refrac- 

 tion are 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 ob- 

 ject 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 tem- 

 perature inversion at a distance of several feet above the surface. 

 This inversion introduces an abnormal change in density, and extra- 

 ordinary 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. Some- 

 times 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 1. 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 difference between this and the condition necessary for looming 

 is that for superior mirage there must 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 un- 

 natural 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 ob- 

 structs the vision, but mock fog or the typical refraction band is 

 often seen under such conditions 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 distances 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 

 observations. The navigator should therefore always be on guard 

 against such a possibility whenever there is a chance that a tem- 



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